compiled by Brad Jahn

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Accretion disk A rotating disk of gas orbiting a star, formed by material falling toward the star (Kuhn, 1998).

Accretionary heating Thermal energy resulting from bolide impacts upon the surface of an object (Planetary Geology for Teachers, 1999).

Achondrites A rare stony meteorite without spherical granules (chondrites). Achondrites represent meteorites that are most like terrestrial rocks (Bates and Jackson, 1984).

Albedo The ratio of the amount of solar radiation reflected from an object to the total amount incident upon it (Welcome to the Planets, 1995).

Albite A white or colorless, lower relative temperature, triclinic mineral of the feldspar group: (NaAlSi₃O₈). It is a variety of plagioclase that commonly occurs in igneous and metamorphic rocks (Bates and Jackson, 1984).

Alluvial fan An outspread, gently sloping mass of alluvium deposited by a stream, esp. in an arid or semiarid region where a stream issues from a narrow canyon onto a plain or valley floor (Bates and Jackson, 1984).

Alpine glaciers A glacier or body of compacted snow and ice, in mountainous terrain. It generally originates in a cirque and may flow down a valley previously made by a stream (Bates and Jackson, 1984).

Amphibole A mineral group with the general formula A₂B₂(Si,Al)₈O₂₂(OH)₂ where A is mainly Mg,Fe,Ca, or Na, and B is mainly Mg, Fe⁺², Al, or Fe⁺³. The most common amphibole minerals are hornblende, tremolite-actinolite and cummingtonite-grunerite (Bates and Jackson, 1984).

Anion A negatively charged atom or molecule resulting from the atom’s gain of an electron (Beatty and Chaikin, 1990).

Anorthite A white or gray, high temperature, triclinic mineral of the feldspar group: (CaAl₂Si₂O₈). It is the most calcic member of the plagioclase series, and occurs esp. in basic and ultrabasic igneous rocks (Bates and Jackson, 1984).

Anorthosite A plutonic rock composed almost wholly of plagioclase (Bates and Jackson, 1984).

Antipodes Two points on the surface of a celestial body that are diametrically opposed to each other. The term may be extended to include a whole region at the opposite end of a body’s diameter, as on Earth, Australia and New Zealand are roughly opposite the British Isles (Bates and Jackson, 1984).

Antipodes/ hills of Mercury "Weird terrain" best describes this hilly, lineated region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers (62 miles) on a side (Planetary Geology for Teachers, 1999).

Aphelion The point in the orbit of a planet, asteroid, comet, or other body that is farthest from the sun (Kuhn, 1998).

Aphrodite Terra (Venus) Approximately the size of Africa, Aphrodite terra straddles the equator, is over 10,000 kilometers (6,214 miles) long, and is made up of four smaller highlands: Ovda, Thetis, Atla, and Ulfrun Regiones. Western Aphrodite Terra is believed to be the site of crustal spreading on Venus, with an estimated separation rate of several centimeters per year (Beatty and Chaikin, 1990).

Apogee The point in the orbit of an Earth satellite where it is farthest from the Earth (Kuhn, 1998).

Apollo asteroids Asteroids that cross the Earth’s orbit (Beatty and Chaikin, 1990).

Arachnoids A coronae with less than a 170 kilometer (106-mile) diameter. It is a circular-to-elongate Venusian feature surrounded by multiple concentric ridges and complex lineations, thought to be formed by hot spots (Planetary Geology for Teachers, 1999).

Archean The earlier part of Precambrian time form about 2500mya to 570 mya (Bates and Jackson, 1984).

Arete A rocky, sharp-edged ridge or spur, commonly present above the snowline in rugged mountains sculptured by glaciers, and resulting from the continued backward growth of the walls of adjoining cirques (Bates and Jackson, 1984).

Asteroid belt Asteroids that orbit the Sun at distances from 2.2 AU to 3.6 AU. This orbital zone between Mars and Jupiter is the location of most of the known asteroids (Kuhn, 1998).

Asteroids Any of the 1000’s of small rocky/metallic bodies that orbit the sun mostly between Mars and Jupiter; also referred to as minor planets (Welcome to the Planets, 1995).

Asthenophere The layer or shell of the Earth below the lithosphere, which is weak and in which isostatic adjustments take place, magmas may be generated, and seismic waves are strongly attenuated. The asthenosphere begins about 100 km below the surface and extends to a depth of about 350 km (Bates and Jackson, 1984).

Atmosphere (Earth) The gaseous envelope surrounding the Earth. 78% nitrogen, 21% oxygen, 0.9% argon, 0.3% carbon dioxide and other trace elements (Bates and Jackson, 1984).

Atom The smallest unit of material which retains its chemical identity (Beatty and Chaikin, 1990).

Atomic mass The total number of protons and neutrons in the nucleus of an atom (Beatty and Chaikin, 1990).

Atomic number The total number of protons in the nucleus of an atom (Beatty and Chaikin, 1990).

Augite A dark mineral of the pyroxene group, (Ca,Na)( Mg, Fe⁺², Al)(Si,Al) ₂O₆. It is an essential constituent of many basic igneous rocks (Bates and Jackson, 1984).

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Baryons A fundamental atomic particle. The known baryons include protons, neutrons and hyperons which are said to have a baryon number of 1 (Beatty and Chaikin, 1990).

Basalt A fine-grained, dark rock made of plagioclase and pyroxene (often with olivine and Fe-oxides), it usually occurs as lava flows or tephra (fragmented lava that makes up cinder cones, spatter cones, etc). Basaltic magma is generated by partial melting of mantle rock, and rises through the crust to the surface (Planetary Geology for Teachers, 1999).

Basin A depressed area with no surface outlet (Welcome to the Planets, 1995).

Batholith A large, generally discordant plutonic mass that has > 40 square miles (100 km²) of surface area and no known floor (Bates and Jackson, 1984).

Belts Thick, rotating clouds that swirl around the planet, blowing in opposite directions in adjacent bands. The darker colored bands are called belts (Beatty and Chaikin, 1990).

Beta Regio (Venus) Best example of an upland dome is Beta Regio located around 280° E. longitude and 30° N. Latitude (Planetary Geology for Teachers, 1999).

Big Bang One theory of the origin of the universe that proposes a tremendous release of energy (i.e.an explosion) called the "big bang" which initiated the formation of matter approximately 15 billion years ago. This is partly suggested by the astronomical observation that the universe is expanding; the expansion is projected backwards in time to some initial state (singularity) (Planetary Geology for Teachers, 1999).

Biosphere All the areas of the Earth occupied by living organisms, including the lithosphere, hydrosphere and atmosphere (Bates and Jackson, 1984).

Biotite A common rock forming mineral of the mica group, K(Mg,Fe⁺²)₃(Al,Fe⁺³) Si₃O₁₀(OH)₂. It is black in hand specimen, brown or green thin section, and has perfect basal (001) cleavage (Bates and Jackson, 1984).

Black hole A celestial object of infinite density whose escape velocity exceeds the speed of light (Kuhn, 1998).

Blowouts A general term for various saucer- or through-shaped hollows formed by wind erosion on a dune or other sand deposit: the adjoining accumulation of sediment derived from the depression where readily recognizable, is commonly included (Bates and Jackson, 1984).

Breccia A coarse-grained, clastic rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix (Bates and Jackson, 1984).

Bright highlands (Venus) Magellan images of highland regions above 2.5 kilometers (1.5 miles) are unusually bright, characteristic of moist soil. However, liquid water does not exist on the surface and cannot account for the bright highlands. One theory suggests that the bright material might be composed of metallic compounds (Planetary Geology for Teachers, 1999).

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Canali (Venus, Mars) Long sinuous forms, Canali are best preserved in regions of subdued relief. They have a high width-to-depth ratio and maintain a remarkably constant width over very long distances. Images reveal the presence of meanders, point bars, cut banks, and abandoned channel segments (Planetary Geology for Teachers, 1999).

Caldera A large, basin-shaped volcanic depression that is more or less circular in form. Most calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano (Bates and Jackson, 1984).

Caloris Basin (Mercury) Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris basin is 1,300 kilometers (800 miles) in diameter and is the largest known structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. The large impactor is believed to be buried below the floor of the crater and contributes to the uneven mass of the planet (Planetary Geology for Teachers, 1999).

Carbonaceous chondrites Asteroids made of undifferentiated silicates with carbon compounds and water (densities ~2.3gm/cc). They represent the primordial material of the Solar System from which planets accreted. Besides an aggregate of carbonaceous dust and hydrated silicates, they also contain chondrules, spherical blobs ranging in size from less than 1 mm to nearly 2 cm that have higher temperatures of melting than the surrounding material. Chemical analyses indicate that, except for volatile elements (H, He, Ne, Ar, N, etc.), the relative chemical proportions of rock-forming elements (e.g. Si, O, Mg, Fe, Ca, Al, etc.) in carbonaceous chondrites closely correspond to the relative proportions in the Sun (Planetary Geology for Teachers, 1999).

