Glossary of planetary geology

Some terminology for planetary geology, the solar System, moons, asteroids, comets, meteorites, exploration, measurement, and related topics.

A3 Tsuchinshan-ATLAS: Comet C/2023, is the brightest comet visible from Earth since Hale-Bopp in 1997, mostly visible to the naked eye in October 2024. During this time the comet had an anti-tail – a tail pointed towards the sun. It has a retrograde orbit of 80,000 years!

Ablation: Erosion of an object as it enters Earth’s atmosphere, usually associated with heat. The term also applies to Earth surface objects that are eroded and commonly shaped by wind. 

Absolute magnitude: The brightness of an object if it was 10 parsecs, or 32.6 light years from Earth. Usually applied to stellar objects. The brighter the object the lower the number according to a logarithmic scale. Magnitude numbers can also be negative for really bright objects, like our own sun. So, the difference in brightness between magnitudes 1 and 2 is 2.512 times. The scale can be applied to stars, planets, and comets. Measured magnitudes (light flux) are usually calibrated against standard stars for which the flux, or magnitude is known accurately.

Absolute zero: The theoretical temperature, measured in degrees Kelvin (K), where the kinetic energy of atomic particles is zero – everything is at complete rest. The temperature value corresponds to -273.15^o C and -459.67^o F. Thus, 0^o C is 273.15^o K.

Accretion: The process where atoms, dust, and larger lumps of matter come together to form a more massive object. The principal forces that result in accretion are gravity – accretion is responsible for star, galaxy, and planetary body formation from an initially dispersed interstellar molecular cloud. Accretion also generate heat and potentially electrically charged plasma gases, that in turn generate increasingly stronger magnetic fields. Accretion collapse appears to generate disc-like structures with nascent stars at the centre.

Achilles Asteroid 588: The first Trojan asteroid to be discovered in 1906 (it has the same orbit as Jupiter). It has an average diameter 133 km.

Achondrites: This group of meteorites lacks chondrules and has igneous textures and compositions that indicate a degree of magmatic differentiation and metamorphism, having formed on all manner of bodies such as planets, moons, and asteroids. Hence, they are more like many basic and ultrabasic Earth lithologies. Mineralogically, they contain pyroxenes, olivines, Ca-rich plagioclase, traces of other silicates, and FeNi Kamacite. Achondrites derived from asteroid collisions with Mars and the Moon have been found on Earth. Lunar-derived achondrites have identical mineralogical and geochemical compositions to lunar rock samples collected by the Apollo missions.

Active seismic experiments: Use of artificial sources of seismic energy are common practices for Earth bound seismic investigations. The method was used for the first time on another planetary body during Apollo missions 14, 16, and 17. Two types of energy source were used: thumpers triggered by small explosive charges, and explosives lobbed a few 100 metres from the stations by grenade launchers.

Active volcanism on Venus: Images obtained by the Magellan spacecraft between 1990 and 1992 reveal a change in shape of a vent on Maat Mons, one of the largest volcanoes on Venus. Possible new lava flows were also identified. These phenomena appear to confirm recent volcanic activity on Venus (Herrick and Hensley, 2023).

Adiabatic: A process where no heat enters of leaves the system but where energy is transferred to mechanical work.

Age of Earth: Current estimates indicated ~4.5 billion years (4.5 Ga). This is based on radiometric dating of moon rocks (4.4 Ga), meteorites (e.g., from Barringer Crater 4.567 Ga), and the oldest (so far) zircons from Jack Hill, Australia at 4.4 Ga. that indicate a differentiated crust had already developed at that time. It is likely that the age of all other planets in the Solar System have a similar age. Note the Murchison carbonaceous chondrite has been dated at 7 Ga, almost 2.5 billion years before the solar system.

Airy isostasy: George Airy’s model of isostasy (1855) where the crust and mantle lithosphere are represented by discrete blocks that move independently (compensate) up or down. The density of each block is the same. The depth of compensation is the depth at which lithostatic pressures beneath the blocks are equal and in this model includes both crust and mantle.

Albedo: A measure of the amount of direct radiation from the sun that is reflected from a surface. Dark surfaces reflect very little radiation; white surfaces (such as ice and snow) reflect the most. On Earth, the ice caps at both poles play a major role in balancing heat radiation from the Sun. Clouds and the sea surface also play an important role in heat transfer to the atmosphere and ocean water masses, balanced by the amount of heat that is reflected back to space.

ALMA: Atacama Large Millimeter/submillimeter Array consists of 66 antennae that collectively gather light in the radio and infrared frequencies. It is located on the Chajnantor Plateau in Atacama Desert at about 5000 m above sea level. The thinner and clearer air at that altitude is ideal for the lower frequency radio signals from colder regions of deep space. ALMA has also imaged several protoplanetary discs around distant stars (future exoplanets).

Alpha Centauri: The closest star system to our Sun, it consists of a double star 4.37 light years away, and a third much smaller star Proxima Centauri that orbits these two. The double stars are named Rigil Kentaurus and Toliman that are about the same size as our Sun, Rigil K a bit brighter, and Toliman about half as bright. Proxima Centauri, a red dwarf, is the closest star at 4.25 light years away. The exoplanet Proxima b orbits Proxima Centauri. The Alpha Centauri system is located in the southern sky and is the most distant of the two ‘Pointer’ stars from the Southern Cross.

Alpha particle: A high energy particle consisting of two protons and two neutrons emitted from a nucleus during radioactive decay. The particle has the same composition as a helium-4 nucleus. The name was coined by Earnest Rutherford during his studies of uranium decay.

Altitude (astronomical): Altitude is the angle in degrees or radians of an object relative to some horizontal coordinate. On Earth, an observer’s view of the sky is measured relative to the horizon – the line connecting them is assumed to be horizontal. Thus, an object on the horizon has an altitude of 0^o, and at its zenith (overhead) it is 90⁰. A useful rule of thumb is the width of a fist held at arm’s length is about 10^o.

Amazonian Period (Mars): The period on Mars from 2.9 Ga to the present day. The period when Mars lost much of its atmosphere, stripped away by the solar wind – Mars magnetic field strength was significantly reduced. Constant sub-zero temperatures and a lack of atmospheric water vapour resulted in very slow weathering. Most of the sand dune seas we developed at this time. The two polar ice caps indicate continuing low temperatures (averaging -63^oC), but there is some evidence that they are melting.

Amino acid: The building blocks of proteins – all proteins contain up to 20 amino acids. All have the same basic structure – a central carbon atom bonded to four groups: Hydrogen, a carboxyl group (the general formula COOH where C=O is a double bond, plus C-OH), an amine group (derivatives of ammonia – NH₃) and an R group (more complex side chains, some of which are soluble in water). The amino acid glycine has been found in comets and asteroids.

Angular velocity: For a rotating body, a measure of the rate of angular change. It is usually stated in radians per unit time, for example the angular velocity for Earth’s rotation about a north-south axis is 1.99 x 10^-7 radians/second. The angular velocity at the equator is the same as that for the poles. cf. linear or tangential velocity.

Antipodal point: A point located on the direct opposite side of Earth. The north and south geographic poles are antipodal points.

Anti-tail: A comet tail that points towards the Sun.

Antumbra: One of three types of shadow cast by a radiant object. The shadow is lighter than the associated umbra – it will only form if the radiant disc is larger than the body causing the shadow. On Earth, the primary example occurs when the Moon is at its greatest distance from Earth. When it is located between Earth and Sun the shadow does not completely cover the sun’s disc such that the dark region is surrounded by a very bright region – this is also called an annular eclipse.

Aphelion: The farthest distance from the Sun (or any star) of an orbiting body. The term coined by Johannes Kepler applies to planets, comets and asteroids that have elliptical orbits around a star. Etymology – Helios, the Greek Sun God.  Cf. Perihelion

Apoapsis: The general term for the most distant point of an orbiting body – it incorporates more specific terms like apogee, aphelion etc.

Apogee: The farthest distance of the Moon from Earth during its elliptical orbit. Cf. perigee

Apojove: The position on a satellite orbit that is farthest from Jupiter’s center. Cf. Perijove.

Apollo missions: There were 17 manned Apollo missions to the Moon, although the first mission ended prematurely with the tragic loss of its crew. Apollo 11 was the first actual landing with Astronaut Neil Armstrong setting foot on the moon on July 20, 1969. Geological samples totaling 382 kg of bedrock, pebbles, dust and regolith were collected on 6 missions. Passive seismometers were installed on Apollo 11, 12, 14, 15 and 16. Active seismic experiments were undertaken at Apollo 14, 16, and 17.

Apparent magnitude: The brightness of an object as it is seen from Earth. Usually applied to stellar objects. The brighter the object the lower the number according to a logarithmic scale. Magnitude numbers can also be negative for really bright objects, like our own sun. So, the difference in brightness between magnitudes 1 and 2 is 2.512 times. Stars farther than 10 parsecs from Earth will have lower absolute magnitudes (will be brighter) than their apparent magnitudes. The scale can be applied to stars, planets, and comets. Measured magnitudes (light flux) are usually calibrated against standard stars for which the flux, or magnitude is known accurately.

Apsis (plural apsides): The general term for the closest and farthest distances of an orbiting body around its parent body. Around our Sun they are specified as aphelion and perihelion. For the Moon-Earth system the terms are apogee and perigee.

Arc minute: The angular measure that is 1/60 of a degree, or 1/ 21,600 of a complete rotation (360^o). An Arc second is 1/60 of an arc minute, or 1/3600 of a degree.

Armillary sphere: Armillary spheres attempted to map the heavens in three dimensions. They were only intended as physical models rather than measurement, but as such they represented attention to detail, and wonderful craftsmanship. They were popular during the Renaissance. One of the better-known spheres was made for Ferdinand I de’ Medici by Antonio Santucci that took 5 years to complete (1593). The sphere contained many of the elements representing the known universe and general theological beliefs. Santucci’s sphere is about 2 m in diameter, has the earth fixed at its centre surrounded by 7 spheres in succession representing the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and the outermost wandering star, Saturn. An eighth sphere guides the viewer to the fixed stars, that includes a band locating the signs of the Zodiac. A ninth sphere, called the Prime Mover, encloses all other spheres; it also contains wire meridians.   All the spheres could be rotated.

Asteroid: A term first used by astronomer John Herschel in 1802 to describe rocky and icy bodies that orbit the Sun. The largest known is Vesta at 530 km diameter, but asteroids as small as 2-3 m have been detected. Over one million have been identified. The orbits of dislodged asteroids occasionally intersect Earth’s orbit. Asteroid impacts physically modify Earth’s crust and add to its geochemical constitution.

Asteroid Belt: The name first used by Alexander von Humboldt in 1850 for the belt of large and small bits of rock, dust, and a dwarf planet (Ceres) that is located between the orbits of Mars and Jupiter. It occupies an orbital region where 18^th century and later astronomers surmised a 9^th planet should be. Some of the larger asteroids were discovered in the early 1800s. The belt is torus shaped – like a donut. It is about 1 AU wide. An early hypothesis suggested the belt contained the remnants of a planet torn apart by Jupiter’s gravity. A more recent explanation considers the belt a remnant from the formation of the solar system – material that never aggregated into a full planet.

Asteroid names-numbers: Asteroids, like other planetary-like bodies, are commonly named after their discoverer or an important historical event. Identification of asteroids and comets are confirmed by the Minor Planet Centre of the International Astronomical Union, given a provisional name and number, and after considering all the data a more permanent name and number.

Astronomical tides: Gravitational forces acting on Earth, Moon and Sun are in a state of balance. Centrifugal forces provide the counterbalance between Earth and Moon; they are the same everywhere on Earth, but the Moon’s gravitational pull changes with distance; it is strongest on the side closest to the Moon, and weakest on the opposite side. Thus, at different points on the Earth surface, there is a slight difference between the two forces. The difference is not enough to upset the overall balance between Earth and Moon, but it is strong enough to create a bulge in the ocean mass; one on the side facing the Moon, the other on the opposite side of Earth. The bulges correspond to high tides.  However, Earth rotates on its axis which means that different parts of Earth experience the bulge at different times. In this simple model, the bulges on opposite sides of the earth mean that there are two tides every 24 hours – semidiurnal tides. The Sun exerts a similar effect on Earth, but its influence on tides is about half that of the Moon. Nevertheless, the Sun’s gravitational force will reinforce that of the Moon during full and new phases of the Moon, resulting in spring tides; the opposite effect, neap tides occur when the gravitational forces are in opposition.

Astronomical unit: (AU) The distance between Earth and the Sun (about 150 million km). It is a convenient measurement unit for objects in the solar system where distances are measured in million of kilometres.

Aurora: High latitude winter nights are frequently interrupted by glowing, twisting and flickering curtains of light, commonly in shades of red, green, and yellow, but also blues. The polar lights (northern lights) are caused by charged particles from the sun interacting with Earth’s magnetosphere, specifically the Van Allen Radiation Belts. The charged particles interact with atoms of oxygen and nitrogen in the upper atmosphere (100 km + altitudes) – excited oxygen atoms produce red and green glows, while nitrogen produces blues. Aurora are common at latitudes above the Arctic and Antarctic circles; the northern events are the aurora borealis, and the southern aurora australis. Aurora have also been observed over the poles of Jupiter and Saturn.

Azimuth: The horizontal angle drawn from a line connecting an observer and an observed object, and some predetermined or cardinal point on the Earth surface – commonly true north. It is used in conjunction with elevation to fix the spherical coordinates of a celestial body. Elevation in this context refers to the angular distance above the observer’s horizon.

Barringer impact crater: Barringer Crater, also known as Meteor Crater, is a 50,000 year old stony meteorite impact excavation on Colorado Plateau, Arizona. It is 1200 m diameter and 180 m deep, with a rim raised about 45 m above the surrounding plain. The bolide was probably 30-50 m diameter. Its current setting is desert but at the time of the impact the region was fully vegetated. It is classed as a simple crater, lacking a central peak and collapsed walls. Fall-back breccia contains shock metamorphic coesite and stishovite.

Barycentre: The centre of mass for two bodies, around which they rotate or orbit. Pluto and its near companion Charon are tidally locked about a barycentre that is 2,126 km from Pluto’s centre. The barycentre for the Earth-Moon system is 4671 km from Earth’s centre, which places it inside the Earth sphere. The barycentre for the Sun-Earth system lies within the Sun.

Baryonic matter: This is the matter we can see or detect, consisting of baryons, the general name for all the atomic particles in an atom nucleus; it is ordinary matter. The universe is composed of about 20% detectable matter – the rest is theoretically dark matter that is not visible or undetectable. Baryons that make up the planets, stars and other objects are largely uncharged (i.e. the atoms retain their electrons). However, most of the detectable mass in the universe consists of charged plasma that forms when electrons are stripped from their atoms, for example the charged particles in nebulae.

Bennu: Asteroid 101955 has peaked astronomers’ interest, in part because it appears to be quite porous and carbonaceous, and because late in the 22^nd century there is a 1 in 2700 chance it will impact earth (the probability will change as we approach the year of closest approach). Its diameter is 492m. The OSIRIS-Rex satellite, launched in 2016, reached Bennu in 2018, successfully landed, and took measurements and samples that are currently winging their way back to Earth, with an ETA in 2023.

BepiColombo mission: A joint ESA and Japan Space Agency (JAXA, launched 2018, will conduct several close flybys of Mercury before entering its primary orbit November 2026. The first flyby was 1 Oct 2021. The flyby on 4 Sept 2024 was 165 km from the planet surface. The spacecraft has two orbiters that will separate once it enters Mercury orbit.

Beta particle: A high energy electron or positron emitted from a nucleus during radioactive decay.

Biosignatures: Chemical compounds and elements that are the products of living metabolic processes and that cannot be synthesized by abiotic processes. For example, molecular oxygen, carbon dioxide, and methane are well known bioproducts but they can also be produced by non-biological processes. Many organic compounds are potential biosignature types although some amino acids have been detected on comets and meteorites (e.g., glutamic acid, glycine, and some aspartic acid, serine, alanine, β-alanine, and γ-amino-n-butyric acid) and amines.

Blood moon: The moon appears red when it is full and in total eclipse.

Blue moon: There are normally 12 full moons observable from Earth. A blue moon is an additional full moon. They occur every 2-3 years, and can occur at different times of the year.

Bolide: A general name for large meteorites or comets that impact Earth or explode during entry to the atmosphere.

Callisto: The second largest of Jupiter’s moons, is the farthest of all the Galilean moons at 1,883,000 km. Its surface is peppered with white-centered, possibly ice-filled craters – hence its spotty white appearance. Most craters appear to be very old, indicating little internally forced change. Gravity surveys also suggest the possibility of a subsurface ocean. It has a very thin atmosphere of CO₂. Callisto has the largest multi-ring crater in the Solar system – the Valhalla crater.

Caloris impact crater: Caloris Planitia (plain) on Mercury at about 1550 km diameter is one of the largest impact structures in the Solar System. It is surrounded by a mountain ring 1-2 km high. It is about 3.8 Ga and thus is relatively young with only a few superimposed craters covering the basin floor.

Carbonaceous chondrites: These are the most primitive meteorites known. The carbon content up to 3-5% consists of carbonates, organic matter, and traces of carbon-bearing minerals like diamond, graphite, and silicon carbide. They tend to be fine grained, with a matrix of clay, water, olivine, oxides, and sulphides. They appear to be low temperature meteorites. The organic content includes amino acids, amines, alcohols, and in rare cases, complex molecular nucleobases which are important ingredients for RNA and DNA. However, it is generally thought that the organic compounds formed abiotically.

Cassini spacecraft: Launched October 15, 1997, Cassini’s primary tasks involved exploration of Saturn, its rings, and moons. The mission began with two gravity assists from Venus (1998-99), one from Earth later in 1999, and one from Jupiter in 2000. It completed 294 orbits of Saturn. Among its discoveries were details of the rings (consisting of ice and rock, some derived from the moons like Enceladus), active geyser plumes on the icy moon Enceladus, methane lakes and hydrocarbon sand dunes on Titan plus sending probe Huygens to Titan’s surface, 6 new moons, observation of storms, jet streams, and lightening on Saturn. Cassini’s grand finale was to plunge through Saturn’s atmosphere on September 15, 2017.

Celestial equator: The imaginary equator that projects from Earth’s geographic equator into space and through the celestial sphere.  

Celestial poles: An imaginary line drawn through the axis of rotation (close to the geographic poles) that intersects the star Polaris, the North Star. The North Star appears to be stationary at night because it is so close to this axis. However, the North Star also moves with a period of about 25,000 years (the time taken to complete a cycle of movement across the sky). Thus, in 10,000 years the North Star will not coincide with Earth’s rotation axis.

Celestial sphere:  For an observer on Earth, an imaginary sphere centred on the celestial equator, and having poles corresponding to the north and south celestial poles (that correspond to the poles of rotation). It is a useful device for determining astronomical time and position.

Centaurs: The duality of Centaurs in Greek mythology is applied to objects that have the characteristics of both asteroids and comets. They are thought to originate in the Kuiper Belt beyond the orbit of Neptune and are dislodged by the gravitational pull of the giant planets. Their orbits between Neptune and Jupiter are managed by competing gravity fields  – they can produce tails when they approach one of these planets.

Ceres: Discovered by Giuseppe Piazzi in 1801. It was originally identified as an asteroid in the Asteroid Belt but in 2006 was elevated to the status of dwarf planet. Gravity measurements made by the satellite Dawn indicate a layered body having an average density of 2.08 gm/cc. The interior includes a salt-water layer that periodically leaks or erupts at Ceres’ surface, producing cryovolcanoes, the most recent being Ahuna Mons which is an almost circular cone about 20km wide at its base and 4000m high. Ice at the surface will sublimate and over time the cryovolcanoes degrade.

Chandrayaan-3 spacecraft: Launched by the Indian Space Agency, inserted into lunar orbit August 2023. It comprises a landing module and rover, tasked to measure chemical and physical properties at the lunar surface.

Chang’e-4 spacecraft: Launched December 8, 2018, China’s lander successfully alighted the Von Kármán crater on the far side of the moon. Communication with Earth is via a relay satellite. It has spectrometer and ground penetrating radar instruments.

Chang’e-5 spacecraft: Launched in 2020, This Chinese spacecraft released a land to the lunar surface December 6, 2020. The lander collected 1.7 kg of lunar soil and rock that was transferred to an Earth return module that landed in Mongolia. The samples are some of the youngest collected at <2 Ga.

Chang’e-6 spacecraft: The Chinese explorer was launched 3 May 2024, landing in the Apollo Crater located within the largest lunar impact structure, on the far side of the Moon, estimated to be 2.5 Ga. Two kilograms of sample were collected and returned to Earth June 25.

Chariklo: One of the largest Centaur objects at 248 km across, orbiting between Saturn and Neptune. Charilko’s icy rings were discovered in 2013.

Charon: The largest of five moons orbiting Pluto, it is an oblate spheroid at 606 km diameter and a density of 1.7 g/cc, marginally less than that of Pluto (2.03 g/cc). Its orbit is tidally locked to Pluto about a barycentre between the two bodies. It is probably composed of a mix of rock, methane, and nitrogen ice. Water ice has also been detected. The red colouration at the north pole is due to organic molecules.

Chelyabinsk meteor: An asteroid estimated at 17-20m wide, exploded about 22 km above the city of Chelyabinsk, Russia on February 15, 2013, producing a shock wave that broke thousands of windows, injured hundreds of people, and was recorded on seismographs on the other side of the world. It entered Earth’s atmosphere at 64,370 km/hour. Pieces of the meteorite were later found scattered over a wide area. Click here for video footage.

Chicxulub impact: The Chicxulub impact (north Yucatan Peninsula) coincides with the 65 million year old Cretaceous-Paleogene boundary (K-Pg) . This event has been strongly implicated in the extinction of dinosaurs, ammonites, and many other faunal and floral groups. The crater is not visible at the surface, but gravity surveys indicate a crater 180 km in diameter. Other lines of evidence for this event include:

• An iridium anomaly at the KPg boundary in many parts of the world.
• Tektites scattered more than 2500 km from the impact site.
• Drilling near the imaged crater rim has intersected melt rock containing quartz crystals with shock lamellae.

Chiron: The first centaur object discovered in 1977, orbiting between Saturn and Uranus. Icy rings around Chiron were discovered in 2015, using occultations with distant stars. The rings appear to be unstable.

Chondrites: Chondrites are a diverse group of meteorites composed mainly of olivine, pyroxene, metallic FeNi compounds, and traces of spinel and anorthite, all in varying proportions and textures. The most common textural attribute is spherical to highly irregular chondrules ranging in size from sub-millimetre to 10 mm and more. An important subgroup is carbonaceous chondrites. They represent the rapid crystallization from ultramafic igneous melts. They are thought to have formed early in the differentiation of the solar nebula.

67P/Churyumov-Gerasimenko: Comet 67P/Churyumov-Gerasimenko is a dumbbell-shaped, porous, fluffy-looking icy body who’s claim to fame was cemented in 2014 when landing craft Philae Lander touched down on the comet’s surface. Launched in 2004 by the European Space Agency consortium, the carrier satellite Rosetta journeyed for 10 years and more than 6 billion kilometres. Although Philae’s life was cut short, it did collect and analyse dust at the comet surface that revealed the presence of solid water-ice (expected), solid carbon dioxide (unexpected), phosphorous, and 16 organic compounds some of which had not been previously detected in comets (methyl isocyanate, acetone, propionaldehyde, and acetamide), plus the amino acid glycine.

Circumstellar disc: Discs of dust, gas, asteroids, comets, and in some cases dwarf planets that surround and rotate around a star, that are remnants of early protoplanetary discs and planet formation. The solar system has three main discs – the asteroid belt, the Kuiper belt and the Oort cloud.

Coesite: An ultra-high pressure polymorph of silica (quartz family) found primarily in meteorite impact rocks and in some eclogites (mantle rocks). It belongs to the monoclinic crystal system (quartz is trigonal), lacks cleavage, and has a density of 3.0 g/cm³ (quartz is 2.65 g/cm³).

Coma: The thin gas atmosphere surrounding a comet nucleus, consisting primarily of water vapour, CO₂, ammonia, methane, and methanol. As the comet approaches the Sun, the gas plus dust particles from the comet surface form the characteristic comet tail.

Comet: Large lumps of primordial ice, solid carbon dioxide, bits of rock, and traces of organic compounds that, it is hypothesized, originate from the Oort Cloud at the outer edge of our Solar System. Some that are dislodged from the cloud enter an elliptical orbit around the Sun.  As they approach the Sun, heating produces a tail of ejected ice and dust; tails can be several million kilometres long. One of the most celebrated is Halley’s Comet that has an orbit period of 76 years. In one of those fortuitous circumstances for science, the 1994 impact of comet Shoemaker-Levy9 with Jupiter was predicted and the aftermath observed. Adding to the excitement, the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko ended with the touchdown of landing craft Philae in 2014. Its etymology is from the ancient Greek kometes meaning a head with long hair, referring to the long comet tail, probably as early as 500 BCE. Until the 18^th C comet sightings were regarded as ill omens, God’s wrath. A sighting on the eve of the Battle of Hastings (1066) ended in disaster for the English. Edmund Halley (1656 to 1742) introduced the scientific concept where comets like the planets orbited the Sun. He calculated Halley’s comet periodicity at 76 years, which is pretty close to more recent calculations (75 years). Mark Twain revelled in the knowledge that he was born under Halley’s comet in 1835.

Comet classification: Based on the duration of their orbit around the Sun: short period comets take 200 years or less, long periods comets >200 years. Single apparition comets appear to make one orbit and then leave the solar system – they are thought to originate in other star systems.

Complex craters: Craters with diameters >10 km fall into this category if they also contain (Grieve & Therriault, 2012):
– A central peak of highly brecciated country rock that in some craters can rise 2 km and more above the crater floor.
– A peak ring in very large craters.
– Collapsed crater walls that result in terraced topography.
The central peak forms by two processes: flow of brecciated country rock from the walls into the middle of the crater; and rebound of the bedrock depressed during the initial impact.

Conjunction: When two objects in the sky appear close together to an observer. Examples include planets, moons, asteroids, stars. The closeness is only apparent. Cf. Opposition

Copernicus: Nicolaus Copernicus was a Polish mathematician and astronomer (1473-1543) who, as part of the Renaissance awakening of curiosity, challenged the Geocentric theory of planetary motion espoused by Aristotle, Ptolemy, St. Thomas Aquinas and the entire Catholic Church edifice, with a proposal that Earth and all its neighbouring planets orbited the Sun. Despite its rationale foundations (observations and mathematical calculations) Heliocentric theory was rejected by Catholic and Protestant authorities alike – ironically, Pope Gregory XIII used Copernicus’ theory to develop the Gregorian calendar in 1582. It would be another 100 years before Galileo resurrected the heliocentric principle.

For anyone interested in this historical episode, Thomas Kuhn The Copernican Revolution Planetary: Astronomy in the Development of Western Thought, 1957, is a good start.

Coriolis effects: The result of (fictitious) Coriolis forces apply to rotating, non-inertial systems like Earth. The forces act orthogonal to the direction of movement such that deflections are to the right of the direction of forward motion in the northern hemisphere, and to the left in the southern hemisphere. Coriolis forces are directly proportional to linear velocity on the same rotating body. Coriolis effects increase towards the poles of rotation and are zero at the equator. The deflections apply to ocean water masses (gyres), contourites, and to weather systems.

Corona: The Sun’s plasma atmosphere, that extends several 1000 km from the surface, eventually transforming to solar winds. Temperatures in the corona exceed 10^{6 o} C, significantly hotter than the surface temperatures of about 5500^oC.

Coronal mass ejection: Formed when a solar prominence breaks from the surface of the Sun. The ejection mass contains remnants of its former magnetic field; the mass expands as it travels outward at speeds from 250 to 3000 km/second. They can be accompanied by solar flares. They can create intense magnetic storms on Earth.

Cosmic radiation: Heavy, high-speed particles, 85% of which are protons, that reach Earth that are generated beyond the solar system. The Sun generates low-energy cosmic rays during solar flares but most of these are captured by Earth’s magnetic field. Lighter elements like charged Li, Be, and B are also present. Some may be ejected by supernovae.

Cosmology: The study of the universe, its origin (big bang?), the formation of stars, black holes,  galaxies, and planetary bodies. It is the conjunction of physics, astronomy, and other natural sciences, and metaphysics.

Curiosity Rover: The 6-wheeled Mars rover landed near Mt. Sharp on August 5, 2012. Its primary task was to look for past environmental conditions that might have supported life. It has analysed the chemistry of >40 soil-rock samples and taken over one million images of Mars surface. The instrumentation is designed to detect possible organic-biological chemical compounds. Analytical instruments include: Gas Chromatograph (analyses gas compositions); Mass Spectrometer (elemental analysis): Tuneable Laser Spectrometer (for detecting water vapour, biogenic or abiotic methane): Sample Manipulation System (organizes samples for low and high temperature analysis); Ovens (high temperature gas extraction).

DART: Double Asteroid Redirection Test. Launched Nov. 23, 2021, from the SpaceX Falcon 9 rocket. It impacted asteroid Dimorphos September 26, 2022, at about 6.6 km/s. The impact changed slightly the moon’s orbit around Didymos. The planned follow-up mission to observe the impacted surface is ESA’s Hera mission, to launch in 2024.

Declination: In astronomy, it is the angle of an object in the sky measured to the celestial equator, which corresponds approximately with Earth’s equator. The angle provides one coordinate equivalent to latitudes. The second coordinate required to fix the objects position is associated with time (GMT or UTC) that corresponds with longitudes. North of the equator the declination angles are positive; south of the equator they are negative. Astronomical declination is usually fixed to some distant star. For geographic measures declination is the difference between true and magnetic north from any point on Earth’s surface. Unlike the astronomical declination, the geographic declination changes as the magnetic pole migrates.

Degree: The primary unit of angular measure, it is based on the Babylonian division of a circle into 360 equal units of arc. One degree equals 60 minutes of arc, and one minute has 60 seconds of arc.

Deimos: The smallest of the two Martian moons, that orbits Mars every 30 hour. It has an odd, almost trapezoidal shape with a maximum dimension of 15 km. It is thought to be a captured asteroid.

Didymos : A binary asteroid. Didymos is the larger of the two (780 m diameter), Dimorphos is its moon (160 m diameter). Dimorphos was the target of a deliberate spacecraft (DART) impact on Sept. 26, 2022, to test the possibility of forcing orbital corrections of an asteroid that could impact Earth. Neither object is considered a near-Earth threat.

Dimethyl sulphide: (DMS) An organosulfur compound, formula (CH₃)₂ S (methylthiomethane) is produced by marine bacteria and phytoplankton, particularly in polar seas. As such it is considered as a possible biosignature for extraterrestrial life on exoplanets. There is a hint that DMS may exist on exoplanet K2-18b based on spectrographic data measured by the James Webb telescope, but this has yet to be confirmed with more robust data.

Dimorphos: The asteroid that was the target for NASA’s DART collision mission. It is about 170 m across and almost 11 million km from Earth. The planned collision on Sept. 26, 2022, at about 22,000 km/hr, was part of NASA’s program to investigate the possibility of forcing near-Earth objects from their trajectory. The evidence from this event indicates a change in asteroid shape (because of a loss of surface material) and orbital period around its binary partner Didymos.

Dinkinesh: A small asteroid in the main Asteroid Belt (between Mars and Jupiter), that is orbited by a contact binary moon (the moon is named Salem); i.e., it is composed of two separate objects that are in contact. This was discovered when NASA’s Lucy Spacecraft flew by on November 1, 2023.

Dione: Saturn’s 4th largest moon at 562 km radius, 377,400 km from its parent, and having an orbital period of 2.74 Earth days. Its density is 1.48 g/cc indicating a composition of mostly water ice and perhaps a rocky core. Surface temperatures average -186^oC. The surface is cratered and cut by deep fractures. Its orbit is tidally locked to Saturn.

Dust devils (on Mars): Dust devils are common on Earth, and it seems on some parts of the Martian surface. They have been observed and measured in Jezero Crater by Perseverance rover. They are small convective vortices, or whirlwinds, usually less than 10 m wide, but up to a kilometre high. They are not associated with storm cells like tornadoes, but with warm air rising from the surface through cold air.

Dwarf planet: According to the IAU (2006), a dwarf planet “… is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite”. Clearing its nearest neighbours would require a sufficiently large gravity field to attract and force collisions.

Eccentricity: Earth orbit is an ellipse where the degree of ellipticity changes by about 5% over periods of 100,000 years. Milutin Milankovitch (1879 – 1958) theorized that the change in orbit, although seemingly small, produces significant differences in solar intensity as earth moves closer or farther from the sun. It is one of the Milankovitch Cycles and is an important determinant of Earth’s climate. The shorter period Milankovitch obliquity and precession cycles are superimposed on the eccentricity cycle.

Eclipse: The partial or total blocking of light of one celestial object by the transit of another. A solar eclipse occurs when the moon’s orbit takes it between Earth and the sun – the moon’s disc is large enough to completely cover the sun. A total lunar eclipse occurs when Earth’s umbral shadow is cast on the moon creating a reddish blood moon. Eclipses are periodic and can be predicted.

Ecliptic: An imaginary plane containing Earth’s orbit, passing through the Sun. The orbits of the 7 other planets and minor planets lie in or close to this plane.

Ejecta: Shattered country rock and melt excavated during a bolide impact. Clast sizes range from very large blocks to dust and vapour. Some of this material fills the crater basin as fall-back breccia. The remainder is spread as an apron around the crater, generally becoming finer grained with distance from the impact site. Small fragments like tektites and spherules of glass melt can be distributed many 100s of km from the impact. Dust and vapour will spread much farther, particularly on planetary bodies that have an atmosphere. On Earth this would include aerosols that could, with large impacts, spread globally through the upper atmosphere.

Emirates Mars Mission: The United Arab Emirates mission, named the Hope Probe, to study the Martian atmospheric circulation and weather, and the dynamics of atmospheric loss to space, particularly oxygen and hydrogen. Launched July 2020. Planned duration is two years from arrival in February 2021.

Enceladus: Enceladus is an icy moon that orbits Saturn within its E ring. It is the 6^th largest moon of Saturn with radius 250 km. Gravity and density modelling suggest it has rocky core, a liquid inner shell, and an icy crust that has a temperature of -201^oC. Analysis of geyser-like plumes that erupt from fractures in the crust indicate the presence of water, derived from the subcutaneous liquid shell.