Carbonates A mineral compound characterized by a fundamental anion structure of CO₃^-2. Calcite and aragonite, CaCO₃ are examples (Bates and Jackson, 1984).

Cassini Gian Domenico Cassini, (1625-1712). An astronomer born in Italy, Cassini later became a naturalized French citizen. He discovered four of Saturn's satellites and observed a dark division in Saturn's ring (the Cassini Division) (Welcome to the Planets, 1995).

Cassini Division A space observed between the A and B segments of Saturn’s rings. The gap is believed to have been created by the gravitational pull from Mimas, one of Saturn’s moons (Planetary Geology for Teachers, 1999).

Cataclasis A special type of metamorphism in which "weathering" involving mechanical processes along fault zones pulverizes mineral grains into smaller grains and dust. This type of metamorphism depends on forces related to tectonics (faulting). Another form of cataclasis is impacting, which involves planetesimal bodies such as asteroids striking the planet with enough energy to pulverize, or even melt, rock (Planetary Geology for Teachers, 1999).

Cation A positively charged atom or molecule resulting from the atom’s loss of an electron (Beatty and Chaikin, 1990).

Cenozoic The latest of the four eras into which geologic time is divided; it extends from the close of the Mesozoic Era, about 65 mya, to the present. The Cenozoic Era is subdivided into the Tertiary and the Quaternary Periods (Bates and Jackson, 1984).

Channels (Venus) Lava channels extending from hundreds to thousands of kilometers in length are conspicuous on the Venusian plains. Simple channels typically show little or no branching. They include long sinuous forms, termed "canali", and sinuous rilles. Canali are best preserved in regions of subdued relief. They have a high width-to-depth ratio and maintain a remarkably constant width over very long distances. Images reveal the presence of meanders, point bars, cut banks, and abandoned channel segments (Planetary Geology for Teachers, 1999).

Chaotic terrain (Mars) Regions of fractured, jumbled rocks that apparently collapsed when groundwater suddenly surged outward (Planetary Geology for Teachers, 1999).

Chemical potential energy Stored energy contained in matter that can be released by chemical reactions (Beatty and Chaikin, 1990).

Chemical weathering The process whereby minerals in rocks are dissolved or changed to other minerals, such as feldspar weathers to a clay mineral (both are alumino-silicates). This type of weathering is enabled by processes such as hydration, oxidation, and ion exchange (Planetary Geology for Teachers, 1999).

Chlorite A group of platy, monoclinic, usually greenish minerals of the general formula: (Mg, Fe⁺², Fe⁺³)₆AlSi₃O₁₀(OH)₈. Chlorites are associated with and resemble micas, they may also be considered as clay minerals. They are widely distributed, esp. in low-grade metamorphic rocks, or as alteration products in ferromagnesian minerals (Bates and Jackson, 1984)..

Chondrites A stony meteorite containing spherical granules, usually about one mm in diameter, consisting chiefly of olivine and/or enstatite or bronzite embedded in a fine matrix of pyroxene, olivine, and nickel-iron with or without glass. They constitute more than 80% of meteorite falls (Bates and Jackson, 1984).

Chromosphere The region of the solar atmosphere between the photosphere and the corona notable for its ruby red light. Temperatures of the chromosphere vary from 4500 K at its base to 8500 K at its top (Kuhn, 1998).

Cirque A deep steep-walled recess or hollow, horseshoe-shaped or semicircular in plan view, situated high on the side of a mountain and produced by the erosive activity of a mountain glacier. It often contains a small round lake (Bates and Jackson, 1984).

Coma A roughly spherical region of diffuse gas which surrounds the nucleus of a comet. Together, the coma and the nucleus form the comet's head (Welcome to the Planets, 1995).

Comet Shoemaker-Levy 9 On July 7, 1992, Comet Shoemaker-Levy 9 made a close approach to Jupiter and broke apart into 21 pieces due to tidal forces inside Jupiter’s Roche limit. On the subsequent pass, each of the pieces of the comet impacted Jupiter (Planetary Geology for Teachers, 1999).

Comets Small, icy bodies that revolve around the sun, usually with long periods, that are made up of a nucleus, coma and two tails- one composed of ions and the other composed of dust. Also, a small celestial body composed at least partially of ices. Comets either orbit the Sun or pass through the Solar System on hyperbolic orbital paths (Welcome to the Planets, 1995).

Composite volcano A volcano that is constructed of alternating layers of lava and pyroclastic deposits, along with abundant dikes and sills. Also, a composite cone (Bates and Jackson, 1984).

Compound A distinct substance formed by the union of two or more ingredients in definite proportions by weight; for example, H₂O (Beatty and Chaikin, 1990).

Compression wave Kinetic energy is transferred by a shock wave that spreads outward from the point of impact (Planetary Geology for Teachers, 1999).

Conduction The transfer of heat in a solid by collisions between atoms and/or molecules (Kuhn, 1998).

Conglomerate A cemented mixture of rounded pebbles transported from various places. Rounding is evidence of stream transport that causes the sharp edges and corners to become worn down as the particle is bounced along with other particles in a stream (Planetary Geology for Teachers, 1999).

Continental crust The rocks of the continental crust underlie the continents; they are equivalent to sial, and range in thickness from about 35 km to as much as 60 km under mountain ranges. The density of the upper layer of the continental crust is ~ 2.7 g/cm³, and the velocity of compressional seismic waves through it are less than ~ 7.0 km/sec (Bates and Jackson, 1984).

Continental glaciers A glacier or body of compacted snow and ice, of considerable thickness and more than 50,000 square kilometers in extent, forming a continuous cover of ice and snow, spreading outward in all directions, and not confined to the underlying topography (Bates and Jackson, 1984).

Continental rift A long, narrow continental through bounded by normal faults, a graben of regional extent, often associated with volcanism (Bates and Jackson, 1984).

Convection Fluid circulation driven by temperature gradients; the transfer of heat by this automatic circulation within gaseous or liquid materials (Kuhn, 1998).

Convection Zone (Sun) The outer 30% of the Sun’s radius where heat is transferred via convection currents. Temperatures continue to drop in this zone from 1.5 x 10⁶ K at the base to 5800 K at the surface or photosphere (Kuhn, 1998).

Convergent Boundary A boundary between two crustal plates that are moving together, with the more dense plate descending beneath the less dense, usually continental, overriding plate (Bates and Jackson, 1984).

Copernicus Polish astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe (geocentric theory), which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It should be noted, however, that the heliocentic idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long ignored.) (Kuhn, 1998).

Core (Earth) The central part of the Earth, beginning at a depth of about 2900 km, probably consisting of iron-nickel alloy; it is divisible into a solid inner and liquid outer core (Bates and Jackson, 1984).

Core (of the Sun) Site of nuclear fusion where hydrogen is fused into helium and thermonuclear energy is released. The core temperature of the Sun is approximately 1.5 x 10⁷ K (Kuhn, 1998).

Core formation The heat driven process whereby materials of higher density sink through less dense material and form the center of a planet. Also, planetary differentiation (Planetary Geology for Teachers, 1999).

Corona (Geology) A circular to elongate feature which is surrounded by multiple concentric ridges. Coronae are thought to be formed by hot spots (Welcome to the Planets, 1995).

Corona (Venus) A 170- to 1,000-kilometer- (106- to 621-mile-) diameter circular-to-elongate Venusian feature surrounded by multiple concentric ridges, thought to be formed by hot spots (Planetary Geology for Teachers, 1999).

Corona: (Sun) The outermost portion of the Sun’s atmosphere. During eclipse it is observed as a white, glowing light extending far into interplanetary space. Corona temperatures are about 1.0 x 10⁶ K near the transition zone where the chromosphere gives way to the corona (Kuhn, 1998).

Covalent bond A chemical bond formed by the sharing of a pair of electrons between atoms (Beatty and Chaikin, 1990).

Crater An approximately circular depression, sometimes surrounded by a raised rim. Craters are typically formed by explosion during meteorite impact (Welcome to the Planets, 1995).

Crater degradation Once a crater has been formed, it will degrade over time as gravity and isostatic rebound modify the crater walls and floor. The central portion of a large crater will rise by isostatic uplift that results from removal of overburden. Walls may collapse by slumping to produce terraced crater walls that form the rings in multi-ring basins. Continued bombardment causes the crater to become less and less distinct from its surroundings. Degradation also can be attributed to lava flows, atmospheric processes such as weathering and erosion, groundwater processes (sapping), and tectonism. Thus old craters may exhibit only remnants of the original crater walls (Planetary Geology for Teachers, 1999).