Ephemerides: Tabulated data giving the orbital position, time, and velocity for astronomical objects (planets, asteroids, comets) and satellites as observed on Earth. A data table is called an Ephemeris.

Equinox: Two days in a year that have an equal duration of day and night when the sun lies directly over the equator: Spring (vernal) equinox on March 21, and autumnal equinox on September 23. Cf, Solstice when the sun is farthest from the equator.

Erg: Ergs, also called sand seas, are vast areas of wind-blown sand, sculpted into sand dunes. Celebrated examples on Earth like the Sahara in North Africa, Atacama in Chile, and Taklamakan in China, occur in arid mid-latitude regions (between 30^o-50^o north and south). Much of the Martian surface is covered by sand and spectacular fields of sand dunes. Saturn’s moon Titan also has fields of dunes but in this case the sand is apparently composed of hydrocarbons.

Eris: A dwarf planet discovered 2005 at the edge of the Solar System. It appears to have an icey crust and rocky core. Mean radius is 1163±6 km (Pluto is 1188 km), a mean density of 2.43 g/cc that is 0.58 g/cc greater than Pluto. It has a single moon – Dysnomia.   Its orbit at aphelion is about 97.5 AU, more than 48 AU farther away from the sun than Pluto.

E.S.A.: European Space Agency. Established 1975, research and missions funded by 22 states and based in Paris.  22 European states dedicated to the exploration of space, established in 1975, and headquartered in Paris. It operates a spaceport in French Guiana but also embarks on joint missions with NASA.

Eta Aquariids: A well known meteor shower derived from material in Halley’s comet, usually seen in Aquarius (its radiant) April-May each year. On average 60 meteors per hour are visible at the peak display. They are some of the fastest meteoroids, travelling at 66 km/second (238,000 km/hour). cf. delta Aquariids.

Euclid telescope: An ESA mission, launched July 2023, its primary objectives to observe and measure dark matter and the expansion of the universe to determine the nature of dark matter, dark energy, and the underlying structure of the universe. The telescope images in the near infra-red and visible part of the light spectrum. It will orbit at Lagrange Point 2 in the Earth-Sun system. Named after the Greek mathematician who founded mathematics of geometry (300 BCE).

Europa: Jupiter’s moon Europa has an icy surface riven by fractures and ice rafts, but very few craters, all indicating a tectonically active, geologically young crust. Imaging of Europa’s magnetic field suggests that it has a subcutaneous liquid ocean beneath the fractured crust. Tidally generated heat maintains the liquid state of the inner ocean.

Europa Clipper: NASA’s spacecraft launched Oct. 14, 2024, planned insertion into Europa orbit April 2030. Its primary tasks are to investigate the subcutaneous ocean beneath the icy crust, and determine if there are the ingredients for life forms. Clipper will make 50 flybys, some as close as 25 km.

Exo-planet: The name given to planets beyond our Solar System. Several thousand exo-planets have been identified in other solar systems using land- and space-based telescopes. To date, over 5000 have been identified. Their physical and chemical characteristics are as variable as we can imagine. They range from massive Jupiter sizes to much smaller bodies, some warmer than Mercury or colder than Pluto, dense or light. But very few occupy a ‘Goldilocks Zone’ that might permit life forms.

Exosphere: An exosphere is the thinnest of atmospheres, where atomic and molecular collisions are few. On Earth, it is the uppermost atmospheric layer. On Mercury it is the only layer having an atmospheric pressure close to zero.

Faculae: Bright spots on the Sun surface, associated with strong magnetic fields, the number of which are related to darker sunspots.

Fall-back breccia: Shattered and brecciated rock generated during an asteroid impact, that is deposited in the associated crater, along the crater rim, or spread outward from the crater. The breccia includes intensely sheared rock fragments, minerals such as deformed quartz with shock laminae, and melt rock.

Filament: See solar prominence.

Fugacity: A thermodynamic function used instead of effective partial pressures for reactions involving gases and gas mixtures and is a function of the chemical potential. It is usually expressed as a natural log – ln(f) = dμ/RT where μ is the chemical potential and R is the Gas Constant R = 8.314 J / mol·K. Thus, the fugacity of oxygen in air is the partial pressure exerted solely by oxygen.

Fukang pallasite: (meteorite). Recovered in Fukang, China in 2000, weighing in at 1003 Kg, it is a rare example of an olivine-Fe-Ni meteorite. The Ni-Fe phase is mostly kamacite, the olivine Forsterite. It probably formed at the core-mantle boundary of a differentiated planetesimal hypothesized to be 400-680 km radius.

Gaia: From the ancient Greek, a Goddess of the earth. Gaia now commonly refers to a personalized version of Earth, the Gaia hypothesis, as a living, breathing thing that is self- regulating through complex interactions and interconnections among the atmosphere, hydrosphere, biosphere, and lithosphere.

Gaia space telescope: Launched by ESA 19 December 2013, tasked with mapping the Milky Way and the largely invisible halo beyond the visible limits of the spiral arms, including its effect on neighbouring galaxies like the Magellanic cloud star clusters. The data set so far is about 1.5 billion stars.

Galactic centre: The centre around which a galaxy rotates, and with it all the stars and their solar systems. The centre of the Milky Way is occupied by a super massive black hole that is about 4 million solar masses in size.

Galaxy: Galaxies are structured collections of stars (of all ages), dust and gas clouds, black holes, and dark matter, all glued together by gravity – the Milky Way spiral has about 400 billion stars. Their diameters are measured in 10s to 100s of thousands of light years. Four principle geometric configurations are: Elliptical clusters (no arms), spiral with arms, peculiar galaxies that are odd-shaped and probably formed by intergalactic collisions, and irregular types that are odd shaped and commonly lack arms (e.g., small clusters like the Magellanic Clouds).

Galilean moons: Four of the largest moons of Jupiter – Io, Europa, Ganymede, and Callisto, discovered by Galileo near the end of 1609. They can be observed with a decent pair of binoculars.

Galileo: Galileo Galilei (1564 –1642) was an Italian Renaissance polymath who made revolutionary discoveries on the motion of bodies (Earth-bound and planetary), and other astronomical phenomena using experiments and observations, frequently with equipment he himself constructed. He also built the first telescope. He was a renowned debater. His discoveries challenged the entrenched Aristotelian, Ptolemaic, and Church views of motion, culminating in his resurrection with proofs of the Copernican theory, that Earth and all the other known planets orbit the Sun. This work was published in 1632 “Dialogues on the Two Chief World Systems”, but in 1633 the Inquisition charged him of being “vehemently suspect of heresy” and he was forced to recant. His tomb and monument lie in Basilica Santa Croce in Florence.

Galileo spacecraft: An exploration satellite launched October 18, 1989 on a 14 year mission, 8 of them orbiting Jupiter. The mission began with a Venus flyby, acquiring radar images through Venus’ clouds. En route to Jupiter Galileo paid the first ever spacecraft visit to an asteroid, first Gaspra, then Ida, both in the Asteroid Belt. During its Jupiter sojourn, Galileo Europa’s subcutaneous ocean, active volcanism on Io including images of eruption plumes, and measurements of moons Ganymede and Callisto indicating liquid salt water in their subsurface layers. Galileo was also witness to the impact of comet Shoemaker-Levy 9 with Jupiter from July 16 – 22, 1994. A small probe was also sent into Jupiter’s atmosphere to measure composition and pressure.

Galileo navigation system: An ESA and partners project that since 2016 has put 38 satellites in Earth orbit (as of May 2024), the latest launched on the SpaceX Falcon 9. It is used primarily for navigation and can be accessed by a variety of devices including mobile phones.

Gamma ray: High energy, short wavelength particle emitted from the nucleus of an unstable isotope. It usually accompanies beta and alpha particle emissions. Gamma radiation is always emitted during nuclear fission in stars and explosive devices. It is one of the most dangerous forms of radioactive particle emission.

Ganymede: The largest of the Jovian moons, with a radius of 2631 km which makes it larger than Mercury and Pluto. It is the 3rd most distant from Jupiter of the Galilean moons, at 1,070,000 km. Ganymede’s density is … It is the only moon in the Solar System that has its own magnetic field and accompanying aurora. Surface temperatures range from -183 to -113 C. It has an iron-rich core, a rocky mantle, and an outer shell of ice and rock. The Ganymede surface is a mix of craters and grooves that may represent some kind of extension in the crust, or a response to bolide impacts.

Gas giants: Very large planets (several times the diameter of Earth) consisting of mostly hydrogen, helium, and lesser proportions of ammonia and methane. Rocky material, if present, is confined to the core. Core material may also be metallic hydrogen or helium. There is no rocky or solid, frozen surface, and the boundary between the planet surface and atmosphere is diffuse. The Jovian gas giants are Jupiter, Saturn, Uranus, and Neptune. All four have orbiting rings. All four have moons, some of which are rocky.

Gaspra 951: The first asteroid visited by the spacecraft Galileo, October 29, 1991. It has an oblong shape with dimensions 18.2 x 10.5 x 8.9 km. It orbits within the Asteroid Belt, and is  probably a fragment from some earlier collision.

Geminids: One of the most active meteor showers derived from the asteroid 3200 Phaethon, usually peaks mid-December every year. Discovered 1862. It is seen in the constellation Gemini (its radiant), hence the name.

Geocentric model of the universe: This is the Earth-centred model around which the Sun, the planets and stars revolve. Notable proponents included Aristotle, Claudius Ptolemy (2^nd C BCE), and St. Thomas Aquinas (1225? – 1274). In western theology and philosophy, the model held sway until the mid-16^th C; in popular constructions it was represented by armillary spheres.   The model assumed doctrinal importance for the Roman Catholic Church to the extent that anyone questioning it was deemed a heretic. The model was seriously challenged by Nicolaus Copernicus (1473-1543) and a bit later by Galileo, both of whom were required to recant their views.

Geographic pole: Geographic poles are points that we have defined by constructing lines of Longitude that converge at the north and south poles – also referred to as true north-south. The projection of these poles onto the celestial sphere intersect at the celestial north (North Star) and celestial south poles.

Geologic Atlas of the Moon: Produced by the USGS at scales from 1:5,000 to 1:5,000,000. They are free to download.

Geostationary orbit: The orbit where a satellite appears stationary to an observer at any location and time. The satellite matches the Earth rotation at the equator. The ideal orbit is 35,786 km at a velocity of 3 km/s. A single GSO at this altitude covers about a third of Earth’s surface.

Gibbous moon: The stage of the Moon’s illumination when it is more than half illuminated but not full.  There are both waxing and waning gibbous moons each month.

GMT: Greenwich Mean Time, established in 1884 is the prime meridian (longitude) to which all international time zones are referenced.

Goldilocks zone: The orbital distance of a planet from its sun where atmospheric temperatures and pressures are in the ‘habitable zone’, and where vapour pressures are sufficient for water to remain liquid. Earth exists in this zone.

Gosses Bluff – Tnorala: Remnants of an Australian, Northern Territories, 142.5 million year old impact structure. The residual crater is 5 km diameter, but originally was probably about 20 km. The bolide, possibly a comet, is hypothesized to have been about 600 m across. The structure is sacred to the Western Arrernte Aboriginal people, and is designated a conservation reserve.

Gravitational constant G: Upper case G is a the mathematical constant in Newton’s Laws of gravity, that expresses the force of gravity between two bodies F = (G. m₁ ). (m₂. R^-2) where m₁ and m₂ are the body masses, and R is the distance between the centre of each body. The units of G are Newtons . m² . kg^-2. G has the universal value of 6.67 × 10^-11 N. m². kg^-2. The acceleration due to gravity (lower case g) can be calculated from this function as g = G.m . R^-2. On Earth g = 9.81 m.s^-2. On Mars g = 3.72 m.s^-2, and on Jupiter g = 25.8 m.s^-2.

Gravity assist: (colloquially called ‘slingshot’). Space craft designed to explore the distant regions of the Solar System require the energy to do so. The initial energy is derived from the launch rocket. However, additional momentum is needed to get to the various planets and this is derived from orbits around Earth, the Sun, and other planets. For example, Voyager 2, launched in 1977, passed by Jupiter (transmitting data and images), increasing in speed during the approach. At a certain point during its orbit, Voyager’s speed and momentum were sufficient to change it to a trajectory that would take it to Saturn. The same process was used at Saturn to put Voyager on course for Uranus, then again from Uranus to Neptune, and finally from Neptune to interstellar space.

Great Blue Spot: The other, more recently discovered spot on Jupiter,  it is a patch of intense, localised magnetic fields.

Great Red Spot: The most iconic structure on Jupiter, the Great Red Spot is a massive storm. It was probably first identified in 1665. It spins counter clockwise, rotating every 4.5 Earth days. The storm is fed by jet streams. Recent measurements by Juno spacecraft indicate the storm drafts extend at least 500 km deep. Measurements of its long axis since the late 1800s indicate that it has lost half its size over the last 100 years, a decrease of about 900 km/year. Its long axis is presently about 16,500 km long.

Greenhouse effect: Greenhouse gases help modulate atmospheric temperatures. On Earth, this is accomplished by small amounts of water vapour, carbon dioxide, methane, nitrous oxide, and more recently certain industrial hydrocarbons in our atmosphere. The most important are CO₂ and CH₄. Both molecules absorb heat at different infrared frequencies, that also are different to the frequencies absorbed by water vapour. Note that water vapour does not cause increased atmospheric temperatures – in this case the proportion of H₂O increases or decreases because of changes in temperature caused by other processes. Despite their low concentrations, CO₂ and CH₄ provide the balance in atmospheric temperatures required to sustain life. In contrast, runaway greenhouse conditions on Venus have resulted in atmospheric temperatures about 450^oC and atmospheric pressures about 92 time that on Earth. Aerosols and volcanic dust also moderate the greenhouse effect.

Habitable Zone: The distance of a planet or exoplanet from its star where conditions are such that water can exist on its surface in liquid form. The conditions require a relatively narrow range of temperatures and atmospheric pressures. Also known as the Goldilocks Zone. Whether an exoplanet is potentially habitable will also depend on many other factors, such as atmospheric composition, and the intensity of incident UV or cosmic radiation.

Halley’s Comet: One of the better known and spectacular comets visible from Earth, discovered in 1758 by Edmond Halley, has a period of 75-79 years. It will next appear around 2061.

Heliocentric: An orbit with the Sun as centre. Nicolaus Copernicus is credited with the discovery that Earth orbited the Sun in 1543. The theory was later confirmed and modified by Tycho Brahe (1546 –1601), Johannes Kepler (1571-1630) and Galileo around 1610 (although Galileo was forced by the inquisitorial church to recant.  Prior to this Earth was understood or assumed to be the centre based largely on theological doctrine throughout greater Europe, India, and the Islamic world, although Greek astronomers had proposed the versions of heliocentricity several centuries before.

Heliosphere: The boundary between the Solar wind and interstellar space that extends about 120 AU from the Sun. It protects the Solar System from cosmic radiation originating elsewhere in the Milky Way.

Hesperian Period (Mars): The period on Mars from 3.7 Ga to 2.9 Ga. Asteroid bombardment had lessened, but volcanism persisted, with sulphate-bearing aerosols and acid rain blanketing the planet. This was a period of extensive sulphate mineral precipitation and sedimentation, and significant atmospheric cooling because of reduced incident solar energy. However, local, short-lived heating caused by asteroid impacts seem to have produced massive flood surges.

HiRISE (High Resolution Imaging Science Experiment) The most successful, high resolution camera to orbit Mars, HiRISE (High Resolution Imaging Science Experiment) images Martian landscapes in the visible and infrared light frequencies. It is capable of resolutions at 30 cm per pixel, or about one metre.  HiRISE is one of 6 instruments on the Mars Reconnaissance Orbiter that arrived on the scene in 2006. It can rapidly process large numbers of images which means we can witness geomorphic processes almost in real time. The lead investigative institution is Lunar and Planetary Laboratory at the University of Arizona.

Host star: The star an exoplanet orbits.

Hubble telescope:  Launched from Space Shuttle Discovery in 1990, it orbits Earth at 535 km altitude, with a complete orbit every 95 minutes. It was designed (and upgraded) to image our Solar System and beyond at light frequencies from ultraviolet to infrared. As of October 2022, it has acquired more than 1.5 million images. It has revolutionized our observations and understanding of space.

Hydrodynamic escape: Takes place when thermally driven escape of light atoms and molecules from a planet atmosphere (e.g., hydrogen, helium), drag heavier molecules with them – removal is primarily via viscous drag and molecular collisions. Most of the energy required is solar, but meteorite impacts can also enhance escape. Escape of atmospheric gases over time can deplete a planet atmosphere (e.g., Mars) or help to maintain a form of equilibrium if the planet can produce new gas components, for example via volcanic exhalations.

Hydrostatic equilibrium (HE):  If the gravitational forces generated by a celestial body (directed to the centre of the body) are balanced by internal pressures that are directed outward, then the body is said to be in hydrostatic equilibrium. It applies to stars, planets, and even galaxies. Maintenance of this equilibrium requires that the body assumes a spherical geometry, although most are oblate spheroids because of gravitational forces exerted by neighbouring bodies. For example, Earth has a slight equatorial bulge because of the gravitational attraction exerted by the Moon. HE is one of the requirements for a body to be designated a planet, or dwarf planet.