Crater Mead (Venus) Mead crater, with a diameter of 280 kilometers (174 miles), is the largest impact crater on Venus. The inner ring is thought to represent the original rim of the crater cavity, while the outer scarp is thought to be the expression of a ring fault that has down-dropped the flank terrace (Planetary Geology for Teachers, 1999).

Cratered southern highlands (Mars) The often densely cratered southern hemisphere of Mars which stands 1 to 3 km above topographic sea level, and is composed of a very ancient crust which is nearly saturated with craters, and younger intercrater plains which appear ancient but have been modified less (Planetary Geology for Teachers, 1999).

Cratered terrain (Mercury) Mercury's major landforms include impact craters which dominate the landscape, cratered terranes, widespread intercrater plains, multiring basins, and some smooth plains that may have been formed by flood basalts. Though an absolute age is not available now, scientists assume Mercury's heavily cratered regions probably formed during the same period of intense bombardment that formed the lunar highlands (Planetary Geology for Teachers, 1999).

Craters with streaks Ray craters with a distinctive radial pattern of fresh ejecta (Planetary Geology for Teachers, 1999).

Crust The outermost layer of the Earth; that part of the Earth above the Moho. It represents less than 0.1% of the Earth’s total volume (Bates and Jackson, 1984).

Crustal upwarps (Mars) Large areas of the Martian surface which stand high in topographic relief due to the expansion of the crust resulting from the presence of an underlying mantle plume or hot spot (Planetary Geology for Teachers, 1999).

Crystal A homogeneous, solid body of a chemical element, compound or isomorphous mixture, having a regularly repeating atomic arrangement that may by outwardly expressed by plane faces (Bates and Jackson, 1984).

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Danu Mountains (Venus) A mountain range situated south of Lakshmi Planum, which shows intense deformation (faulting and folding) of the planet's crust, similar to deformed rocks seen in mountain belts on Earth (Planetary Geology for Teachers, 1999).

Dactyl A small asteroid which measures 1.6 x 1.4 x 1.2 km and orbits a larger asteroid, Ida. Dactyl is the first evidence that asteroids may have tiny moons (Beatty and Chaikin, 1990).

Deflation basins A topographic basin resulting from the removal of material from a beach, desert or other land surface by wind action (Bates and Jackson, 1984).

Deimos The irregularly shaped, dark colored moon of Mars. Deimos is smaller than its companion moon, Phobos. Both orbit very close to Mars, have short periods of revolution (1.3 days for Deimos) and are believed to be captured asteroids (Kuhn, 1998).

Dendritic river systems Networks of interconnecting dry river beds that exhibited dendritic (from the Greek, meaning "treelike") drainage patterns characteristic of water-carved channels (Bates and Jackson, 1984).

Deposit Material of any kind that has accumulated through the activities of water, wind ice or other agents (Bates and Jackson, 1984).

Desert pavement A residual accumulation of coarse rock fragments on a surface after the finer material has been blown away by winds. Also, lag deposits (Bates and Jackson, 1984).

Devana Chasma (Venus) A north-south trough located in the Beta Regiones highland regions in the equatorial zone (Planetary Geology for Teachers, 1999).

Differentiated Refers to meteorites that are differentiated, i.e., they have igneous textures because they were once molten. All other meteorite types (achondrites, stony-irons and irons) are undifferentiated (Planetary Geology for Teachers, 1999).

Divergent Boundry A boundary between two crustal plates that are moving apart, with new oceanic type lithosphere being created in the seam. Also, accreting plate boundary (Bates and Jackson, 1984).

Domes An uplift or anticlinal structure, circular or elliptical in outline, in which rocks dip gently away in all directions. Doming results in extension of the crust above, and leads to predominantly extensional tectonic features, such as fracture belts, troughs, grabens, and rifts (an area of extension) (Bates and Jackson, 1984).

Dominant ices (Solar System) Water (H₂O) ice, CO₂ ice and NH₄ ice (Planetary Geology for Teachers, 1999).

Double chain A class or structural type of silicate characterized by the linkage of the SiO₄ tetrahedra into linear chains by the sharing of oxygens. In a double chain or band, e.g. amphiboles, half the SiO₄ tetrahedra share three oxygens and the other half share two (Bates and Jackson, 1984).

Drumlins A low, smoothly rounded, elongate hill of compact glacial till, or rarely other kinds of drift, built under the margin of the ice and shaped by its flow or carved out of an older moraine by re-advancing ice; its longer axis is parallel to the direction of movement of the ice (Bates and Jackson, 1984).

Dunes (longitudinal, star, transverse, and barchan) A mound, ridge, or hill of wind-blown sand, either bare or covered with vegetation (Bates and Jackson, 1984).

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Eistla Regio (Venus) A region centered at 12.3 degrees north latitude, 8.3 degrees east longitude, with an area 160 kilometers (96 miles) by 250 kilometers (150 miles). Its prominent circular features are volcanic domes, 65 kilometers (39 miles) in diameter with broad, flat tops less than one kilometer (0.6 mile) in height. Sometimes referred to as "pancake" domes, they represent a unique category of volcanic extrusions on Venus formed from viscous (sticky) lava. The cracks and pits commonly found in these features result from cooling and the withdrawal of lava (Planetary Geology for Teachers, 1999).

Ejecta Material thrown out from and deposited around an impact crater (Welcome to the Planets, 1995).

Electron The negatively charged, almost mass-less particle that orbits the nucleus of an atom (Kuhn, 1998).

Element A substance that consists of atoms, each with the same atomic number (Bates and Jackson, 1984).

Elysium uplift (Mars) A region at the center of a huge bulge in the Martian surface formed by a series of fluid lava flow eruptions that carpeted its flanks and slowly built up the high features we see today, and is evidence for tectonism on Mars. The Elysium uplift was likely caused by upwelling mantle material (Planetary Geology for Teachers, 1999).

Enstatite A common rock forming mineral of the orthhopyroxene group, (Mg,Fe)SiO₃. It is isomorphous with hypersthene. It is an essential constituent of many igneous rocks (Bates and Jackson, 1984).

Eolian features Related to wind deposits and associated effects. Including wind streaks, sand dunes, ripples, deposits of loess, and yardangs (Bates and Jackson, 1984).

Eolian systems Pertaining to the wind, esp. said of such deposits as loess and dune sand, of sedimentary structures such as wind formed ripple marks, or erosion and deposition accomplished by the wind (Bates and Jackson, 1984).

Erratic A rock fragment carried by glacial ice, deposited at some distance from the outcrop from which it was derived, and generally resting on bedrock of different lithology. Size ranges from pebbles to house-sized blocks. Also, glacial erratic (Bates and Jackson, 1984).

Escarpment A long, more or less continuous cliff or relatively steep slope facing in one general direction, produced by erosion or faulting (Welcome to the Planets, 1995).

Esker A serpentine ridge of roughly stratified gravel and sand that was deposited by a stream flowing in or beneath the ice of a stagnant or retreating glacier and was left behind when the ice melted (Bates and Jackson, 1984).

Extrusive rocks (volcanic) Magma that erupts onto the surface is called lava. It cools very quickly because the temperature at the surface is relatively low, and crystals will probably cease growth because the liquid is quenched. Volcanic glass is a type of quenched lava, and typical volcanic rocks are rhyolite, basalt, and andesite (Planetary Geology for Teachers, 1999).

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Fault-bounded troughs A long linear depression defined and bordered by a crack or break in the crust of a planet along which slippage or movement can take place (Welcome to the Planets, 1995).

Fault A crack or break in the crust of a planet along which slippage or movement can take place (Welcome to the Planets, 1995).

Feldspars A group of abundant rock forming minerals of the general formula: MAl(Al,Si)₃O₈, where M can be K,Na,Ca,Ba,Rb,Sr, or Fe. Feldspars are the most widespread of any mineral group and constitute 60% of the Earth’s crust, they occur in all types of rock (Bates and Jackson, 1984).

Ferromagnesian Containing iron and magnesium, applied to mafic minerals, esp. amphibole, pyroxene, biotite and olivine (Bates and Jackson, 1984).

Flood basalts A term applied to those basaltic lavas that occur as vast composite accumulations of horizontal or sub-horizontal flows, which, erupted in rapid succession over great areas, have at times flooded sectors of a body’s surface on a regional scale; and are generally thought to be the product of fissure eruptions (Bates and Jackson, 1984).

Fluvial channels Channels associated with rivers and streams that are produced by the action of flowing water, e.g. erosion (Bates and Jackson, 1984).

Fluvial processes Processes pertaining to rivers and streams that are produced by the action of flowing water, e.g. erosion (Bates and Jackson, 1984).

Foliation An important texture in metamorphic rocks, caused by alignment of platy minerals such as mica, or compositional layering. It is the manifestation of directed stresses that occur within the crust (Planetary Geology for Teachers, 1999).

Framework silicates A class or structural type of silicate characterized by the sharing of all four oxygens of the SiO₄ tetrahedra with neighboring tetrahedra, and with a Si:O ratio of 1:2. Quartz, SiO₂ is an example (Bates and Jackson, 1984).

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Galileo Galileo Galilei, (1564-1642). Italian mathematician, astronomer, and physicist. First astronomer to use a telescope to observe the skies and discoverer of the major (Galilean) moons of Jupiter, craters on the Moon, the phases of Venus and sunspots. (Kuhn, 1998) (Welcome to the Planets, 1995).

Galileo Spacecraft The Galileo mission consists of two spacecraft: an orbiter and an atmospheric probe. The probe descended through the cloud layers of Jupiter and relayed information about the pressures, chemical compositions, cloud densities, and other data before it was eventually destroyed by the high temperatures and pressures (Planetary Geology for Teachers, 1999).

Gaspra An "S-type", main belt asteroid of silicaceous chemical composition. 951 Gaspra, photographed by the Galileo spacecraft in 1991, is a generally gray, irregular object measuring 18 x 11 x 19 km. The surface is heavily cratered and fractured with a dusty regolith similar to Deimos. Sharp-edged craters and ridges appear bluish while low-lying areas are reddish (Beatty and Chaikin, 1990).

Geologic history of Mars The early geologic history of Mars is believed to be similar to that of Mercury and the Moon, however volcanism persisted well into the last half of solar system history and may continue today (Planetary Geology for Teachers, 1999).

Geologic history of Mercury (5 stage) 1. Initial planetary material coalesced form the solar nebula. 2. Heating from accretion of planetesimals and other space debris. 3. Planetary differentiation into layers of increasingly denser material from the surface to the core. 4. Catastrophic impact and stripping away of much of the surface and the upper part of the mantle. 5. Additional accretion and formation of a rigid crust through heat loss to surrounding space (Beatty and Chaikin, 1990).

Geomorphology The study of the external structure, form, and arrangement of rocks in relation to the development of landforms (Bates and Jackson, 1984).

Glacial systems The formation, movement and recession of glaciers or ice sheets (Bates and Jackson, 1984).

Glass beads Small, rounded pieces of silicate glass, thought to have become airborne during terrestrial impact events, also, tektites (Bates and Jackson, 1984).

Global escarpment A long, more or less continuous cliff or relatively steep slope of global proportions, facing in one general direction, separating two level or gently sloping surfaces, and produced by erosion and/or faulting (Beatty and Chaikin, 1990).

Globular cluster A spherical group of up to hundreds of thousands of stars, found in the halo at the margin of galaxies (Kuhn, 1998).

Gneiss A high-grade, metamorphic rock that has experienced intense recrystallization and segregation of compositional layers (Planetary Geology for Teachers, 1999).

Golubkina (Venus) A 30.1 kilometer (19 mile) diameter crater characterized by terraced inner walls and a central peak, typical of large impact craters on the Earth, the Moon and Mars. The terraced inner walls take shape late in the formation of an impact crater, due to the collapse of the initial cavity created by the meteorite impact. The central peak forms due to the rebound of the inner crater floor. This crater is named after the Russian sculptor Anna Golubkina (Planetary Geology for Teachers, 1999).

Graben An elongated, relatively depressed crustal unit or block that is bounded by faults on its sides (Welcome to the Planets, 1995).

Gravitational potential energy Stored energy in a system resulting from the force of gravity (Beatty and Chaikin, 1990).

Great Red Spot (Jupiter) A complex storm moving in a counter-clockwise direction. At the outer edge, material appears to rotate in four to six days; near the center, motions are small and nearly random in direction (Beatty and Chaikin, 1990).

Ground ice Wedge-shaped, foliated ice produced in permafrost, occurring as a vertical or inclined sheet, dike or vein tapering downward, and measuring form a few mm to as much as 6 meters wide and from 1 meter to > 30 meters high. It originates by the growth of hoar frost or by the freezing of water in a narrow crack or fissure produced by the thermal contraction of permafrost (Bates and Jackson, 1984).

Ground water That part of the subsurface water that is in the zone of saturation, including underground streams. Loosely, all subsurface water as distinct from surface water (Bates and Jackson, 1984).

Gula Mons (Venus) A 3 kilometer (1.86 mile) high volcano, located at approximately 22 degrees north latitude, 359 degrees east longitude (Planetary Geology for Teachers, 1999).

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Halides A mineral compound characterized by a halogen such as fluorine, chlorine, iodine, or bromine as the anion. Example, Halite, NaCl (Bates and Jackson, 1984).

Halley, Edmond (1656-1742) English astronomer. In 1758, he accurately predicted the return of a comet previously observed in 1531, 1607, and 1682. The body was subsequently named Halley's Comet (Welcome to the Planets, 1995).

Herschel, Sir William (1738-1822) British astronomer. He built a reflecting telescope of superior quality and with it discovered planet Uranus. He also discovered some of the satellites of Uranus and of Saturn (Welcome to the Planets, 1995).

Horn A high pyramidal peak with steep sides formed by the intersecting walls of three or more cirques formed by alpine glaciation (Bates and Jackson, 1984).

Hornblende The most common mineral of the amphibole group (Ca,Na)_2-3 (Mg,Fe⁺²,Fe⁺³,Al)₅(Al, Si)₈O₂₂(OH)₂. It has a variable composition, and may contain potassium and appreciable fluorine. It is commonly black and occurs in distinct monoclinic crystals or in columnar, fibrous, or granular forms (Bates and Jackson, 1984).

Hubble Space Telescope (HST) A 98 inch, Earth orbiting reflecting space telescope which is equipped with infrared, x-ray and other electromagnetic sensors. Named after famous American astronomer, Edwin Hubble (Kuhn, 1998).

Hummocky Uneven; describing a terrain abounding in irregular knolls, mounds, or other small elevations (Welcome to the Planets, 1995).

Hydrologic system (Mars) Water cycling during past wet episodes on Mars would have had many components. A thick atmosphere most likely carried a substantial amount of water evaporated from lakes and seas. That water vapor would, in turn, condense into clouds and eventually precipitate. Rain formed in this way would have created surface runoff, and much of this water would have percolated into the ground. Snowfalls might have accumulated to form glaciers, which in turn would have discharged their meltwaters into glacial lakes. Hydrothermal circulation, associated perhaps with sites of volcanism, could also have brought water to the surface from reservoirs deep underground (Planetary Geology for Teachers, 1999).

Hydrosphere The waters of the Earth, including surface and subsurface water. All the oceans, snow, ice and glaciers (Bates and Jackson, 1984).

Hypersthene A common rock forming mineral of the orthopyroxene group, (Mg,Fe)SiO₃. It is isomorphous with enstatite. It is an essential constituent of many igneous rocks (Bates and Jackson, 1984).

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Ice (dominant Solar System) Water (H₂O) ice, CO₂ ice and NH₄ ice (Planetary Geology for Teachers, 1999).

Ice age A loosely used synonym for glacial epoch, or time of extensive glacial activity (Bates and Jackson, 1984).

Ice shelves A sheet of very thick ice with a level or gently undulating surface, which is attached to the land along one side but most of which is afloat and bounded on the seaward side by a steep cliff (ice front) rising 2 to 50 meters or more above sea level (Bates and Jackson, 1984).

Iceberg A large, massive piece of floating or stranded glacier ice of any shape, broken from the front of a glacier into a body of water. An iceberg has the greater part of its mass (4/5 to 8/9) below sea level (Bates and Jackson, 1984).

Ida An "S-type", main belt asteroid of silicaceous chemical composition. 251 Ida exhibits an irregular, heavily cratered surface and measures 55 x 24 x 20 km making it the largest member of a compositionally similar group called the Koronis Family. Ida rotates every 4.6 hours on its axis and was first photographed by the Galileo spacecraft in 1993. At about 100 km from its center, Ida is orbited by the small satellite, Dactyl, which measures 1.6 x 1.4 x 1.2 km. Dactyl is the first evidence that asteroids may have tiny moons. Crater density on Ida is 5 times that of Gaspra from which it is inferred that Ida is the older of the two (Beatty and Chaikin, 1990).

Igneous Said of a rock or mineral that solidified from molten or partly molten material, e.g. from a magma; also applied to processes related to the formation of such rocks (Bates and Jackson, 1984).

Impact basin A depression formed by the impact of a meteorite, and also a depression around the orifice of a volcano (Bates and Jackson, 1984).

Impact craters A depression formed by the impact of an unspecified projectile, esp. a crater formed on the Earth, planet or moon surface where the nature of the impacting body is unknown (Bates and Jackson, 1984).