Hyperion: (a child on Gaia and Uranus) The potatoe-shaped moon of Saturn that looks like a sponge. Discovered 1848, its maximum length is 410 km. The surface is pocked by deep craters. Density is less than 0.5 kg/m³ and hence may be composed of very porous ice. The average surface temperature is -180^oC.

Iapetus: Saturn’s 3^rd largest moon, 3,561,300 km distant, with a radius of 730 km. Its density is 1.2 g/cc and composition predicted to be mostly water ice with some rock. It is tidally locked to Saturn. Its leading hemisphere (the one facing into the direction of orbital motion) has very low albedo (very dark) while its trailing hemisphere has very high albedo.

Ice rafts: Sheets of ice that have been broken by extension and contraction, where rafted fragments are thrust against and over one another. On Earth, rafting develops during successive periods of freeze and thaw of thick polar sea ice. Ice rafting has also been observed in the outer crust of Jupiter’s moon Europa, and Saturn’s moon Enceladus.

Ida 223: 243 Asteroid Ida was also visited by spacecraft Galileo on August 28, 1993. It is the first asteroid found to be orbited by its own moon. It has a cratered, regolith covered surface. It is probably composed of silicates.

Impact breccia : Breccia formed by intense fracturing and shock metamorphosed rock during a hypervelocity impact, that also has a clastic matrix (an important difference from impact melt rock). Breccias may also include fragments of impact melt rock. It includes impact foot-wall breccia (e.g., from crater margin collapse) and fall-back breccia. Shock-deformed crystals are an important component. The term is broadly synonymous with suevite.

Impact crater: A bowl-shaped depression caused by the excavation and ejection of rock or ice during a bolide impact. Ejected material is pushed or falls back forming raised crater rims. The rims of large craters can collapse. Some ejecta spreads much farther in a ray-like distribution. Finer dust and aerosols are also spread afar, particularly on planets that have an atmosphere. Large craters commonly have central mounds or peaks that form by a combination of rim collapse and rebound of rock beneath the crater immediately following impact. Three basic crater morphologies are recognised: simple, complex, and peak ring or multi-ring types. The moon has more than 10⁶  craters, many of them superimposed one on the other. Mars is estimated to have at least 300,000. On Earth, the number of confirmed craters is about 190 – we have received as many impacts as other planets in the solar system, but surface degradation by weathering and structural dismemberment, burial by sediment, and crustal recycling (plate tectonics) have rendered them indecipherable or removed them from the rock record.

Impact gardening: The process where a planetary surface is reworked multiple times by meteoroid impacts. It is commonly manifested as brecciated regolith where individual rock fragments may be derived from several impact events.

Impact melt rock: Melt rock formed by heat generated during a hypervelocity impact. Melt rock may be clast-rich, clast-poor, or completely free of clasts (clasts generated by the impact), and they can be further subdivided into glassy or varying degrees of crystallinity. Note that melt rocks containing clasts should not be confused with impact breccias that may contain clasts of melt rock.

Impact velocity: The velocity of a bolide at impact depends on its initial velocity (in space), the gravitational potential of the planetary body, and the density of an atmosphere (if present). When a bolide hits Earth’s atmosphere, it’s a bit like hitting a brick wall – most will burn up (meteors). The minimum impact velocity on Earth is 11.2 km/second which is equal to the escape velocity. The average impact velocity is 18 km/sec (64,800 km/hour), but can be as high as 53 km/sec (190,800 km/hour). Bolide approach velocities range up to 72 km/sec (259,200 km/hour). Lunar impact velocities tend to be slower because gravity is only 16.5% of that on Earth.

Impactite: A broad term that the has been defined as “all rocks affected by one or more hypervelocity impact(s) resulting from collision(s) of planetary bodies” (Stöffler and Grieve, 2007). The term encompasses impact melts, impact breccias, and shocked rocks. The latter includes shock metamorphic structures like shatter cones, and shock-deformed quartz and other crystalline phases.

Inertia: Inertia is loosely defined as a force that resists the change in motion of a body. The term was coined by astronomer Johannes Kepler (17th century); his erstwhile colleague Galileo demonstrated its qualities by experimenting with balls rolling along sloping surfaces. However, it was Isaac Newton who codified the properties of inertia in his three Laws of Motion.

Ingenuity Helicopter: A small dual blade helicopter that arrived on Mars attached to Perseverance Rover, making its first flight on April 19, 2021. It completed 72 flights (the mission was for 5 flights), completing 128.8 flying minutes, covering 17.0 km, and reaching altitudes of 24.0 m. It ceased operation on Jan. 18, 2024, probably during a crash landing.

InSight lander (Mars): Launched on May 5, 2018, InSight landed on a flat plain named Elysium Planitia on November 26, 2018. It had two primary tasks: to record seismic events that would improve our understanding of Mars’ internal structure, and to measure heat flow. The heat flow probe failed. Power failure brought a close to the mission on December 15, 2022. Over 1300 seismic events were recorded including one at magnitude 4.2 that was probably the result of an asteroid or comet impact. The seismic events provide the basis for estimates of layer thickness and density: the (probable) molten core has an 1,890-1,790 km radius. The outer crust (top of the lithosphere) is 30-72 km thick.

International Astronomical Union (IAU): A non-governmental organisation dedicated to the promotion and safeguard the science of astronomy. It also works to define fundamental astronomical and physical constants, and nomenclature.

International Space Station (ISS): One of the more successful,  international, cooperative space ventures, involving at least 15 countries,  spearheaded by Europe, USA, Russia, Canada, and Japan. It was constructed with modular components while in orbit, between 2008 and 2011, and has been continually occupied since 2000. It orbits at 400 km altitude at a speed of 25,000 km/hour, completing an Earth orbit every 90 minutes. Astronauts living on the station conduct experiments in the physical sciences (e.g. fluid dynamics in low gravity, materials science), life sciences (e.g., biomedical, biotech), remote sensing, technology development (e.g., robotics), and education.

Interstellar comet-asteroid: An object that originates outside the solar system and is not bound to a star by gravity. The first interstellar asteroid identified was the wierdly-shaped ʻOumuamua in 2017. In 2019 the comet C/2019 Q4 (Borisov) entered our solar system and is considered a potential interstellar object – it’s velocity is about 150,000 km/hour which is fast enough to escape the Sun’s gravitational pull and leave the solar system.

Interstellar space: Defined as the region beyond the influence of a star’s magnetic field and solar wind (charged particles). In our solar system it is beyond the Heliosphere.

Io: The third largest moon of Jupiter, it is the only one with active volcanism – over 400 eruptive centres have been mapped. Io’s density is 3.5g/cm³ and modelling suggests it has a core of iron or iron sulphide, an inner shell of solid and molten rock, and a crust of sulphur and extruded silicate rock. Its atmosphere is mostly sulphur dioxide. Io’s elliptical orbit brings it to about 420,000 km from the gas giant. The resulting gravitational forces produce huge tides such that the surface bulges up to 100m.  These tidal forces generate enough heat to melt rock and sulphur. The Galileo Explorer mission imaged active eruption plumes extending more than 300 km above the surface.

Ionization: The gain or loss of an electron(s) from an atom creating an ion. Atoms that gain one or more electrons are negative (anions); those that lose 1 or more electrons are cations. The number of electrons gained or lost corresponds with the ion charge number (e.g., Na⁺, Ca²⁺). Ions are formed during chemical reactions, particularly in aqueous conditions, and by high velocity impacts from other charged particles or photons.

Ionosphere: That part of Earth’s upper atmosphere at 60-800 km altitude where extreme solar UV and X-radiation ionizes atoms and molecules forming a broad layer of charged particles. The overall density of the region varies with solar intensity. The ionosphere is less dnese on the night side of Earth. The ionosphere is also layered with respect to particle density. The different layers (labelled D, E, and F the highest) influence different communications radio frequencies, for example the F region reflects High Frequency (HF) signals.   

Iron meteorites: These metallic meteorites are composed of iron and nickel alloys, principally kamacite, an isometric FeNi crystal with about 92% iron, and Taenite, a slightly harder, isometric FeNi crystal form with 25-40% Ni. Both these crystal forms are only found in meteorites. Crystal growth commonly produces Widmanstätten bands. The meteorites are thought to have originated from the cores of planets that were fragmented by collisions during the early differentiation of the solar nebula. Cf. Stony-iron meteorites and stony meteorites.

Iridium anomalies: Stony meteorites commonly contain elevated concentrations of platinum group elements compared with those normally found on Earth. One of these elements, Iridium (Ir), is an important indicator of ancient bolide impacts. It was first discovered in 1980 by Walter and Luis Alvarez in a thin clay layer at the Cretaceous-Tertiary boundary in Italy. Ir concentrations in the clay are about 500 parts per billion, compared with values around 0.1-0.3 ppb in Earth rocks. Similar anomalies were subsequently in clays of the same age in other parts of the world. The Alvarez’ concluded that the Ir spike was caused by fallout from an asteroid impact – the Chicxulub impact. Iridium anomalies have since become a standard indicator of ancient bolide impacts.

Isostasy:   Isostasy describes the tendency to equilibrium of a lighter lithosphere floating on a more dense and perhaps more ductile mantle asthenosphere; it describes the state of balance between the lithosphere and asthenosphere. The two foundational models are Pratt Isostasy that allows density to vary from one lithospheric block to another, and Airy Isostasy where density is the same across all blocks. A later modification of the theory recognises the elastic nature of the lithosphere, allowing it to bend under loads – this is flexural isostasy, also known as regional isostasy. At equilibrium the lithosphere and asthenosphere are isostatically compensated. A well-known example of isostasy in action is the glacio-isostatic rebound of those regions of Earth’s surface that were covered by thick ice sheets during the last glaciation.

JAXA: Japan Aerospace Exploration Agency. Formed in 2003 it is Japan’s primary organization for space missions and research.

J Webb telescope: Launched December 25, 2021, the new telescope will augment Hubble with longer wavelength and more sensitive infrared views of the universe. It is capable of imaging in marvelous detail some of the oldest galaxies in the universe, and the birth of stars and planets in dust clouds and nebulae. Unlike Hubble, J Webb will orbit the Sun in concert with Earth, maintaining about 1.5 million km from and in direct line-of-sight with Earth (Earth lies between the Sun and the J Webb). The side facing the Sun will be at 85^oC; the side opposite -225^oC. The principal collaborators are NASA, ESA, and the Canadian Space Agency.

Jezero Crater: A 45 km diameter crater in the Isidis Planitia region of Mars, and the landing site of Perseverance Rover. Remnants of an ancient delta prograded from the crater margin – these sedimentary rocks are one of the prime objectives for the Perseverance project, in particular looking for sedimentary structures that provide evidence of ancient flowing water, and any chemical-sedimentary evidence that might indicate ancient life.

Jodrell Bank: A telescope in Cheshire, England designed to detect radio signals from astronomical objects. It was completed in 1945; it has a 76 m diameter moveable disc  – the 3^rd largest moveable telescope in the world.  

Jovian rings: Saturn’s rings were first observed in 1610 by Galileo. They consist of rock, ice, and dust. Some of the Saturnian moons also reside with the ring system. The rings around Uranus were discovered in 1977, those around Jupiter in 1979, and Neptune in 1989. Saturn’s rings are a few metres thick and extend about 130,000 km above the planet equator. The moon Enceladus orbits Saturn within Ring E and appears to be shedding new material to those rings. Dust and small fragments that are ejected from the inner moons during meteor impacts, are added to Jupiter’s rings.

JUICE: Jupiter Icy Moons Explorer, The European Space Agency’s spacecraft that will examine the Jovian icey moons Europa, Ganymede, and Callisto. Launched April 14, 2023. It will be sling-shot to Jupiter from an Earth-Venus orbit. It is expected to arrive July 2031. The final part of the JUICE mission will involve orbits of Ganymede ranging from 5000 km to 200 km, after which it will impact with the Ganymede surface.

Julian calendar: Proposed during the reign of Julius Caesar (46 BCE)and based on an older Egyptian solar calendar it divided the year into 12 months with 365.25 days, with a leap year every 4^th year. However, the calculation was in error by 11 minutes and 14 seconds, and by the end of the 16^th C this had accumulated such that the beginning-end of seasons (based on equinox) had shifted by several days. In 1582 Pope Gregory XIII restored the calendar to align with the seasons – this is the Gregorian calendar that is in current use by most states (England adopted it in 1752) except a few eastern Orthodox churches. The current difference between the Julian and Gregorian calendars is 13 days.

Juno spacecraft: Launched August 5, 2011, arriving at its first orbital position around Jupiter on July 4, 2016. Its orbits are highly elliptical extending from the outer limits of the powerful Jovian magnetosphere to its cloud tops. The mission has been extended from its initial 5 years. Juno’s science includes the Jovian physical, chemical and magnetic structure, cloud and storm dynamics, the rings, and flybys of several Jovian moons.

Jupiter’s Trojans: A group of asteroids and planetesimals that occupy the same orbit as Jupiter. There are two main groups that librate around Jupiter’s stable Lagrange Points L4 and L5. There are several hypotheses that relate their origin to early planetary collisions, captured planetesimals from the giant planet zone, or captured from the primordial Kuiper Belt. See Bottke et al., 2023. Open Access.

Kelvin: The SI temperature scale that begins at absolute zero (−273.15 °C); zero degrees C is 273.15^o K and 100^o C is 373.15^o K. Named after William Thomson (Lord Kelvin).

K-Pg boundary: Formerly called the K-T, or Cretaceous-Tertiary boundary, the Cretaceous-Paleogene boundary marks the extinction event at the end of the Mesozoic, that saw the demise of dinosaurs and many marine invertebrate groups, like the ammonoids. Two explanations for the extinction event are: the Chicxulub impact at about 65 Ma (the most popular explanation), and volcanic CO2 emissions from the Deccan Traps that caused abrupt and dramatic atmospheric heating, ocean acidification, and mercury toxicity.

Kepler, Johannes: (1571-1639). German astronomer, who learned about Copernicus’ heliocentric system while at Tubingen University. He made a major contribution that couched the ratios of planetary orbits in simple geometric terms – initially based on circles drawn inside a triangle and round the triangle apices; the ratio of their radii matched the orbits of Jupiter and Saturn. He developed the mathematics of orbital motion, expressed in his three Laws, that were later used by Isaac Newton in his development of the theory of gravity (Kepler new nothing about gravity). He is best known for three Laws; Kepler’s first two Laws were formulated in 1609 in his Astronomia Nova and the third in a later publication in 1621 – Epitome Astronomiae. The laws also apply to spacecraft, comets, and asteroids.

Kepler’s First Law: Planetary orbits around the Sun are elliptical with the Sun located at one of the foci. This Law replaced Copernicus’ theory that orbits were circular.

Kepler’s Second Law: That planetary orbits along the ellipse slow with increasing distance from the Sun. This Law was based on empirical evidence, in particular observations of Mars over more than a decade. He determined this geometrically by measuring the areas of two line segments drawn from a planet to the Sun (at a focal point), demonstrating that the travel times along the orbit are equal when the two areas are equal. Note that the distances along the orbit arc in each case are not equal – best demonstrated when they are at opposite ends of the ellipse.

Kepler’s Third Law: The third Law was, again based on geometric arrangement of orbits in a heliocentric system, and generally stated as the square of a planet’s orbital period is proportional to the cube of the length of the semi-major axis of its orbit (in an ellipse the semi-major axis is at right angles to and bisects the major axis).

Kuiper Belt: A torus- or donut-shaped region of large and small icy bodies that lies outside the orbit of Neptune. Pluto lies within the Kuiper Belt. Fragments and planetesimals in the belt are probably remnants from the formation of the Solar System. It is different to and lies inside the Oort Cloud.

Lagrange points: When a body is in stable orbit around a larger mass, (e.g., the Earth and Sun) there is a balance between the gravitational forces (between both bodies), and centrifugal forces acting on the smaller orbiting body. In any two body system, there are five locations on the orbital plane where both forces are equal – these are the Lagrange Points. These points are useful for spacecraft because only minor adjustments are required to maintain a stable orbit. For example, the J Webb Telescope orbits Earth at Lagrange Point 2 (L2) in the Earth-Sun system, at a distance of about 0.1 AU. In this case, L2 is on the outside of the straight line joining the Sun and Earth (L1 lies between the Sun and Earth). However, L1 and L2 are not completely stable positions and J Webb Telescope requires minor adjustments every 23 days.

Latitude: Also called parallels. Imaginary lines that describe the angular location north or south of the equator. Zero latitude corresponds to the equator. Ninety degrees north and south correspond to the north and south geographic poles respectively. Lines of latitude describe small circles that intersect lines of longitude (meridians) at right angles. The tropics of Cancer and Capricorn represent the most northerly and southerly latitudes respectively (23.43°) where the sun can be positioned directly above an observer. The Arctic and Antarctic circles at 66.56° north and south respectively. The Antarctic circle represent the limit where the the sun will not set during the southern summer solstice or rise during the winter solstice. The reverse timing applies to the Arctic circle.

Leading hemisphere: For a tidally locked satellite, it is the face that is forward, towards the direction of orbital motion.

Leonid meteor shower: Appears every 33 years from an apparent point in Leo. The next sightings should be in 2032.