Impact events An individual bolide (comet, meteorite, or asteroid) impact and the associated geological, biological and climatic changes that result (Beatty and Chaikin, 1990).

Impact glass Glassy material produced by a complete or partial fusion of target rock by the heat generated from the impact of a large meteorite and occurring in and around the resulting crater (Bates and Jackson, 1984).

Impact melts During an impact event, a compression wave spreads outward from the point of impact. Engulfed by the shock wave, the colliding meteoroid and the surface rocks are melted and partially vaporized (Beatty and Chaikin, 1990).

Inner core (Earth) The central, solid part of the Earth’s core, extending from a depth of about 5100 km to the center (6371) of the Earth; its radius is about 1/3 that of the whole core and is effectively de-coupled from the mantle due to the molten nature of the surrounding outer core. Its density is about 12 g/cm³ (Bates and Jackson, 1984).

Intercrater plains (Mercury) Mercury was larger and closer to the Sun, therefore its surface cooled more slowly. For a long time after the crust started forming, meteoric debris was able to break through the soft crust and allow lava to erupt and obliterate older craters. This resulted in the plains seen today between the craters (Planetary Geology for Teachers, 1999).

Interferometry A procedure that allows a number of telescopes to be used as a single, large telescope by taking into account the time differences at which individual waves from an object strike the individual telescopes (Kuhn, 1998).

Internal structure (Jupiter) Jupiter is thought to have very rare, liquid metal hydrogen surrounding its core, which could give the planet its magnetic field. Gaseous clouds may become dense enough at Jupiter’s center to support a solid rocky core (Beatty and Chaikin, 1990).

Internal structure (Saturn) The internal structure of Saturn consists of a gas outer layer with traces of ice, a middle layer of liquid metal hydrogen, and, possibly a rocky core (Beatty and Chaikin, 1990).

Intrusive rocks (plutonic) Rocks formed beneath the surface where magma tends to cool slowly and develop large crystals. Mineral growth is often enhanced by aqueous fluids that can help carry the chemical components necessary for rapid crystal growth. Common examples of plutonic rock are granite, diorite, gabbro, anorthosite, and syenite, with rock names depending on the relative proportions of constituent minerals and their bulk chemistry (Planetary Geology for Teachers, 1999).

Ion A charged atom or molecule resulting from the atom’s loss or gain of an electron (Kuhn, 1998).

Ionic bond A chemical bond formed by the electrostatic attraction between positive and negative ions; for example, NaCl (Beatty and Chaikin, 1990).

Iron A meteorite consisting essentially of iron with up to 30% of nickel in solid solution (Bates and Jackson, 1984).

Iron core, Large (Mercury) Mercury's large metallic core, three-fourths the size of the planet, makes it much denser than the Moon and the most iron-rich planet in the solar system. Surrounding the metallic core is a rigid outer shell, or lithospheric mantle, that is about 500-600km thick (The New Solar System, 1990)

Ishtar Terra (Venus) A highland area in the northern hemisphere (about the size of Australia) which stands several kilometers above the average planetary radius. It is separated from the surrounding rolling uplands by relatively steep flanks, and the western portion is a vast plateau, Lakshmi Planum, some 2000 kilometers across. Eastern Isthar contains Maxwell Montes, the highest topographic feature on Venus (Planetary Geology for Teachers, 1999).

Island arc A curved chain of islands, e.g. the Aleutians, rising from a deep sea floor and near the continents. Its curve is generally convex toward the open ocean (Bates and Jackson, 1984).

Isostacy The condition of equilibrium, comparable to floating, of the units of the lithosphere above the asthenosphere. Also, isostatic compensation (Bates and Jackson, 1984).

Isotope Atoms of the same element with different numbers of neutrons in their nuclei. Also, the same atomic number but a different atomic mass (Beatty and Chaikin, 1990).

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Jovian Of or relating to the planet Jupiter. Also designating the outer, Jupiter-like gas giants which in addition to Jupiter, include Saturn, Uranus and Neptune (Kuhn, 1998).

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Karst A type of topography that is formed over limestone, dolomite or gypsum by dissolution, and that is characterized by sinkholes, caves, and underground drainage (Bates and Jackson, 1984).

Kettle lake A body of water occupying a depression in glacial drift, esp. in outwash and a kame field, formed by the melting of a detached block of stagnant ice that was buried in the drift. Walden’s Pond is and example (Bates and Jackson, 1984).

Kinetic energy The energy of motion. Kinetic energy is transferred by a shock wave that spreads outward from the point of impact (Beatty and Chaikin, 1990).

Kreep basalts An acronyum for a type of basaltic lunar rock first found by Apollo 12. The fines and breccia of this lunar basalt are characterized by an unusually high content of potassium (K), rare earth elements (REE), phosphorus (P) and other trace elements. The KREEP basalts are compositionally different form the iron-rich mare basalts (Bates and Jackson, 1984).

Kuiper Belt A region beyond the orbit of Neptune from which the short period comets (>200 years) are theorized to originate (Kuhn, 1998).

Kuiper, Gerard Peter (1905-1973) An American astronomer born in the Netherlands. He studied the origin of the solar system, planetary atmospheres and proposed the Kuiper Belt as the source of short period comets (Welcome to the Planets, 1995).

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Lag deposits A residual accumulation of coarse rock fragments on a surface after the finer material has been blown away by winds. Also, desert pavement (Bates and Jackson, 1984).

Lakshmi Planum (Venus) A high volcanic plateau situated nearly 5 kilometers (3 miles) above the mean radius of Venus and surrounded by mountain belts (ranges), Akna Montes (west), Freyja Montes (north), Maxwell Montes (east), and Danu Montes (south). Two large volcanic calderas, Colette and Sacajawea, are located in the center of Lakshmi Planum and are surrounded by long volcanic flows (Planetary Geology for Teachers, 1999).

Large Magellanic Cloud Hazy, cloud-like patches in the sky of the Earth’s southern hemisphere which are gigantic groups of stars forming separate galaxies (Kuhn, 1998).

Lava Magma that erupts onto the surface (Planetary Geology for Teachers, 1999).

Lava Channels (Venus) Lava channels extending from hundreds to thousands of kilometers in length are conspicuous on the Venusian plains. Simple channels typically show little or no branching. They include long sinuous forms, termed "canali", and sinuous rilles. Canali are best preserved in regions of subdued relief. They have a high width-to-depth ratio and maintain a remarkably constant width over very long distances. Images reveal the presence of meanders, point bars, cut banks, and abandoned channel segments (Planetary Geology for Teachers, 1999).

Lava flows A lateral, surficial outpouring of molten lava from a vent or fissure; also, the solidified body of rock that is so formed (Bates and Jackson, 1984).

Lavinia Planitia (Venus) A topographic basin consisting of extensive, probably basaltic, lava flows and could be compared to the Earth’s ocean basins in respect to extent and composition. Features seen within the lowlands include broad bowl-like depressions, flood lavas (similar to the Columbia River Basalts on Earth), lava channels that can extend for hundreds of kilometers, and compressional ridges (Planetary Geology for Teachers, 1999).

Limb The apparent outer edge of a lunar or planetary disk (Welcome to the Planets, 1995).

Lineations Linear topographic features that may depict crustal structure (Welcome to the Planets, 1995).

Liquid metal hydrogen A liquid metal form of hydorgen which exists only at high pressures (over 4 million bars) and is theorized to exist near the core of Jupiter (Planetary Geology for Teachers, 1999).

Lithosphere The solid portion of the Earth, as compared with the atmosphere and the hydrosphere. In place tectonics, a layer of strength relative to the underlying asthenosphere. It includes the crust and part of the upper mantle and is of the order of 100 km in thickness (Bates and Jackson, 1984).

Loess A blanket deposit of buff-colored calcareous silt, homogeneous, nonstratified, weakly coherent, porous, and friable. It is considered to be windblown dust of Pleistocene age (Bates and Jackson, 1984).

Lower mantle (Earth) That part of the mantle that lies below a depth of about 1000 km and has a density of 4.7 g/cm³, in which the seismic velocity increases slowly with depth (Bates and Jackson, 1984).

Lowlands (Venus) These large plains consist of extensive, probably basaltic, lava flows and could be compared to the Earth’s ocean basins in respect to extent and composition. Features seen within the lowlands include broad bowl-like depressions, flood lavas (similar to the Columbia River Basalts on Earth), lava channels that can extend for hundreds of kilometers, and compressional ridges. The largest lowland feature is Atalanta Planitia, which is located east of Ishtar Terra (~165° E Long., 65° N Lat.) and is about the size of the Gulf of Mexico (Planetary Geology for Teachers, 1999).