Light pollution: The ever-increasing problem of incident and reflected light generated by cities and orbiting satellites interfering with the operation of Earth-bound telescopes.

Light year: A unit of length used for astronomical distances, usually from Earth to stars and galaxies. Light travels through a vacuum at 299,792 km/second (186,282 miles/sec). Thus, the distance traveled in one year is 9.46 x 10¹² km (5.87 x 10¹² miles). The nearest star Proxima Centauri is ~4.25 light-years away.

Lithophile elements: One of the Goldschmidt classification groups of elements that readily bond with oxygen. This means they tend to be concentrated in Earth’s crust and probably the crusts of other rocky planets – common examples are Na, Ca, Mg, Si, Al, K. as well as some of the transition metal elements like Fe, Mn. cf. siderophiles.

Longitude: A measure of a line on the surface of the Earth that gives a position in degrees as east or west. Imaginary lines of longitude converge at the two poles. Lines of longitude describe great circles. Longitude also denotes time, where one hour is equivalent to 15^o east-west. The zero line of longitude, the prime meridian set internationally in 1884, is located at Greenwich, England. East of the prime meridian time increases by 1 hour for every 15^o; west it decreases by 1 hour. Thus, 12 noon at Greenwich corresponds to 12 hours later at 180^o east from Greenwich.

Lookback time: The time elapsed from when a star, galaxy, or planet emits light and its detection on Earth, expressed in light years (or minutes). The Sun has a lookback time of 8 minutes, Pluto averaging 5.5 hours, and Andromeda Galaxy 2.5 x 10⁶ light years. Deep space galaxies have lookback times of billions of light years.

Love waves: Seismic surface waves that propogate like S waves but only generate side to side ground movement; they are also attenuated in fluids. They are slower than P and S body waves. Cf. Rayleigh waves.

Low Earth orbit: Orbits generally less than 2000 km, and at any inclination to Earth ‘s axis. LEOs are used by imaging satellites, and the International Space Station.

Lucy Spacecraft: A NASA mission launched October 16, 2021 to study three asteroids in the Asteroid Belt (e.g., it flew by asteroid Dinkinesh on November 1, 2023 ) and eight Trojan asteroids (that share Jupiter’s orbit). It will arrive at the first Trojan on April 20, 2025.

Luminosity: A measure of an object’s brightness expressed as energy or power output (e.g. watts). In astronomy, it is usually expressed with reference to a standard known luminosity, such as the Sun. It is related to absolute magnitude of a star by the expression:

M = -2.5Log (Observed luminosity/Standard luminosity)

Lunar core: Precise thickness of the lunar crust is uncertain because of ambiguities in the seismic data and models of rotation. Seismic S waves are strongly attenuated which means that either the entire core is liquid, or that it has a liquid outer shell. Recent models tend to favour a small liquid core ranging from 300 to 350 km diameter, surrounded by a liquid shell 90 to 220 km thick. All evidence indicates an iron-rich core. Core density is probably in the region of 7.8 kg/m³.

Lunar crust: Thickness estimates vary from 30 km to 43 km depending on the gravity or seismic model used. Thickness values for the far side are about 15 km greater than the near side. The average bulk density is 2550 kg/m³.  The most common rock types are basalt and anorthosite.

Lunar datum: The datum used in geodetic surveys, based on the lunar average radius. The datum is an imaginary sphere of radius 1737.4 km.

Lunar eclipse: When the moon moves into Earth’s shadow, such that Earth lies exactly between and in line with the Moon and Sun. A total lunar eclipse can only occur on a full moon. A partial eclipse occurs when Earth’s umbral region covers part of the moon. A penumbral eclipse occurs when the more diffuse penumbra shadow covers part of the moon – this type of eclipse is usually more difficult to observe.

Lunar mantle: Precise thickness of the lunar mantle is uncertain because of ambiguities in the seismic data. However, from the base of the lunar crust to about 1200 km P wave velocities are consistently about 6.8 km/s and S wave velocities about 4.5 km/s. Seismic S waves are strongly attenuated below this depth which may indicate partial melting in the lower mantle or outer core.  Deep Moonquakes range from 700-1100 km depth. The mantle is probably silicate-rich, composed of olivine- and pyroxene-rich rocks, with relatively consistent densities of 3.4 to 3.5 kg/m³.

Lunar mare: (singular maria, from Latin for ‘seas’): Extensive plains covered in basalt over about 15% of the lunar surface, most filling large craters from very old impacts. Most mare occur on the Earth side of the moon. They formed between 3.9 and 3.1 billion years ago. Basalt melts were probably generated at each impact. The eruptions were mostly effusive; evidence in the form of glass globules in lunar regolith suggest that fire fountains were important eruptive mechanisms. Wrinkling of lava flow tops has also been imaged. Samples collected by the Apollo missions show vesicularity of some basalts, indicating degassing during eruption.

Lunar near side / far side: The near side of the moon is visible from Earth; the far side is never visible from Earth.

Lunar Reconnaissance Orbiter: A lunar satellite operating since September 2009, tasked to produce detailed topographic and geodetic surveys of the lunar surface, identify possible future landing sites, measure surface temperatures and UV albedo, particularly across the lunar polar regions.

Lunar Seismic Profiling Experiment: An active seismic experiment at Apollo 17 station (1972) using explosive charges, and four geophones – three in a 100 m sided triangle about a central geophone. The aim was to profile the lunar structure to several kilometres depth.

Lunar stratigraphy: The recognition of distinct geologic periods on the Lunar surface is based primarily on superposition of craters and ejecta blankets, and the products of volcanism, augmented by a small number of radiometric dates from Apollo mission samples. There are 5 main periods:

• Pre-Nectarian: 4.533 – 3.920 Ga.
• The beginning of the Nectarian (3.920 – 3.850 Ga) is taken as the age of Nectaris impact with its ejecta blanket providing a useful stratigraphic marker. A period of intense bombardment.
• The Imbrium Period (3.85 – 3.20 Ga) Bombardment intensity continued, beginning with the Imbrium impact. There was significant basaltic volcanism.
• The Eratosthenian, 3.20 – 1.10 Ga; the lower age limit of this period is less precise because it is based on the erosional morphology of craters (i.e., erosion by other impacts), whereas the upper age limit is based in part on how fresh and bright the excavated rock appears.
• Copernican Period, 1.10 Ga to present. The Copernican crater near the southern edge of the near-side in the Imbrium Basin, is 93 km diameter and has spectacular, bright rays extending to 300 km from the impact site.

Lunation: A lunar month measured as the time between two new moons. Equals 29 days, 12 hours, 44 minutes, and 2.8 seconds.

Maat Mons: One of the largest volcanic edifices on Venus, with a peak elevated about 5 km above the surrounding plains. Recent vent activity has been identified on a shield volcano that is part of the Maat Mons complex, from radar images acquired by the Magellan spacecraft in 1990-1992.

Magellan spacecraft: A NASA mission, launched 1989 from Space Shuttle Atlantis, arrived in Venus orbit August 1990. Its primary task was to image the Venus surface over four years using radar mapping to penetrate Venus’ thick cloud cover. Almost 100% of the surface was mapped, in addition to gravity measurements over about 95% of the planet.

Magellanic Clouds: Two of the closest galaxies to the Milky Way, they are relatively small, irregular clusters of stars and nebulae. The Large Magellanic Cloud (LMC) is about 160,000 light years away; the Small cloud (SMC) about 200,000 light years. They are thought to be satellites of the Milky Way. They are always observable with the naked eye in the southern hemisphere because they are close to the south celestial pole.

Magnetic field: Earth’s magnetic field is generated by a hot (4000-5000^oC), fluid-like, iron-nickel rich outer core that moves slowly around a solid iron inner core. The magnetic field is forced into a tear-drop shape by solar winds, with the head of the ‘drop’ towards the sun (extending about 65,000 km), tapering over 600,000 km away from Earth. The magnetic field protects us from harmful components of the solar spectrum, like X-rays. The field has North and South poles that occasionally reverse over geological periods of 10⁴ -10⁵ years. The field intensity also waxes and wanes.

Magnetic pole: Points north and south where lines of equal magnetic intensity converge. The strength of Earth’s magnetic field depends on the movement of the fluid iron-nickle outer core around the solid iron inner core. Because of this movement, the magnetic field also moves. Thus, the magnetic poles traverse back and forth across their respective polar regions. The magnetic polarity can also reverse – and has done multiple times in the geological past.

Magnetic reversal: Reversal of Earth’s magnetic field has occurred many times, and over the last few million years this has happened about every 200,000 to 300,000 years.  The last reversal took place 780,000 years ago; this is called the Brunhes-Matuyama Reversal. Reversals are recorded by iron-bearing minerals in volcanic and sedimentary rocks where the minerals act as tiny magnets – the direction of polarity (i.e. magnetic N and S) is locked in mineral at the time of lava solidification or sedimentation, and this remnant magnetism can be measured.

Magnetosphere: The region of space around a planet where the magnetic field interacts with the solar wind. The impact of the solar wind on the Sun-side of Earth sets up a bow wave, or shock wave that is relatively close to Earth. On the opposite side of Earth, the magnetosphere streams to a point 10s of 1000s of kilometres away. The magnetosphere is responsible for limiting the impact of cosmic rays that are harmful to life and would also strip away Earth’s atmosphere.

Manicouagan impact structure: A well preserved, Late Triassic impact crater in northern Quebec, Canada, about 90 km diameter and outlined by a moat-like lake that surrounds the central uplift. It has been dated at 215 million years. The crater occurs within a Precambrian metamorphic-igneous terrane. The central uplift is composed of anorthosite that is surrounded by impact breccia and a 55 km wide melt sheet of differentiated monzonite and monzodiorite that are up to 1.4 km thick. Shock metamorphic structures include breccia, shatter cones, and shock-deformed quartz and feldspar crystals. The bolide was probably on the order of 5 km diameter. The impact may be implicated in the end-Triassic extinction event.

Mare (plural maria): Broad, relatively flat impact plains underlain by extensive basalt lava flows and lava tubes (these are the large dark splodges visible from Earth). Some are multi-ringed structures. They were originally interpreted as oceans or seas by enlightenment astronomers, hence the name – mare). Most formed from catastrophic impacts 3.1 to 3.9 billion years ago.

MARS 2: The Soviet Union vehicle that crashed while attempting a landing on Mars in 1971.

MARS 3: The second of two attempts by the Soviet Union to land a vehicle on Mars. MARS 3 landed successfully on December 2, 1971 but only functioned for about 20 seconds and did not transmit any data.

Mars Express: The first European Space Agency Mars orbiter, launched in 2003 and still operating. It has provided detailed images of the Martian surface, images that have been used to characterise landing sites for future landing craft. It discovered ice water in the polar ice caps, possible lakes beneath the ice caps, and has provided valuable information on atmosphere composition and weather conditions.

Marsquake: Analogous to earthquakes and moonquakes, caused by fault-related tectonics within the martian crust and mantle (low frequency events), plus numerous very small magnitude events caused by thermal contraction and expansion at the surface. The only records of marsquakes are from Mars InSight lander and its seismometer SEIS experiments from 2019 to 2022.

Mars Reconnaissance Orbiter: Arriving in 2006, the primary mission of this spacecraft was to explore the role of water on the early Martian surface. It began orbiting in 2006. It has 6 cameras and analytical tools: HiRISE imaging camera (High Resolution Imaging Science Experiment); CTX (wide angle imaging to give c9ntext to HiRISE; MARCI (Colour Imager to monitor clouds and dust storms); CRISM (Compact Reconnaissance Imaging Spectrometer for Mars); MCS (Mars Climate Sounder) to monitor atmospheric changes; SHARAD (Shallow Radar) to see below the Martian surface.

Martian canals: In 1888 Italian astronomer Giovanni Schiaparelli produced a detailed map of Mars showing features such as seas, islands, and other landmasses, and a network of ‘canali’, or channels.  Canali was misinterpreted in English as canals, and with it the connotations of intelligent life on Mars entered popular belief. We now know that many of Schiaparelli’s canali are actual channels, gullies, and canyons that have remarkably similar morphologies to water-worn features on Earth.

Mass: A measure of the amount of matter in an object. The SI unit is the kilogram (kg). The mass of an object is not dependent on the local force of gravity (g) – mass does not change even if the value of g changes. For example, a 1 Kg mass on Earth weighs 9.8 N; on the Moon the same mass weighs 1.62 N (Lunar value of g is 1.62 m.s^-2), and on Jupiter it weighs 24.79 N where g = 24.79 m.s^-2. Mass is not the same measure as weight.

Medium Earth orbit: Orbits between about 2000 km, the outer limits of low Earth orbits, and Geostationary orbits. The orbital path can be at any inclination to Earth’s axis. It is most commonly used for navigation and communication satellites.

Mercury’s tail: The closest planet to our Sun has a very thin, tenuous, sodium-dominated atmosphere that at certain times of the year is stripped by solar radiation pressure into a visible comet-like tail that contains sodium – the faint orange colour is due to excited sodium atoms. The tail length is about 40,000 km long.

Meridian: An imaginary line of arc on the celestial sphere that passes through an observer’s zenith and the two celestial poles. Cf. Nadir.

Mesosiderites: A type of stony-iron meteorite that consists mostly of brecciated fragments of crustal basalt, gabbro, and pyroxenite, plus crystals of olivine and orthopyroxene, and less commonly plagioclase. The matrix consists of finer grained cataclastic fragmentals and impact melt. Mesosiderites are thought to have formed from reconstituted fragments during asteroid collisions.

Messenger spacecraft: Launched by NASA on August 3, 2004, it is the only spacecraft to have orbited Mercury (entered the orbit March 11, 2011). Its mission was to map the planet surface – in the process it discovered water ice and took detailed measurements of the magnetic field. The mission ended in 2015 when it took its final dive into Mercury’s rocky surface.

Meteor: Another name for shooting star, it refers to a meteoroid that burns upon entry to a planetary atmosphere. Cf. meteorite.

Meteor shower: A display of shooting stars that occurs when Earth’s orbit takes it through the remnants of a comet or asteroid. Most meteoroids are small bits of rock and dust. They originate from a radiant – a particular point in the sky. There are over 100 well documented showers, and many more suspected. For example, the Aquariid shower is caused by a pass though remnants of Halley’s comet from 390BC. Other prominent showers are the Perseids that can be seen every year and are derived from the tail of Comet Swift-Tuttle, and the Geminids derived from asteroid Phaethon. Meteor visibility depends on the timing in the night sky, and on the Moon’s phase because of light interference. The American Meteor Society publishes a calendar of shower events.

Meteorite: A meteor that impacts a planet’s surface. There are three main types: iron meteorites, stony-iron meteorites, and stony meteorites. There are several important subdivisions of these three, such as chondrites, carbonaceous chondrites, and pallasites.

Meteoroid: Rock and ice fragments that move through space, ranging in size from dust to massive asteroids. Cf. meteor.

Metre/meter: The SI spelling of the measurement is metre. The American equivalent is meter. However, English meter also refers to an instrument or gauge. Meter is also a part of speech.

Milankovitch cycles: Milutin Milankovitch was a Serbian mathematician and engineer (1879-1958) and a contemporary of Alfred Wegener. He developed mathematical models to explain the variations in solar insolation that is an important driver of earth’s climate. He was particularly interested in the periodicity of glaciations. Milankovitch theory describes three kinds of orbital cycle: Precession, Obliquity and Eccentricity, each of which is influenced by gravitational interactions between the earth, sun and moon, and to a lesser extent the planets. Milankovitch cyclicity is commonly invoked to help explain changing climates and sea levels.

Milky Way: The galaxy in which we reside is a barred spiral with two primary arms extending from the end of a central bar of stars. It is about 100,000 light years wide (9.5 x 10¹⁴ km) and contains a billion or more stars. Our solar system is located about 2/3 of the way (26,000 light years) from the Galactic Center in one of the arms. Our Sun and the Solar System rotate about the galactic centre, completing a circuit every 250 million years. The nearest large galaxy is Andromeda (M31), about  2.5 million light years away. There are two smaller clusters nearby – the Greater and Lesser Magellanic Clouds.

Mimas: One of the smallest of Saturn’s moons, it consists almost entirely of ice. Mimas stands out because one of its craters, Herschel Crater, is 130 km diameter, about a third of Mimas diameter. The crater walls are 5 km high and a central peak that is 6 km high. The impact forming this structure must have come close to completely fragmenting the moon.

Minor Planet Center: This is the official organization that considered the identification and naming of minor bodies in the Solar System (dwarf planets, asteroids, comets, moons, near Earth objects). It operates under the auspices of the International Astronomical Union (IAU) and is funded by NASA. It is also responsible for publication of ephemerides.

Moonquake: Seismic tremors and upheavals from quakes, meteoroid and artificial impacts. The equivalent of an Earthquake.

Murchison meteorite: A carbonaceous chondrite that was observed to explode of as it entered Earth’s atmosphere on 28 September, 1969, over the eponymous Australian Victorian town. Several bits of the meteorite were found. Hundreds of organic compounds have been identified including amino acids, macromolecules, nitrogen- and sulphur-bearing compounds. All the organic compounds are considered to have formed abiotically, but such compounds may have provided the biochemical seeds for the beginning of life of Earth. Very small grains of silicon carbide have been dated at 7 billion years – almost 2.5 billion years older than the Solar System.

Nadir: An imaginary point on the celestial sphere immediately below an observer – the opposite of zenith.

NASA: The National Aeronautic and Space Agency was formed in July 29, 1958, in part a response to the successful launching and orbit of the USSR’s Sputnik satellite. It’s primary aim was to “accelerate work on human and robotic spaceflight, and is responsible for scientific and technological achievements that have had widespread impacts on our nation and the world.” For a complete history check out the various commentaries and videos in the link above.

Nasa shuttle era: The five shuttles beginning with Columbia in 1981, and continuing successively with Challenger, Discovery, Atlantis and Endeavour, provided launch and recovery trips to space for 135 missions, the program ending in 2011. Missions included satellite recovery and repair, experimental work, and was instrumental in the construction of the International Space Station. Each shuttle was capable of a glide landing on a runway.

Near Earth objects (NEO): Asteroids and comets having orbits that approach Earth to within 1.3AU units (about 200 million km) are considered a potential risk. The risk of impact is based on probability. There are three categories based on bolide size: <140 m (low risk), 140-1000 m (low to moderate risk), and >1000 m (disastrous). Near Earth Objects with orbits that bring them to within 0.05AU to earth (7,500 km), and are large enough to survive entry, are classified as Potentially Hazardous Objects.

Nebula: Vast interstellar clouds of dust and gas (mostly hydrogen). Many are the remnants of  stars that have exploded at the end of their life, but the clouds then become nurseries for new stars. The Crab Nebula, about 10 light years across and 6500 light years away has several nascent stars; likewise the Eagle Nebula that is up to 70 light years wide and contains the iconic Pillars of Creation.

New Horizons mission: The NASA mission to fly-by Pluto, its moons, and beyond to the Kuiper Belt, launched January 2006. The fly-by began July 14, 2015, and the Kuiper Belt dumb-bell shaped object Arrokoth on January 1, 2019. Its closest approach to Pluto was 7,800 kilometers. En route it flew by Jupiter’s moon Io, Europa, and Ganymede, including a gravity-assist by Jupiter in February 2007. The spacecraft is about 8.8 billion km from Earth as of December 5, 2023.

Newton: The SI unit of force that accelerates a mass of 1 kg at 1 m.s^-2. Units are kg.m.s^-2 and dimensions MLT^-2.

Newton’s Laws of motion: Isaac Newton codified the properties of inertia in his three Laws of Motion – apparently Newton credits Galileo with the discovery. The 1st Law, also called the Law of Inertia, states that the motion of a body will not change unless an external force acts on it (i.e., to accelerate, decelerate, or change its direction). The 2nd Law quantifies the relationship between an external force F, mass (m) and acceleration (a) as F = ma. And the 3rd Law states that when an external force is applied, there will be an equal and opposite force that resists the change in motion, i.e., an inertial force – also called the Action-Reaction Law.

Noachian Period (Mars): The period on Mars from 4.1 Ga to 3.7 Ga. Asteroid bombardment continued. There were vast outpourings of basaltic lava from volcanic centres like the Tharsis region, including the massive edifice of Olympus Mons. Ocean masses may have persisted into this period. Many of the drainage channels and associated features also formed at this time, presumably because of precipitation. Sediment derived by these processes would have been transported to basinal regions. Most of the crossbedded sandstones and laminated mudrocks imaged by various Mars rovers date from this period.

North Star: The northern hemisphere star Polaris lies on an imaginary line projected from Earth’s rotational poles. Thus, the position of the star in the night sky remains the same. It is also called the celestial star and lies on the celestial sphere.

Nuclear fission: The splitting apart of atoms under bombardment by electrons releasing energy as heat, radiation and light.

Nuclear fusion: A natural multi-step process in the core of stars where hydrogen is converted to protons (positively charged plasma) that in turn transforms to helium under intense heat (15 million degrees C) and pressure. The process is initiated when two protons fuse together. One of the protons transforms to a neutron, forming deuterium that in turn collides with another proton to form Helium-3 (2 protons + 1 neutron). The process is complete when two He-3 nuclei collide to form helium-4 (2 protons + 2 neutrons). Each fusion step releases heat and light.

Nusku volcano (Io): One of the many, recently active volcanoes on Jovian moon Io. An August 2024 flyby by Juno imaged a huge red ring around the volcano, about 1100 km diameter, indicating a massive eruption probably sometime in April 2024.

Obliquity: Obliquity is the combined effects of axial precession and ecliptic precession that cause earth’s tilt to move between 21.5^o and 24.5^o (the tilt angle is measured against the ecliptic). It is one of the Milankovitch cycles. The time to complete one tilt cycle is 41,000 years.  The changes in obliquity can impact the severity of Earth’s seasons.  At the lowest tilt the sun is distributed a bit more evenly so that seasonal variations are likely to be more clement. At the highest tilt, seasonal variation is likely to be more dramatic. Earth’s tilt is presently 23.4^o and is on the decreasing part of the cycle.

Occultation: In astronomy, an occult occurs when observation of a body is prevented by a second body passing between them. Common examples are solar eclipses or when one of the planets is eclipsed by the moon.

Odysseus lander: The first commercial landing on the Moon on February 22, 2024, on the relatively steep slope of crater Malapert A, South Pole region. The landing itself was a success, except one of the legs broke on rough terrain and the lander fell over but continued to send data and images. It was launched aboard a SpaceX Falcon 9 rocket.

Olympus Mons: The largest known volcano in the Solar System and part of the Tharsis Montes volcanic region on Mars. It has the iconic shield form, 624 km in diameter, 25 km high, a caldera crater 80 km in diameter, and a 4-6 km high scarp on its outer rim.

Oort Cloud: The Oort Cloud contains the most distant bodies in the Solar System at distances ranging from 2000 to 200,000 AU (3.2 light years). It contains mostly icy bodies described as planetesimals. Icy bodies are distributed through the cloud in two regions: an inner disc, and an outer sphere – both are located beyond the heliosphere which means the Sun’s gravitational influence is very weak to non-existent. Some comets visible from Earth may have been dislodged from the cloud.

Orbital velocity: The velocity of a body required to keep it in orbit, either around the barycenter if the two bodies are approximately the same size, or the centre of mass if the parent body is much larger.

Opposition: In astronomy, the situation when two celestial bodies are in exactly opposite positions in the sky. When viewed from Earth, a full moon is opposite the Sun with Earth approximately in the middle. Opposition can only be observed for planets outside Earth’s orbit. Thus, Jupiter is in opposition when it rises in the east as the sun sets in the west – again Earth is in the middle.

Orbit: The path described by a body that moves around a (larger) parent body. Orbits are commonly elliptical in planetary, star, and satellite systems. Orbits are stable when gravitational forces are balanced by centrifugal forces.

Orbital period: The time taken for a body to complete one rotation around its parent. In planetary systems it refers to planet-star, planet-moon, asteroid/comet -sun, and in some cases comet-planet systems.

Orbital resonance: A resonance between satellites occurs when the difference in their orbital periods can be expressed as an integer. Good examples of this relationship are three of the Jovian Galilean moons – Io, Europa, and Ganymede, whose orbital period ratios are 4:2:1 respectively. This relationship means that each moon influences the gravity response of the other two moons and the degree of eccentricity of their orbits.

Orcus: A dwarf planet and one of the largest Kuiper Belt objects, discovered in 2004, along with its moon Vanth form a binary system. Estimates of its diameter range from 958.4 ± 22.9 km to 892 -942 km based on albedo measurements. Vanth is about 260 -293 km diameter. Its orbital period is similar to Pluto at 245.18 years. Its density is about 1.5 g.cm^-3 indicating a composition of rock and water ice.

OSIRIS-Rex satellite: (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) Launched September 8, 2016, it reached Bennu in 2018 spending a year mapping the surface. It successfully landed and collected measurements and samples, departing for Earth May 10, 2021. ETA at the Utah Test and training Range (near Salt Lake City) is September 24, 2023.

ʻOumuamua: A cigar-shaped, interstellar, asteroid-like rock that entered our solar system at about 20^o to the ecliptic, was discovered October 2017. It was sling-shot away from the Sun at about 137,000 km/hour, fast enough to break free of the Sun’s gravity and take it out of the Solar System.

Ozone layer: On Earth, the atmospheric layer in the lower part of the stratosphere (15-40 km above sea level) that contains the greatest concentrations of ozone (O₃) – ozone is more stable at these altitudes than lower in the troposphere. It is responsible for filtering UVB light (280-320 nanometres wavelength). UV radiation at these wavelengths can be damaging to DNA.

P & S waves (seismology): Seismic body waves generated by an impulse (earthquake, TNT, meteor impact) that travel through Earth from the energy source. P waves push and pull materials in the same direction as the propagated waves (also called compressional waves). S waves, or shear waves produce sideways motion – motion at right angles to the propagation direction. Shear waves do not travel through liquid. P waves travel fastest (up to 7.97 km/sec in upper mantle rocks) and are the first to appear on a seismogram. See also Surface waves.

Pallasites: One of two types of stony-iron meteorites that consist of olivine crystals encased in the FeNi metal compounds Kamacite and Taenite. They may represent a more differentiated part of the cores of ancient planets.

Parsec: The distance of a body from the Sun, calculated when the angle between the Sun and Earth, subtended by the body is 1 arcsecond (1/3600 of a degree). The distance between Earth and the Sun is 1 AU. Therefore, a parsec corresponds to 3.086 x 10¹³ km, or 3.26 light years.

Partial solar eclipse: When the Moon’s penumbral shadow traverses Earth when the moon is between Earth and the Sun – i.e., the umbral shadow does not fall on Earth cf. total eclipse.

Pascal: (Pa) The SI unit of pressure; 1 Pa = 1 N.m^-2 or 10^-5 bars. It has dimensions M.L^-1.T^-2

Passive seismic experiments: A common method of earthquake and impact investigation on Earth, the method was used for the first time on another planetary body during Apollo 11 in 1969, and subsequent Apollo missions until the experiments were terminated in 1977. More than 13,000 significant seismic events were recorded including shallow and deep moonquakes, meteoroid impacts, artificial impacts, and diurnal thermal events. A passive seismometer was also installed during the InSight program on Mars from 2018 to 2022.

Paterae: From Latin meaning dish-shaped, a depression, or crater. Used by early astronomers to describe such features on planetary surfaces, like Mars, that subsequently have been identified as volcanic craters and calderas. They are also known on Jupiter’s Moon Io.

Peak ring craters: These form in very large craters where the central peak collapses and brecciated rock flows outward to form a blocky, roughly concentric ring. They are far less common than simple or complex craters. The only known example on the moon is the 900 km diameter Mare Orientale. (Grieve & Therriault, 2012)

Penumbra: The region of diffuse shadow outside the umbra.

Periapsis: The general term for the closest point of an orbiting body. More specific terms include perihelion, perigee, preijove.

Periaston: The point in the orbit of one component of a binary system where it is nearest the other component. In the Solar System, Pluto and its moon Charon are probably a binary system; dwarf planet Orcus and its moon Vanth are binary.

Perigee: The closest distance of the Moon from Earth during its elliptical orbit. Cf. apogee

Perihelion: The closest distance from the Sun (or any star) of an orbiting body. The term coined by Johannes Kepler applies to planets, comets and asteroids that have elliptical orbits around a star. Etymology – Helios, the Greek Sun God. Cf. Aphelion

Perijove: The position on a satellite orbit that is closest to Jupiter’s center. Cf. Apojove, Perigee.

Periodic comet: Comets having orbital periods less than 200 years. The classification is indicated by a ‘P’ in the comet name. The link above is to the University of Maryland/NASA list of periodic comets (more than 800 of them).

Perseids: A prominent meteor shower that peaks mid-July to early September. It originates from the constellation Perseus (its radiant), from comet 109P/Swift-Tuttle.

Perseverance Rover: Launched July 30, 2020, landing in Jezero crater February 18, 2021. The site was chosen because it contains exposures of ancient delta – river deposits. Its primary mission was to explore the Geology, Astrobiology (possible ancient life), Sample Caching (collect and package samples for collection by future missions – 38 sample tubes available), look for possible sites for future human missions, and technology development using the Ingenuity Helicopter.

Phaethon – Asteroid 3200: Discovered in October 1983 this object is 5.1 Km in diameter and a highly elliptical orbit at 22.2^o to the ecliptic, that takes it very close to the Sun – almost 30 million kilometres, to a more distant point between Mars and Jupiter. Its orbital period is only 524 days. It is a Near Earth Object (NEO); its next encounter with Earth in 2093 will be as close as 0.0198 AU (2.962 million km). The tail it develops on approach to the Sun is composed of sodium rather than rocky dust. The Geminids meteor shower (4th-17th December) is thought to be sourced from Phaethon as it approaches the Sun. The name derives from Phaeton, the Son of the Greek Sun God Helios.

Phases of the Moon: The constantly changing positions of the moon as it is viewed from Earth, describes a cycle beginning with a new moon when the Moon lies between Earth and the Sun, and ending with the full moon when Earth lies in the middle. In the northern hemisphere a waxing moon will become increasingly illuminated from right to left, and a waning moon decreasingly illuminated from right to left (i.e., it is illuminated on the left). The opposite rule applies in the southern hemisphere. The phases in a full lunar cycle (29.5 days) for the northern hemisphere are: a waxing crescent (illuminated on the right); 1^st quarter; waxing gibbous; full moon; waning gibbous (illuminated on the left); last quarter; waning crescent; new moon.

Phobos: The larger of the two Martian moons, discovered in 1877, it is very close to the Mars surface, orbiting about every 8 hours. Its orbit is decaying at about 1.8 m/100 years. Its largest dimension is 27 km. The surface is pockmarked by craters and groove-like structures that are hypothesized to have formed by tidal forcing.

Phoebe: Saturn’s outermost moon, is 106.5 km diameter, discovered in 1898 by William Pickering. Its orbital distance is 12,952,000 km; the orbit is retrograde and out of the plane of Saturn’s rings. It is possibly a Centaur, a object captured by Saturn from the Kuiper Belt. It was one of the first objects in the Saturn system encountered by Cassini.

Photosphere: The outer, visible layer of the Sun; it is about 100 km thick. This layer contains the Sunspots, faculae, and at greater telescopic magnification, the granules.

Planet: The 2006 IAU definition “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.” The expression ‘cleared the neighbourhood’ refers to the gravitational attraction and impact of asteroids, comets, and dust that reside nearby.

Planetary differentiation: The process of separation of elements and minerals into distinct compositional layers. Dense minerals will tend to sink (gravitational differentiation). Fully differentiated rocky planets and planetesimals commonly have Fe-Ni cores, surrounded by silicate-rich mantle layers; olivine is a common mineral in mantle rocks. Accumulation of lighter silicates such as quartz will occur in outer crustal layers. Differentiation may begin during initial planetary accretion, but will continue with partial melting of mantle rocks, and magmatic processes in the crust.

Planetary orbits, axial rotation, obliquity, moons, distance from the Sun, and rings: Information in this chart has been sourced from multiple websites.

Planetary surface temperatures, pressures, and atmospheres: Information in this chart has been sourced from multiple websites.

Planetary radii, densities and gravity: : Information in this chart has been sourced from multiple websites.

Planetesimal: Accretion of gas and dust during the early stages of planetary nebula evolution, eventually leads to bodies, or planetesimals with gravitational fields large enough to draw in their nearest neighbours. The planetesimals may be broken up by asteroid bombardment (the resulting fragments subsequently available for accretion to other planetesimals), or they may continue to accrete leading to fully fledged planets. Larger planetesimals may be capable of generating enough internal heat to cause partial melting, and differentiation of a core, inner shell, and an outer crust. Planetesimals are regarded as the building blocks of planets. Theoretically, many of the asteroids in the Asteroid Belt are planetesimals left over from accretion in the early nebula.

Polar orbits: A class of satellite Low Earth orbit in the plane of the two poles, or within about 20^o of the polar axis.

Pole star: See North star (Polaris).

Ppb: Parts per billion. For example nano-gram/gram, or 0.001 ppm.

Ppm: Parts per million. For example μg/g (microgram/gram), or 0.0001 mass%.

Ppt: Parts per trillion. For example 10^-12 gram/gram, or 0.001 ppb.

Pratt isostasy: John Pratt’s model of isostasy (1855) where the crust is represented by discrete blocks of different density that move independently (compensate) up or down (his block model only considers the crust and not the mantle). Pratt’s model distinguishes oceanic from continental crust (unlike the Airy model). The depth of compensation is where lithostatic pressures beneath the blocks are equal and in this model it is the common depth at the base of the crust.

Precession: Precession describes the Earth’s wobble as it spins on its axis of rotation. The time taken to complete a single wobble cycle is 19,000 to 23,000 years. Earth is an oblate spheroid with its longest diameter at the equator – precession is caused by this equatorial bulge. It has the shortest period of the three main Milankovitch cycles. Precession cycles are superimposed on obliquity cycles.

Pre-Noachian Period: The period on Mars from 4.5 – 4.1 billion years ago, from its accretion to the formation of differentiated crust, and hypothesized cooling and condensing of its atmospheric water vapour to form oceans and lakes. It was also a period of intense asteroid bombardment.

Pressure: Defined as Force per unit area, or SI units N.m^-2; it has dimensions M.L^-1 T^-2. Also called stress.