Lunar Magma Ocean The outer several hundred kilometers of the lunar mantle became molten yielding a Lunar Magma Ocean (LMO) that began to cool and differentiate like a thick intrusion of mafic magma. Dense ferromagnesian minerals, such as olivine and pyroxene, crystallized and sank to the deeper mantle. Less dense calcium plagioclase feldspar minerals accumulated into anorthosite masses (along with minor amounts of ferromagnesian minerals) that rose and concentrated near the surface. These masses formed the early lunar crust of which the highland regions are made (Planetary Geology for Teachers, 1999).

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Magma A 3-phase system of silicate liquid, solid minerals, and perhaps a vapor phase. It takes on various forms including pods, lenses, diapirs, veins, or dikes depending on viscosity, density, and the forces applied to it by surrounding rock (Planetary Geology for Teachers, 1999).

Magnetosphere (Earth) The confines of the Earth’s magnetic field, modified by the influence of the solar wind. On the sunlit side, the magnetosphere is nearly hemispherical with a radius ~10 Earth radii under quiet conditions and ~6 Earth radii during solar storms. Opposite the sunlit side, the magnetosphere extends in a ‘tail’ of several hundred Earth radii (Kuhn, 1998).

Main Belt asteroids Asteroids that orbit the Sun at distances from 2.2 AU to 3.6 AU. This orbital zone between Mars and Jupiter is the location of most of the known asteroids. Also, asteroid belt (Beatty and Chaikin, 1990).

Manicougan impact stucture A 70 km diameter impact crater in Quebec, Canada. The 210 million year old structure has a central peak of shock-metamorphosed rock, surrounded by a thick layer of frozen impact melt that pooled on the original crater floor (Beatty and Chaikin, 1990).

Mantle (Earth) The zone of the Earth below the crust and above the core, it is divided into the upper mantle and the lower mantle, with a transition zone between (Bates and Jackson, 1984).

Mantle plume A buoyant mass of hot, partially molten mantle material that rises to the base of the lithosphere. A persistent pipelike body of hot material moving upward from the planetary mantle into the crust. Its surface expression may be a hot spot (Welcome to the Planets, 1995).

Mare A dark, low-lying lunar plain, filled to some depth with volcanic rocks (Welcome to the Planets, 1995).

Mariner 10 Only one spacecraft, Mariner 10, has ever visited Mercury. It made three passes near the planet in 1974 and '75. It snapped pictures of half of the planet's surface, measured temperatures, and discovered a weak magnetic field. American and European space agencies are drafting plans for future missions to Mercury, but none will take place before the next century (Planetary Geology for Teachers, 1999).

Mass extinction The total disappearance of many species or higher taxon, such that they no longer exist anywhere (Bates and Jackson, 1984).

Mass movement Unit downslope movement of a portion of the land surface, as in creep, landslide or slip (Bates and Jackson, 1984).

Maxwell Montes (Venus) Maxwell Montes in Ishtar Terra is the highest peak on Venus. It towers more than 11 kilometers (7 miles) above the lowland plains

(Planetary Geology for Teachers, 1999).

Mead crater (Venus) Crater Mead, with a diameter of 280 kilometers (174 miles), is the largest impact crater on Venus. The inner ring is thought to represent the original rim of the crater cavity, while the outer scarp is thought to be the expression of a ring fault that has downdropped the flank terrace (Planetary Geology for Teachers, 1999).

Mercury’s landforms Mercury's major landforms include impact craters which dominate the landscape, cratered terranes, widespread intercrater plains, multiring basins, and some smooth plains that may have been formed by flood basalts. Though an absolute age is not available now, scientists assume Mercury's heavily cratered regions probably formed during the same period of intense bombardment that formed the lunar highlands (Planetary Geology for Teachers, 1999).

Mesas A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes (Bates and Jackson, 1984).

Mesozoic An era of geologic time, from the end of the Paleozoic to the beginning of the Cenozoic Era, or from about 225 mya to about 65 mya. It includes the Triassic Period (230 mya to 190 mya), the Jurassic Period (190 mya to 140 mya) and the Cretaceous (140 mya to 65 mya) (Bates and Jackson, 1984).

Metallic bond A chemically bonded solid that consists of positive cores of atoms held together by a "sea" of electrons (Beatty and Chaikin, 1990).

Metamorphic Any rock derived from pre-existing rocks by mineralogic, chemical and/or structural changes, essentially in the solid state, in response to marked changes in tempeature, pressure, shearing stress and chemical environment, generally at depth in the Earth’s crust (Bates and Jackson, 1984).

Metasomatism Metamorphic rocks formed by the addition or subtraction of chemical components. Hot, aqueous fluids can be generated by magma that heats groundwater or gives off primary water. Such hydrothermal fluids can alter rocks that surround intrusive bodies. This is how some economic mineral deposits are formed (Planetary Geology for Teachers, 1999).

Meteorite Any solid object from interplanetary space that has fallen to the Earth’s surface without being vaporized by frictional heating during its passage through the atmosphere; a stony or metallic object large enough to reach the ground. Most meteorites are believed to be fragments of asteroids and to consist of primitive solid matter similar to that from which the Earth was originally formed (Bates and Jackson, 1984).

Meteorite crater An impact crater formed by the falling of a large meteorite onto a surface (Bates and Jackson, 1984).

Meteoroid Any relatively small fragment of solid material associated with a meteor and made luminous as a result of friction during its passage through the Earth’s atmosphere (Bates and Jackson, 1984).

Mica A group of monoclinc minerals of the general formula (K,Na,C,)(Mg,Fe,Li,Al)_2-3(Al,Si)₄O₁₀(OH,F)₂. It consists of complex phyllosilicates with perfect basal cleavage, which split into thin elastic laminae and range from colorless to black (Bates and Jackson, 1984).

Microcline A white, gray, brick-red or green mineral of the alkali feldspar group: (KAlSi₃O₈). It is the fully ordered, triclinic modification of potassium feldspar and is dimorphous with orthoclase, being stable at lower temperatures, it usually contains sodium in minor amounts. Microcline is a common rock forming mineral of granitic rocks and pegmatites, and is often secondary in orthoclase (Bates and Jackson, 1984).

Mineral A naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties (Bates and Jackson, 1984).

Moons Any of the 63 (so far known) smaller objects that revolve around the planets (Kuhn, 1998).

Moraines A mound or ridge of unstratified glacial drift, chiefly till, deposited by direct action of glacial ice (Bates and Jackson, 1984).

Morphology The study of structure or form (Welcome to the Planets, 1995).

Mountain belts A group of mountain ranges exhibiting certain unifying features, such as similarity in form, structure, and alignment, and presumably originating from the same general causes. Cf: orogenic belt, mountain chain (Bates and Jackson, 1984).

Multiring basins Multiring basins have diameters over 300 kilometers and often contain mare lava flows that filled the crater floor after the crater formation. These craters look like a bulls-eye, with low ridges radial around the crater (Beatty and Chaikin, 1990).

Muscovite A mineral of the mica group: KAl₂(AlSi₃)O₁₀(OH)₂. It is colorless to pale brown, and is a common mineral in gneisses and schists, in granites and pegmatites, and in may sedimentary rocks, esp. sandstones (Bates and Jackson, 1984).

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Neutrino An elementary particle that has little or no rest mass and no charge which carries energy from a nuclear reaction (Kuhn, 1998).

Nucleus (atomic) The central, massive part of an atom consisting of protons and neutrons (Kuhn, 1998).

Nucleus (comet) The frozen core of a comet which contains almost the entire cometary mass and is located in the comet's head (Welcome to the Planets, 1995).

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Oceanic crust The oceanic crust underlies the ocean basins and is equivalent to the sima. The oceanic crust is about 5 to 10 km thick, has a density of 3.0 g/cm³, and compressional seismic wave velocities exceeding 6.2 km/sec (Bates and Jackson, 1984).

Olivine A green or brown orthorhombic mineral, (Mg,Fe)₂SiO₄. It consists of the isomorphous solid-solution series forsterite-fayalite (Bates and Jackson, 1984).

Olivine phenocrysts The relatively large and ordinarily conspicuous crystal of the mineral olivine generated early in prophyritic igneous rocks (Bates and Jackson, 1984).

Orthoclase A white, pink or gray mineral of the alkali feldspar group: (KAlSi₃O₈). It is the partly ordered, monoclinic modification of potassium feldspar and is dimorphous with microcline, being stable at higher temperatures; it usually contains sodium in minor amounts. Orthoclase is a common rock forming mineral; it occurs esp. in granites, acid igneous rocks and crystalline schists, and is usually perthitic (Bates and Jackson, 1984).

Outer core (Earth) The upper zone of the Earth’s core, extending from a depth of 2900 km to 5100 km, and including the transition zone. It is presumed to be liquid because it sharply reduces compressional wave velocities and does not transmit shear waves. Its density ranges from 9 to 11 g/cm³ (Bates and Jackson, 1984).