Protoplanetary disc: A (flattened) disc of gas (probably >90%), dust, asteroid-like bodies and other planetesimals surrounding a young star. Gravitational attraction eventually forms larger planets, the number and composition of which depends on the disc mass and composition. As a planet grows, it eventually develops its own gravitational field and continues to attract disc material. Hubble, JWST, and ESO’s ALMA array in Atacama have imaged several protoplanetary discs around exoplanets. Gaps in some discs may indicate that dust in those regions as accreted into planets. Cf. Circumstellar discs.

Proxima b: An exoplanet closest to Earth at about 4.25 light years, discovered in 2016. It orbits the red dwarf Proxima Centauri, the small star that orbits the double stars in the Alpha Centauri system. Its orbit is 0.0485 AU from its star, and although probably in the potentially ‘habitable zone’, it is bombarded by UV radiation intense enough to strip most atmospheric molecules from its surface.

Pseudotachylite: A dark, fine-grained rock, commonly identified by its glassy groundmass and quench textures, that can be found along fault planes or as veins associated with bolide impact shock metamorphism. Pseudotachylite composition is usually similar to the country rock it intrudes and is derived by partial or complete melting of the host rock. The veins commonly contain breccia.

Psyche asteroid probe: The name of the probe and the asteroid (16 Psyche); the asteroid of thought to be composed of metal. The probe is designed to conduct composition experiments on arrival, and also to test the efficacy of using laser-based communications over space travel distances – called Deep Space Optical Communications. Such communications are potentially much faster than standard methods. One of the early test beamed to Earth a video of a ginger cat “Taters” at 267 megabits/second.

Radar shape models: The method uses radar signals to measure an asteroid range (echo time) and radial velocity (Doppler frequency). The signals produce 2D images of the asteroid surface that can be combined into 3D models.

Radian: An angular measure commonly used in mathematical expressions involving rotation and moving bodies, for example angular velocity. 2π radians is equivalent to 360^o.

Radiant: The point in the sky from which meteor showers originate and radiate from. The radiants for most showers coincide with constellations, from which the showers commonly get their name (e.g., the Perseids in Perseus).

Rayleigh waves: Seismic surface waves that produce a rolling ground motion with vertical and horizontal components of movement. They are slower than P and S body waves. Cf. Love waves.

Regolith: A general term describing the products of physical and chemical weathering, resulting in a veneer of soil and granular material including breccia. Regolith associated with meteoroid impacts will usually contain melt glass and evidence of shock metamorphism. Examples of extraterrestrial regoliths have been observed  and/or sampled on the Moon, Mars, asteroid Bennu, and comet 67P/Churyumov-Gerasimenko, and some other planetary moons.

Retrograde motion: Motion of a body that is opposite the so-called normal or prograde motion. All planets orbit the sun counter-clockwise relative to the North Star; most planets also rotate on their axes in the same direction, except Venus and Uranus that rotate clockwise or retrograde. The asteroid Bennu also has a retrograde spin.

Rhea: Saturn’s second largest moon, with radius 764 km, orbiting about half a million kilometres from its parent. Its density is 1.23 g/cc, and gravity 0.264 m/s², and with relatively high albedo indicates that its shell is probably ice water and core a mix of ice and rock. Surface temperatures range from -174^oC to -220^oC. The orbit is tidally locked to Saturn; orbit period is 4.5 Earth days.

Right ascension: One of the coordinates on the celestial sphere (along with declination) used to located astronomical objects. It is the equivalent of longitude and usually measured as time where 1 hour corresponds to 15^o of rotation. The word “Right” in the term refers to Earth’s counterclockwise rotation.

Rocky planets: Planets that have a solid silicate or iron core, molten or solid shells, and a solid crust. In our Solar System this includes Mercury, Venus, Earth, and Mars. Pluto is a combination of ice and rock. Some of the Jovian planets have moons that are rocky, or are combinations of rock, ice, and liquid layers.

Rotational poles: Two points where the axis of rotation of a spheroid intersects the surface. On Earth the poles correspond closely to the geographic poles.

S1000a: The first major meteoroid impact on Mars recorded by the InSight seismometer SEIS on September 18, 2021, for which there was visual confirmation of its crater from before-and-after images taken by HiRISE orbiter. The crater was 130 m diameter. The impact produced a marsquake of magnitude 4.0-4.1, that produced a recorded P wave that was diffracted by Mars core-mantle boundary (Duran et al., 2022).

Saturn’s rings: All the gas giant planets and a few asteroids have rings; Saturns are the most spectacular. They are composed of particles ranging from dust to moon-like rocky fragments. They extend 282,000 km from the planet surface but on average are only about 10 m thick. Rings or ring groups have been labelled in order of discovery – the succession, from the closest to the surface is D, C, B, Cassini Division, A, F, G, and E ring. Cassini orbiter discovered that material in the E ring is probably sourced from vapour plumes ejected by moon Enceladus. An additional, very faint ring is located much farther out in the orbit of moon Phoebe. The rings may only be a few hundred million years old – very young compared with the age of Saturn and the Solar system.

Sedna: A dwarf planet and the most distant object in the Solar System, it is 90 AU from the Sun (about 12.9 billion km). It is 3 times more distant than Pluto. Sedna has a highly elliptical orbit. It was discovered 14 November 2003 by Caltech and Yale astronomers. The name is derived from the Inuit god of the sea.

SEIS: The broad-band frequency seismometer deployed by InSight lander on Mars surface at Elysium Planitia, 2019. More than 1300 high and low frequency, tectonic and impact quakes were recorded over its four years of operation.

Seismic coda: Seismic wave scattering can produce a kind of ringing, or echo effect that is referred to as seismic coda (coda is a term used in music composition to indicate a repeated theme). It is an important contribution to recorded lunar and Martian seismicity, probably because of intense fracturing in the shallow crust, that can mask other important signals.

SETI Institute: Search for Extraterrestrial Intelligence. The SETI Institute was incorporated in California on November 20, 1984 and began operations on February 1, 1985. It began as a NASA project, but has expanded to look for life and intelligent life in other solar systems. NASA and the National Science Foundation remain SETI partners, in addition to private and philanthropic funding.

Shatter cones: Shatter cones are a distinctive structure formed by high pressure events such as asteroid impacts. They occur as overlapping, 3-dimensional cones, pointy end up.  They are usually associated with shocked quartz and melt breccias.

Shock lamellae: Common in minerals like quartz where parallel laminae indicate deformation, or breakage of the crystal along multiple, parallel planes during extreme high- pressure events. They are distinctive because cleavage is usually absent in quartz crystals.

Shock metamorphism: Irreversible structural and chemical changes to rocks caused by the passage of a shock wave from hypervelocity impacts, events that take a fraction of a second to a few seconds. The shock wave generates compression and heat – pressures range from a few GPa to 1000 GPa (gigapascals), and temperatures from a few 100^o C to several 1000^o C. With the passage of the shockwave, increases in pressure and temperature see changes to the target rocks pass through brittle failure and cataclasis, solid-state mineral phase transformations (e.g., quartz to coesite and stishovite), melting, and vaporization. The deformation of crystals like quartz and zircon produces parallel ‘shock’ lamellae. Other products include shatter cones, melt rock as sheets and distributed fragments like tektites.

Shock wave (impact): The instantaneous pressure wave generated during a bolide impact, that expands outward in an approximately hemispherical geometry, decreasing in intensity with distance from the impact. It is responsible for brecciation, mineral phase transformations, melting, and vaporization of country rock.

Shoemaker-Levy 9: Comet Shoemaker-Levy 9 was discovered March 24, 1993 by astronomers David Levy, and Carolyn and Eugene Shoemaker. The comet was not a single body, but a string of bodies. It was found to orbit Jupiter with a 200-year period, but the orbit was decaying and calculations indicated it would collide with Jupiter, beginning with the first and nearest comet fragment on the 16^th July, 1994, and ending July 22^nd. The largest fragment was two kilometres diameter and had an approach speed of 60 km/second, or 216,000 km/hour. Instead of craters, the impacts left a string of greyish, circular to crescent-shaped patches that developed from clouds of dust and aerosols ejected into the Jovian stratosphere. One patch was 12,000 Km across – equivalent to the diameter of Earth. The impacts produced massive fireballs that soared rapidly to 3000 km.  The fireballs, visible for up to 80 seconds had initial temperatures of 23,700°C (42,700 °F). The ambient temperature at the top of Jupiter’s cloud is −143°C (−226°F).

Shooting star: The common name given to the bright trails caused by meteors burning up as they enter Earth’s atmosphere.

Sidereal day: A sidereal day is the time taken for Earth to rotate once on its axis relative to distant stars (that are assumed to be fixed). We regard the stars as being fixed to an imaginary celestial sphere at some great distance (the poles of the celestial sphere are the celestial poles). For an observer on Earth, this means it is the time taken for stars to appear in the same position in the sky from one night to the next. A sidereal day is 23 hours 56 minutes 4.091 seconds – it is about 4 minutes shorter (faster) than a solar day (24 hours). The difference between sidereal and solar time is due to Earth’s orbit around the Sun,where it has moved 1 day farther along its orbit during the period of rotation.

Sidereal month: The time taken for the moon to complete one revolution around Earth relative to distant, fixed stars (see sidereal day). A sidereal month is 27.3 days. It is about 2 days shorter (faster) than a solar month because over this time Earth has moved farther along its orbit around the Sun – a bit like playing catch-up.

Siderophile elements: Literally iron-loving elements, they include the high density transition metals that bond with iron in solid and molten states. They can also bond with sulphur and carbon. As such they tend to concentrate in Earth’s core and to a lesser extent the mantle. They re rare in the crust. Most, except for Fe and Mn have a low affinity for oxygen. The list includes Ag, As, Bi, Cd, Cu, Ga, Ge, Hg, In, Pb, Po, S, Sb, Se, Sn, Te, Tl, Zn – Sulphur is also a volatile element and at Earth’s surface combines with oxygen to form sulphate anions.

Sif Mons: A volcano on Venus that appears to have been active recently, discovered from analysis of radar data collected during surface mapping by the Magellan spacecraft from 1990 to 1992. Morphological changes to the volcano slopes and vent were detected in images collected at different times during the 1991-1992 mission.

Simple crater: (Grieve & Therriault, 2012) The smallest and structurally simplest impact craters, ranging from sub-millimetre diameter pits to 10 km. They have raised rims, a surrounding ejecta blanket, and basin-filling non-melt breccia – some of the larger versions may have melt breccia. There may be some instability in the crater walls, particularly in the larger craters.  On the moon they commonly are superimposed on larger craters. The best-preserved example on Earth is Barringer Crater in Arizona.

Single apparition comets: Comets that orbit the Sun only once then leave the solar system. They are thought to originate in other star systems.

Sol: A solar Martian day, in Earth time equivalent to 24 hours, 39 minutes, 35 seconds. A Martian year is about 668 sols.

Solar Cycle 25: Based on sunspot history, the Solar Minimum for Cycle 24 was December  2019. This heralds the beginning of Cycle 25, that is expected to peak in 2025. This also means that the incidence and intensity of solar flares will increase.

Solar day: For an observer on Earth, it is the time taken for Earth to complete one rotation about its axis, relative to the sun. It is about 4 minutes slower than a sidereal day.

Solar eclipse: The shadow cast on Earth when the Moon lies in a straight line between the Sun and Earth. This can only occur during a new moon. A total eclipse occurs in the moon’s umbral region; partial eclipses are observed in the penumbral region. The narrow umbral region will track across Earth’s surface as it rotates. Solar eclipses are visible from somewhere on Earth approximately two times a year.

Solar elemental abundances: See Solar system elemental abundances

Solar flares: An eruption of electromagnetic radiation from the sun, usually above sun spots. They appear as bright spots on the Sun surface. Electromagnetic and atomic particles are accelerated into space and if the burst is strong enough it can disrupt radio communications on Earth and increase the intensity of aurora. Flares last a few hours to a few days. There are three types of flare: X-class, are very large and very disruptive; M-class can result in weaker electromagnetic storms; C-class where effects on Earth are minimal.

Solar mass: A unit of measurement, approximately equal to the mass of the Sun (about 2 x 10³⁰ kg), that is used to compare the masses of other stars, nebulae, black holes, galaxies, and star clusters.

Solar mass-fractions: The fractions of major elements that make up our sun and other stars. There are three values: X for hydrogen, Y for helium, and Z for all the remaining elements heavier than helium. The values are normalized, and for the outer convective zone of our Sun X = 0.7389, Y= 0.2463, and Z = 0.0148. On this basis, all elements other than H and He make up only 1.48 % of the Sun. (K. Lodders, 2019).

Solar month: For an observer on Earth, it is the time for the moon to complete one revolution around Earth relative to the Sun. The solar month is 29.5 days, about 2 days slower than a sidereal month because Earth has moved farther along its orbit around the Sun.

Solar prominence: Also called a filament. An eruption of flowing plasma extending from the surface of the Sun into the corona, commonly in an arch-like structure that is anchored to the Sun. Their extent is usually measured in thousands of kilometres. These events can last months. Cf. Solar flares that are not attached to the Sun surface. If the prominence breaks free of the surface it becomes a coronal mass ejection.

Solar System: A collection of planets, dwarf planets, moons, asteroids, comets, and dust that orbit a star. Our Solar System has an average-sized star at its centre, 8 planets, 5 dwarf planets (including Pluto), at least 200 moons, an Asteroid Belt between Mars and Jupiter, the Kuiper Belt containing icy bodies that lie outside the orbit of Neptune, and the most distant Oort Cloud of dust and planetesimals that extends beyond the heliosphere. It seems that most stars in the universe have solar systems.

Solar system elemental abundances: Also Solar elemental abundances. The abundance of all elements in the entire solar system. The elemental abundances of the Sun are usually taken as a proxy for the Solar system because it comprises >99% of the system. The abundance values are also taken as an approximation of the original composition of the interstellar molecular cloud from which the sun and planetary bodies accreted (see review by K. Lodders, 2019; link above).

Solar wind: Charged particles generated by the Sun’s corona that travel outwards at high speed. Solar winds react with the magnetospheres of planets, focusing at their magnetic poles. On Earth, this creates the aurora borealis and aurora australis in both polar regions.

Solstice: Twice a year the sun reaches its highest and lowest positions relative to the celestial equator on the celestial sphere; i.e., the sun is farthest from the equator (cf. the equinox). This is usually December 21 and June 21. Winter solstice in the northern hemisphere corresponds (December 21) to the shortest day of the year; in the southern hemisphere it is the summer solstice and the longest day of the year. This relationship exists because Earth’s orbit is elliptical and its spin axis is tilted.

South Pole Aitken Basin: The largest lunar impact basin, or mare, on the southern extent of the lunar far side. It has some of the deepest points on the Moon surface at 8.2 km below the lunar datum. it is about 2500 km diameter.

Southern Cross: The four principal stars in the constellation Crux form a cross in the southern hemisphere sky that has been an important navigation point of reference to the south celestial pole for centuries. To determine south, draw a line connecting the longest diagonal between the head and foot of the cross. Extend the line an additional 4 times the length of the diagonal, towards the horizon – the end of this projection lies directly above the geographic south pole.

Southern Delta Aquariids: A prominent meteor shower usually seen mid July – August each year (cf. the Eta Aquariids), also originating from Aquarius (its radiant). The origin of the shower is suspected to be comet 96P/Machholz that was discovered in 1986.

Spectroscopy: An analytical method that can identify chemical elements and compounds based on the light spectra they emit. When atoms of a particular element or compound are heated, they emit light at certain frequencies (wavelengths). These frequencies can be measured and compared with standard frequencies of known compounds. In this way, solar and interstellar gases, planet, exoplanet and comet atmospheres can reveal their composition. The method is also used in many scientific and industrial investigations on Earth.

Speed of light: 299,792 km sec^-1, 186,180 miles sec^-1

Sputnik 1: Was the first orbiting satellite, launched 4 October 1957 by the Soviet Union. The announcement was made by TASS. It was small, spherical, measuring 58 cm diameter and weighing in at 83.6 kg. It’s orbit was elliptical, up to 900 km from Earth. The event also launched a new era in space exploration, and the USA-USSR space race.

Stishovite: An ultra high pressure polymorph of silica that transforms from quartz during meteorite impacts. It has a density of 4.3 g/cm³ (quartz is 2.65 g/cm³), has 2 planes of cleavage, and belongs to the tetragonal crystal system. Cf. quartz, coesite.

Stony meteorites: The most common kind of meteorite, stony meteorites, are composed of silicate minerals, some FeNi metals, and in rare cases carbon compounds (carbonaceous chondrites). The group is subdivided into two main types: chondrites, made up of silicate mineral globules derived from melts, and achondrites that show evidence of melting.

Stony-iron meteorites: One of the three main types of meteorites, having nearly equal amounts of metal and silicate crystals. The metals are primarily the FeNi compounds Kamacite and Taenite. There are two main groups of stony-irons: Pallasites that consist of FeNi and olivine crystals, and Mesosiderites that are breccias composed of FeNi metal fragments and silicate fragments. Cf. Iron meteorites, stony meteorites.

Suevite: An impact breccia that contains clasts of brecciated bedrock, glassy or crystalline melt rock, and shock-deformed crystals such as quartz and zircon. The matrix is clastic but may also contain small fragments of melt glass.