Outflow channels (Mars) Structures are more than 200 kilometers wide and can stretch for 2,000 kilometers or more. These channels emanate from what is called chaotic terrain, regions of fractured, jumbled rocks that apparently collapsed when groundwater suddenly surged outward (Planetary Geology for Teachers, 1999).

Outwash Sand and gravel deposited by meltwater streams in front of the end moraine to the margin of an active glacier (Bates and Jackson, 1984).

Ovda Regio (Venus) The tectonic fabric of this region trends predominantly east-west. The large radar-dark areas are probably tectonically formed basins that have been filled in by fluid lava flows, thus presenting a smooth surface to the Magellan radar system (Planetary Geology for Teachers, 1999)

Oxides A mineral compound characterized by the linking of oxygen with one or more metallic elements, such as Cuprite, Cu₂O (Bates and Jackson, 1984).

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Paleozoic An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic Era, or from about 570 mya to about 225 mya (Bates and Jackson, 1984).

Patera A shallow crater with a scalloped, complex edge (Welcome to the Planets, 1995).

Patterned ground A group term for the more or less symmetrical forms such as circles, polygons, nets, steps, and stripes that are characteristic of, but not necessarily confined to, surficial material subject to intensive frost action (Bates and Jackson, 1984).

Peak-ring craters A central uplift characterized by a ring of peaks rather than a single peak; peak rings are typical of larger terrestrial craters above about 50 kilometers (30 miles) in diameter (Planetary Geology for Teachers, 1999).

Perigee The point in the orbit of an Earth satellite where it is closest to the Earth (Kuhn, 1998).

Perihelion The point in the path of a planet, asteroid, comet, or other body that is closest to the sun (Welcome to the Planets, 1995).

Permafrost Permanently frozen soil or subsoil, occurring in arctic, sub-arctic, and alpine regions. Its thickness ranges from 30 cm to > 1000 meters, and it underlies about 1/5 of the Earth’s land area (Bates and Jackson, 1984).

Phanerozoic That part of geologic time represented by rocks in which the evidence of life is abundant, and includes the Cambrian and later time (Bates and Jackson, 1984).

Phlogopite A magnesium rich mineral of the mica group: K(Mg,Fe)₃AlSi₃O₁₀(OH,F)₂. It is yellowish brown to brownish red and usually occurs in crystalline limestones as a result of dedolomization (Bates and Jackson, 1984).

Phobos The irregularly shaped, dark colored moon of Mars. Phobos is larger than its companion moon Deimos. Both orbit very close to Mars, have short periods of revolution (0.3 days for Phobos) and are believed to be captured asteroids (Kuhn, 1998).

Photon The smallest possible amount of electromagnetic energy of a particular wavelength. This bundle or quanta of energy is called a photon (Kuhn, 1998).

Photosphere The visible "surface" of the Sun with temperatures of about 5800 K and an average thickness of 400 km. The part of the solar atmosphere that radiates light into space (Kuhn, 1998).

Physical weathering The breaking down of rocks by mechanical processes into pieces that get smaller and smaller until they may eventually become sand, silt, or dust-sized particles (Planetary Geology for Teachers, 1999).

Plagioclase A group of triclinic feldspars of general formula: (Na,Ca)Al(Si,Al)Si₂O₈. At high temperatures it forms a complete solid-solution series from Albite (NaAlSi₃O₈) to Anorthite (CaAl₂Si₂O₈). The series is subdivided and named according to the mole fraction of the Anorhtite component. Plagioclase minerals are among the most common rock forming minerals (Bates and Jackson, 1984).

Planet Any of the nine (?) so far known, large objects that revolve around the Sun. The designation of Pluto as a planet is in dispute among astronomers (Kuhn, 1998).

Planetary differentiation The heat driven process whereby planets develop concentric layers of differing composition based on the density of the materials involved, with heavier elements sinking toward the core and lighter elements remaining near the surface (Planetary Geology for Teachers, 1999).

Plate tectonics A theory of global crustal displacement in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of torsionally rigid bodies that interact with one anther at their boundaries, causing seismic and volcanic activity along the boundaries (Bates and Jackson, 1984).

Plateau Any comparatively flat area of great extent or elevation (Welcome to the Planets, 1995).

Plutonic rocks (intrusive) Rocks formed beneath the surface where magma tends to cool slowly and develop large crystals. Mineral growth is often enhanced by aqueous fluids that can help carry the chemical components necessary for rapid crystal growth. Common examples of plutonic rock are granite, diorite, gabbro, anorthosite, and syenite; the rock names depend on the relative proportions of constituent minerals and the bulk chemistry (Planetary Geology for Teachers, 1999).

Polar dune fields (Mars) A vast "sea" of sand dunes surrounds the north polar cap. The dunes are classic forms known as barchan dunes, small, crescent-shaped hills, and transverse dunes, with ridges that resemble coalesced barchans. These dunes are similar in size and shape to familiar sand dunes found in desert regions on Earth. These two varieties form from winds that persistently come from a single direction (in this case, from the southwest) (Planetary Geology for Teachers, 1999).

Polar ice caps (Mars) Stacks of layered sediments and ice which are at least 4-6km thick in the north and 1-2km thick in the south. Cratered plains surround the ice caps and seem to extend beneath them, indicating that the ice covers the older craters and is, therefore, one of the youngest features on the planet. During the Martian winter, the ice caps grow in extent and are covered with CO₂ frost and ice. The summertime brings with it receding ice caps and sublimation of the CO₂ ice (Planetary Geology for Teachers, 1999).

Polymorph The characteristic of a chemical substance to crystallize in more than one form, e.g. rhombic and monoclinic sulfur (Bates and Jackson, 1984).

Proterozoic The entire Precambrian ranging from about 4500 mya to 570 mya and includes the Hadean and the Archean (Bates and Jackson, 1984).

Proton The massive, positively charged particle in the nucleus of an atom (Kuhn, 1998).

Pulsar A celestial object of small angular size that emits pulses of radio waves with a regular period between about 0.03 and 5 seconds (Kuhn, 1998).

Pyroxene A group of the common rock forming minerals with the general formula, ABSi₂O₆, where A is chiefly Mg, Fe⁺², Ca or Na, and B is Mg, Fe⁺² or Al (Bates and Jackson, 1984).

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Quartz Crystalline silica, an important rock forming mineral, SiO₂. It is, next to feldspar, the most common mineral, occurring either in transparent hexagonal crystals or in crystalline or cryptocrystalline masses (Bates and Jackson, 1984).

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Radiative Zone The inner 70% of the Sun’s radius where heat from the fusion reaction is radiated toward the surface at temperatures of 1.5 x 10⁷ K at the base of the zone to temperatures of 1.5 x 10⁶ K at the top of the zone (Kuhn, 1998).

Radiogenic heating Thermal energy resulting from the decay of radioactive isotopes. Long-lived nuclides are still present in the Earth and other planetary bodies, although in lower abundance than when the Solar System formed. This allows for sustained long-term heating during planetary evolution (Planetary Geology for Teachers, 1999).

Rampart craters (Mars) Impact features characterized by a mud-like flow of overlapping lobes out and away from the crater caused when the heat and pressure of an impact fluidized the local ground water or ground ice that was trapped within the Martian surface (Planetary Geology for Teachers, 1999).

Refractory minerals Minerals that can resist high temperatures, also minerals which exhibit high melting points (Bates and Jackson, 1984).

Regolith The fragmented and unconsolidated rock material, whether residual or transported, that nearly everywhere forms the surface of the land and overlies the bedrock. It includes rock debris of all kinds (Bates and Jackson, 1984).

Relief The maximum regional difference in elevation (Bates and Jackson, 1984).

Rhea Mons (Venus) A large shield volcano (similar to Hawaii or Olympus Mons) (Planetary Geology for Teachers, 1999).

Rift valley An elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike (Bates and Jackson, 1984).

Rille (Moon) One of several trench-, or crack-like valleys up to several hundred km long and 1-2 km wide commonly occurring on the Moon's surface (Welcome to the Planets, 1995).

Rings A thin, disk-like accumulation of very small grains of rock, ice and ice covered rock which probably originated with the tidal force induced breakup of nearby moons. The rings lie within the Roch limit of the planet and rotate according to Kepler’s laws (Planetary Geology for Teachers, 1999).

Ripples Formations in fine-grained sediment caused by the movement of water or wind over a depositional surface. They can be asymmetrical indicating flow direction, or they can be fairly symmetrical indicating an oscillating current (Planetary Geology for Teachers, 1999).

Rock forming minerals Said of those minerals than enter into the composition of rocks, and determine their classification. The more important rock forming minerals include quartz, feldspars, micas, amphiboles, pyroxenes, olivine, calcite and dolomite (Bates and Jackson, 1984).