Sudbury impact structure: The Sudbury impact structure presents as an eroded remnant of a 1840 million year old, 150-260 km diameter basin in Ontario, Canada. The depth of erosion provides a view of the deformed crust beneath the impact structure,  as well as shallower deposits like fall-back and impact breccias and post-impact basin fill. Typical shock metamorphic structures include shatter cones, shock lamellae in Quartz, and a melt sheet (Sudbury Igneous Complex). Iridium anomalies have also been recorded in the ejecta deposits. Sediment that fills the basin (Onaping Formation) includes fall-back breccia that was subsequently hydrothermally altered by remnant heat from the impact-generated igneous complex.

Summer solstice: The time of year when either the north or south pole has maximum tilt towards the Sun corresponding to the longest day – June 20 or 21 in the Northern hemisphere and December 21 or 22 in the Southern hemisphere. For each, there is an opposite winter solstice.

Sun spots: Dark regions of the Sun’s outer photosphere that have slightly cooler temperatures where convection is temporarily inhibited. They were some of the first solar features discovered by Galileo who described and mapped their appearance over several years. Sun spots move across the surface of the Sun. They increase in number during a solar maximum that cycles every 22 years – 11 years to the solar maximum, and another 11 years to the corresponding solar minimum (also called the Schwabe Cycle). The cycle is caused by the reversal of the Sun’s magnetic field where over one 11 year period North becomes South, and over the following 11 years this reverses.

Sun-synchronous orbit: A variation on Polar orbits at altitudes of 600-800 km, that are synchronous with the Sun, such that from a particular location, a satellite will be seen at the same local time every day. This kind of orbit allows an observer to see the same locations on Earth at regular periods.

Super moon: A full or new moon that coincides with, or is close to perigee (closest to Earth).

Supernova: During the death throes of a star, the dense, hot core collapses so rapidly that it creates a shock wave, causing the outer part of the star to explode. The collapse and subsequent explosion can take place in seconds. Gas and charged particles expand rapidly outwards. The supernova that formed the Crab Nebula, a well-known example, was witnessed as a bright object during day light by Chinese observers in 1054 AD. The Crab Nebula is 6500 light years away, and now is 6 light years in diameter.

Surface waves (seismic): Seismic waves generated by tectonic quakes and meteoroid impacts that are confined to the shallow crust (cf. body waves). The two principal types are Rayleigh waves and Love waves. Both are higher frequency seismic responses than P and S waves, and usually arrive at a seismograph after the body waves.

109P Swift-Tuttle: One of the largest comets considered to be a near-Earth object. Its nucleus is about 26 km wide. The orbital period is 133 years – the last closest approach to the Sun was 1992 and its next approach 2125. It is responsible for the Perseid meteor shower.

Synodic day: See Solar day.

 

Tektite: Globules of glass melt a few millimetres in diameter usually attributed to the heat generated by asteroid impacts. Their size means that they can be distributed well beyond the impact site.

Terminator line: The line or boundary between night and day that moves across a planetary body. For example, the delineation of light and dark of a crescent moon. The narrow band along the terminator is useful for viewing relief and shadows of morphological features such as craters and mountains.

Tethys: The 5^th largest moon of Saturn with radius 533 km, a distance of 294,660 km, and orbital period 1.89 days. Its density is 0.97 g/cc indicating a composition almost entirely of water ice. Surface temperatures average -187^oC.

Tharsis Montes: The Tharsis region of Mars that contains twelve shield volcanoes, the three largest, from north are Ascraeus Mons, Pavonis Mons and Arsia Mons. They are smaller edifices than Olympus Mons, located about 1200 km to the north, but still large by terrestrial standards at 375 km to 475 km in diameter, and up to 15 km from bast to summit. All are located within the Tharsis bulge and are the youngest volcanoes on Mars.

Thermal equator: The latitude where Earth’s surface temperatures are a maximum. It is currently about 10^o north of the geographic equator, because the northern hemisphere averages 1.2^o to 1.5^o higher than the southern hemisphere. There is some seasonal variation in its location. There have been major latitudinal shifts in its position over the past 100,000 years, that have led to significant, frequently abrupt climate change.

Tidal forces: Gravitational forces acting among planetary bodies and between planets and their parent star. These forces distort the planetary spheroids that are expected to form under conditions of pure hydrostatic equilibrium. Thus, planets and moons commonly assume oblate spheroidal shapes. Earth’s oceanic tides are a product of tidal forces in our Sun-Earth-Moon system. Earth’s equatorial bulge is also a product of the Moon’s gravity. Tidal forces can also generate internal heat – it is hypothesized that Europe’s subcutaneous oceans maintain a liquid state because of gravitational friction.

Tidal friction: Competing gravitational forces between two planetary bodies can produce elastic body distortion and frictional heat. Maintenance of liquid water in   subcutaneous oceans or inner layers of moons like Europa and Enceladus requires a heat source and frictional heating is currently the preferred mechanism.

Tidal locking: For some planet-moon systems, the rotation of a moon about its axis takes the same time as one orbital rotation about its parent planet. This means the moon always shows the same side when viewed from the planet. This is the case for the Earth-Moon system. All the large moons of Saturn and Jupiter are tidally locked. Tidal locking is a consequence of the gravitational interaction between the two bodies. It also applies to some stars and their planets, and to some binary stars. Have a look at NASA’s animation.

Time zones: There are 24 time zones on Earth, defined by longitudinal meridians each 15^o, or one hour apart. The Primary Meridian passes through Greenwich, England. The concept for universal time zones was developed by a Canadian railway engineer, and in 1884 his ideas were adopted at a conference in Washington DC. However, it is up to country jurisdictions to decide how the time zones are used. For example, Russia has 11 time zones, contiguous USA has 5, and China has 5 zones but uses only one. Likewise, the International Date Line theoretically follows 180^o longitude, but has been drawn with several twists and turns to account for geography and National boundaries.

Titan: Titan is the largest of Saturn’s moons with a radius of 2575 km, about 1.5 times that of our Moon. Its orbit is tidally locked to Saturn. It is the only moon in the Solar System to have a thick atmosphere: 95% nitrogen, some methane and a whiff of organic compounds and CO₂. It has a rocky core and inner shells of water-ice and brine. The outer crust is icy with solid and liquid hydrocarbons. Titan’s seas and lakes consist of liquid methane (CH4) and probably ethane (C2H6). Cassini’s radar images reveal intricate coastlines and embayments, islands, headlands, and drainage systems.

Titius-Bode Law: This is an empirical rule, rather than a Law. The rule approximates, in geometric progression, the orbit radius of planets and is written as D=0.3(2)⁽ⁿ⁻¹⁾ + 0.4 where D is the distance in astronomical units and n is the number of the planet in order outward from the sun, beginning with Venus (= 1) – the rule does not work for Mercury and Neptune. It also predicts a planetary body with radius between Mars and Jupiter, corresponding to the asteroid belt. The rule was initially stated as a footnote by Johann Titius in 1766, and subsequently published in 1768 by Johann Bode.

Torino scale: Adopted by the IAU in 1999, the scale is used to express the likelihood of an asteroid-comet collision with Earth, and the scale of destruction that might ensue. Zero on the scale is an improbable risk, 10 is a certainty of impact with potentially catastrophic consequences.

Total solar eclipse: When the moon’s umbral shadow is located between Earth and sun (umbral region completely blocks sunlight). Cf. penumbral shadow.

Trailing hemisphere: For a tidally locked satellite, it is the face that is backward, facing away from the direction of orbital motion.

Transit method: The method commonly used to detect the existence of planets (exo-planets) in orbit around distant stars. When viewed from Earth (or space telescope), the light of a star will be dimmed when one of its planets passes in front of it. If the transit is repeated and the time taken between successive dimming events is the same, then the presence of a planetary body is regarded as highly probably. The size of the exo-planet is also related to the amount of dimming.

Trans-neptunian objects: (TNOs) Objects that orbit the Sun beyond Neptune’s orbit. They are also called Kuiper Belt Objects (KBOs).

Trojan asteroids: Asteroids that occupy the same orbit around the Sun as Jupiter. They occupy Jupiter’s Lagrangian points that effectively keeps them in a stable orbit. NASA’s Lucy Spacecraft is en route to visit eight of the Trojans, arriving April 2025.

Tsuchinshan-ATLAS: Comet Tsuchinshan-ATLAS (Comet C/2023 A3) was independently discovered by the Chinese Purple Mountain Observatory in January 2023, and the Asteroid Terrestrial-impact Last Alert System (ATLAS) on February 22, 2023 – hence the double-barrel name. The comet has a retrograde orbit (i.e. clockwise relative to the North Star), and a period possibly as long as 80,000 years. Its perihelion was 0.39 AU; its aphelion is unknown (The Sky Live).

Tunguska: The largest impact in recent history occurred over Tunguska, Siberia on June 30, 1908.   The meteorite was probably 50-100m in diameter and exploded in the air (hence no crater), the shock wave devastating 100s of square kilometres of forest. There has been considerable debate over the origin of the Tunguska event, including gas explosions.

Tycho Brahe: Tycho Brahe’s legacy to astronomy and science is centred on his development and calibration of instruments, and his methodological approach to observation and interpretation. He was a 16^th century astronomer whose observations of the moon, stars, and comets helped to dismantle the theory that planets resided on spheres, are centred on Earth, and that the “heavens” were immutable. By observing planetary orbits in their entirety, he was able to demonstrate perturbations in their journeys across the sky and that these orbits deviated from perfect circles. He did, however, disagree with the Copernican theory (1543) of a Sun-centred solar system.

Umbra: The cone of deep shadow on the side of Earth not facing the Sun. The distance to the cone apex will vary because of Earth’s elliptical orbit. A total lunar eclipse occurs when the Moon is positioned within the umbra. Cf. Penumbra.

USGS Mars geology maps: The USGS Astrogeology Science Center in Flagstaff, Arizona has released a series of contour geology maps at 1:200,000 scale (1 cm equivalent to 2 km). Such maps provide detail sufficient for comparison with terrestrial analogues of geological and geomorphic features such as volcanic edifices and lava flows, deltas, and river channels.

UTC: Coordinated Universal Time; The successor to GMT, it is based on a 24 hour clock, although the prime meridian remains at Greenwich. The high degree of precision results from use of atomic clocks that are occasionally adjusted to solar time by a leap second. It has been in use since 1972. IT is coordinated by the Bureau International des Poids et Mesures (BIPM)

Vacuum: A region devoid of matter. Space is close to a perfect vacuum, but even in interstellar space there are probably a few atoms in every cubic metre. Degrees of vacuum are usually stated as low (about 10^-2 pascal), and high, and ultra-high vacuum < 10^-7 pascal.

Valhalla impact crater: The largest multi-ring crater in the Solar System occurs on Jupiter’s moon Callisto. It is 4000 km diameter, compared with Callisto’s 4800 km diameter. The rings have been imaged as troughs, or large cracks in the moon’s crust. A bright, high albedo central region about 600 km in diameter may have been filled with excavated water ice.

Valles Marineris: Nick-named the Grand Canyon of Mars, it extends for 4000 km around about 1/5 of Mars equator, and is up to 7 km deep (the Grand Canyon in USA is about a quarter as long and deep). It is generally thought to have formed during cooling of Mars’ crust. However, there are many tributary-like valleys that may indicate modification by surface water erosion. some images also show the possible effects of slope failure and landslide run-out.

Van Allen radiation belts: Two concentric belts composed of charged particles that surround Earth and are located within the magnetosphere. Named after their discoverer James Van Allen, their extent was first observed by NASA’s Explorer 1 in 1958. The outer belt consists primarily of protons (hydrogen), and the inner belt of particles formed by interaction of cosmic rays with Earth’s atmosphere. They extend to 58,000 km from Earth. Most of the particles are derived from the solar wind, some from beyond the solar system. Knowing the extent and radiation strength of the belts is important for orbiting satellites and probes because instrumentation can be damaged.

Vernal equinox: The astronomical definition for the beginning of spring, where the Sun lies exactly above the equator such that day and night are of equal duration. In the northern hemisphere this falls on March 20 or 21, and in the southern hemisphere September 22 or 23. C.f. summer solstice.  

Vesta: One of the largest asteroids in the Asteroid Belt, is an oblate spheroid with a diameter of 530 km. Vesta’s surface is pockmarked by craters caused by smaller asteroid impacts.

Viking 1: The NASA Mars spacecraft that entered Mars orbit June 19, 1976. Viking 1 lander alighted Mars surface June 20, 1976 – the first successful landing where data and images were transmitted until the end of the mission November 11, 1982. Instruments recorded atmospheric conditions, soil sampling and chromatograph analysis, and a seismometer that failed to operate.

Viking 2: The sister Mars orbiter to Viking 1, that entered orbit August 7, 1976, with its lander arriving Sept. 3, 1976. Lander instruments included an imaging System, gas Chromatograph Mass Spectrometer for soil samples, a seismometer, X-ray fluorescence spectrometer, a weather station, and a robotic sampling arm. The lander operated until April 1980.

Voyager 1 spacecraft: One of the early but hugely successful probes, launched September 5, 1977 (about 2 weeks after Voyager 2), its mission was to image Jupiter, then slingshot to Saturn to look at the rings and the moon Titan. Its trajectory around Saturn eventually took it on a course 35^o above the ecliptic plane, a course it maintained (except for minor adjustments) to eventually leave the heliosphere in 2012 and begin its journey through interstellar space. Voyager 1 has been active for 45 years. It has traveled about 160 AU from Earth.

Voyager 2 spacecraft: Launched August 20, 1977, this probe, like its sister, was tasked to orbit Jupiter and Saturn, but its journey evolved quite differently with successive gravity assists to change its trajectory for trips to Uranus, Neptune, and finally interstellar space. Voyager 2 discovered new moons around Jupiter, Uranus, and Neptune, volcanism on Jupiter’s moon Io, measured wind speeds and magnetic fields on Uranus and Neptune. Its trajectory eventually took it beyond Neptune at an angle of 45^o below the ecliptic (opposite that of Voyager 1), entering interstellar space beyond the Solar System in 2018.

Vredeforte impact structure: Vredeforte Dome in South Africa contains remnants of the oldest recognisable impact structure on Earth. It is 2023 million years old. It’s present circular structure and indicate a crater 190 km in diameter, but local geology suggests it could have been 250-280 km. Successive periods of erosion provide us with a view deep into its structure. Characteristic shock metamorphic structures include shatter cones, shocked quartz and zircon, pseudotachylite breccia, and melt rock. Recent modelling indicates a couple of impact scenarios: the bolide was 25 km in diameter and impacted at 15 km/second, or was 20 km diameter with a velocity of 25 km/s.

Water-ice polymorphs: Water-ice has 19 crystalline types and three amorphous phases, or polymorphs, that form under different conditions of temperature and pressure. All the crystalline phases of ice involve hydrogen-bonding of water molecules with four neighbouring water molecules. Type Ih ice is the kind we see at Earth’s surface in glaciers, icebergs, hail, and snow. It has a hexagonal crystal structure. Type Ih is stable from 0^o to -200^oC and pressures to about 100 MPa. Several other types of ice form at greater pressures, crystallizing in cubic, tetragonal, orthorhombic, and monoclinic forms. For example, the ice on the inner layers of moons like Titan and Enceladus, probably have tetragonal Type VI ice that forms at pressures 10,000 times those normally seen on Earth.

Weight: Weight is the force of gravity acting on an object; it has the units of force (SI unit is the Newton), and the same dimensions – MLT^-2. Weight is calculated from Newton’s Second Law that is expressed as force F = Mass x Acceleration – in this case acceleration due to gravity. Weight ≠ Mass. For example, a 1 Kg mass on Earth weighs 9.8 N; on the Moon the same mass weighs 1.62 N (Lunar value of g is 1.62 m.s^-2, and on Jupiter it weighs 24.79 N where g = 24.79 m.s^-2.

Widmanstätten bands: A crystallization structure characteristic of, and found only in iron meteorites, where the FeNi minerals Kamacite and Taenite form an interleaved pattern. The crystallization texture probably formed under conditions of slow cooling in the FeNi cores of planets.

X-rays: Part of the natural electromagnetic spectrum having wavelengths between about 0.1 Å and 100 Å, of greater energy than UV light.

Zenith: The highest point on the celestial sphere that the sun, Moon, planets or stars reach. The Sun reaches its zenith at midday – this does not mean it is directly overhead, that only happens between the tropics of Cancer and Capricorn.

Zodiacal light: A diffuse, cone- or pyramid-shaped light that appears just before dawn sunrise or just after sunset and in the same part of the sky as the setting or rising sun. It is caused by reflection from interplanetary dust. It is also called the false dawn and false sunset, best seen at either equinox.

Zone of Avoidance: A region in space lying in the plane of the Milky Way in which more distant galaxies are obscured by interstellar dust. It was originally named the Zone of Few Nebulae in 1878 by Richard Proctor, and later renamed by Edwin Hubble. However, galaxies have been identified using infrared and radio light frequencies.

Links to some excellent websites

EarthSky – Updates on your cosmos and world.

The Planetary Society

Astronomy.com

NASA

ESA

International Astronomical Union, IAU

SpaceWeatherLive.com

Astronomy education and outreach (Univ California San Diego)

Encyclopedic Atlas of Terrestrial Impact Craters Springer, 2019.

Planetary and Space Science (Journal)