Roche limit The Roche limit was first described by Edouard Roche in 1848. The Roche limit is the closest distance an object can come to another object without being pulled apart by tidal forces. If a planet and a moon have identical densities, then the Roche limit is 2.446 times the radius of the planet. A large moon orbiting inside the Roche limit will be destroyed. The Earth's Roche limit is 18,470 km (11,470 miles). If our Moon ever ventured within this Roche limit, it would be pulled apart by tidal forces and the Earth would have rings. The four gaseous outer planets have their ring systems inside of their respective Roche limit (Planetary Geology for Teachers, 1999).

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Sand sea A very large sand mass; an erg. From a Hamitic word areg or ergh for such a sand mass in the Sahara (Bates and Jackson, 1984).

Sapping Spring fed channels that are created when soft sediment layers are removed by underlying water erosion (Planetary Geology for Teachers, 1999).

Scarp A cliff or steep slope of some extent that may form a marked topographic boundary (Welcome to the Planets, 1995).

Schiaparelli Giovanni Virginio Schiaparelli (1835-1910). Italian astronomer at the Milan Observatory who reported markings on Mars which he called "canali" (Welcome to the Planets, 1995).

Schist A foliated, metamorphic rock that usually contains micas oriented sub-parallel to one another (Planetary Geology for Teachers, 1999).

Sea ice Any form of ice originating from the freezing of sea water, thus excluding icebergs (Bates and Jackson, 1984).

Sediment Generally, solid, fragmented material transposrted and deposited by wind, water, or ice, chemically precipitated from solution, or secreted by organisms, and that forms layers of loose, unconsolidated form, e.g. sand mud and till. Solid material that has settled down from a state of suspension in a liquid (Bates and Jackson, 1984).

Sedimentary A layered rock resulting from the consolidation of sediment, e.g. a clastic rock such as sandstone, a chemical rock such a salt, or an organic rock such as coal. Usually includes pyroclastic rocks, such as tuff (Bates and Jackson, 1984).

Shearing The motion resulting from stresses that cause or tend to cause contiguous parts of a body to slide relatively to each other (Welcome to the Planets, 1995).

Shepherd satellites (moons) A shepherd moon is a satellite that constrains the extent of a planetary ring through gravitational forces (Planetary Geology for Teachers, 1999).

Shield A large region of exposed basement rocks, commonly with a very gently convex surface, surrounded by sediment covered platforms; e.g. Canadian Shield. Shield rocks are usually Precambrian in age (Bates and Jackson, 1984).

Shield volcano A broad volcanic cone with gentle slopes constructed of successive nonviscous, mostly basaltic, lava flows (Welcome to the Planets, 1995).

Sif Mons (Venus) A large shield volcano (similar to Hawaii or Olympus Mons). Its dark central caldera, which may contain pools of lava, is surrounded by extensive flow features that seem to cascade down its flanks. The volcano has a diameter of 300 kilometers (180 miles) and a height of 2 kilometers (1.2 miles) (Planetary Geology for Teachers, 1999).

Silicates A compound whose crystal structure contains SiO₄ tetrahedra, either isolated or joined through one or more of the oxygen atoms to form groups, chains, sheets, or three dimensional structures with metallic elements (Bates and Jackson, 1984).

Silicic volcanism Said of silica rich igneous rocks or magma, with silica contents > 65% (Bates and Jackson, 1984).

Single chains A class or structural type of silicate characterized by the linkage of the SiO₄ tetrahedra into linear chains by the sharing of oxygens. In a simple chain, e.g. pyroxenes, two oxygens are shared (Bates and Jackson, 1984).

Single element minerals A mineral composed of a single substance that cannot be decomposed into other substances except by radioactive decay (Bates and Jackson, 1984).

Sinkholes A circular depression in a karst area. Its drainage is subterranean, its size is measured in meters to tens of meters, and it is commonly funnel shaped (Bates and Jackson, 1984).

Slumping A landslide that results from the downward sliding of rock debris as a single mass, usually with a backward rotation relative to the slope along which the movement takes place (Welcome to the Planets, 1995).

Smooth plains (Mercury) (Volcanic or ejecta?) The smooth plains seen on Mercury resemble lunar maria and are likewise, sparsely cratered, but may have formed from different processes than those on the Moon. Mercury's smooth plains might have been caused by large amounts of ejecta from the impacts that cratered the surface and formed the Caloris Basin. On the other hand, a volcanic origin is possible and would liken Mercury's plains to the Moon's maria (Planetary Geology for Teachers, 1999).

SNC meteorites Rare achondrites about 1.3 billion years old believed to be pieces of Mars. They are classified according to the original group of specimens used to chemically and lithologically suggest a Martian origin (Beatty and Chaikin, 1990).

SNC meteorites General name for three groups of meteorites with relatively young ages (slightly over 1 billion years old) that probably came from Mars. The groups are the Shergottites, Nakhlites, and Chassigny (Bates and Jackson, 1984).

Solar energy Thermal energy generated by the fusion reaction of the Sun (Kuhn, 1998).

Solar Prominences The eruption of solar material beyond the disk of the Sun (Kuhn, 1998).

Solid solution A single crystalline phase that may be varied in composition within finite limits without the appearance of an additional phase (Bates and Jackson, 1984).

Spectroscopy The use of an instrument that produces an array of visible light ordered according to its constituent wavelengths or colors (Kuhn, 1998).

Spicules A narrow jet of gas that is part of the chromosphere of the Sun and extends upward into the corona (Kuhn, 1998).

Spin-orbit coupling/ orbital resonance For many decades, scientists thought the same side of Mercury always faced the Sun. But in 1965, astronomers discovered that Mercury completes three full turns on its axis for every two orbits around the Sun. Because of this motion, we see the same side of Mercury each time the planet comes closest to Earth. It is the only planet in our solar system with an orbital:rotational resonance ratio of 2:3. This resonance ratio is caused by Mercury’s mass not being evenly distributed throughout the planet. One side is more massive than the other. Because of this, the Sun exerts a torque (a turning force) on the planet, especially at perihelion. After countless revolutions this has resulted in the rotational period of the planet being coupled with its revolution period (Planetary Geology for Teachers, 1999).

Spokes (Saturn) Faint, dark areas perpendicular to the rings that seem to grow and shrink. Little is known about the spokes, but they may be charged particles that float above the actual ring plain (Planetary Geology for Teachers, 1999).

Stable platform The part of a continent that is covered by flat-lying or gently tilted sedimentary rocks, underlain by a complex of rocks that were consolidated during earlier deformations. The platform is part of the craton (Bates and Jackson, 1984).

Stony A general name for meteorites consisting largely or entirely of silicate minerals (chiefly olivine, pyroxene and plagioclase). Stony meteorites resemble ultramafic rocks in composition, and they constitute more than 90% of meteorites seen to fall (Bates and Jackson, 1984).

Stony-irons As their name implies, the stony-irons are comprised of differentiated igneous silicates plus iron and nickel metal. They are fairly rare (~1% of falls and ~5% of finds) and thus are valuable to scientists as well as collectors. The stony-irons provide a glimpse of the region within a planetary body near the core-mantle boundary where metallic core components have not entirely separated from the silicate mantle (Bates and Jackson, 1984).

Structurally bonded water Water molecules held within the geometric pattern of crystal lattices. Amphibole is an example. The water held between the layers of atoms in sheet silicates such as smectite is termed interstitial water (Bates and Jackson, 1984).

Subduction zone A long, narrow belt in which one lithospheric plate descends beneath another usually of lower density, e.g. along the Peru-Chile trench (Bates and Jackson, 1984).

Sulfates A mineral compound characterized by the sulfate radical, SO₄ (Bates and Jackson, 1984).

Sulfides A mineral compound characterized by the linking of sulfur with a metal, such as Galena, PbS (Bates and Jackson, 1984).

Sunspots A magnetically induced feature of the photosphere that is temporarily cool and dark compared to surrounding regions (Kuhn, 1998).

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Tarns A small, deep, commonly circular glacial lake occupying a cirque; it is fed by runoff from the surrounding slopes and dammed by a lip of bedrock or by a small moraine (Bates and Jackson, 1984).

Tectonic Relating to the deformation of the crust of a moon or planet, the forces involved in or producing such deformation, and the resulting forms (Welcome to the Planets, 1995).

Tektites Small, rounded pieces of silicate glass, thought to have become airborne during terrestrial impact events (Bates and Jackson, 1984).

Terminator The line separating the illuminated and unilluminated parts of a celestial body; the dividing line between day and night as observed from a distance (Beatty and Chaikin, 1990).

Terrestrial Belonging to the class of planets that are similar to the Earth in density and composition (i.e. Mercury, Venus, and Mars) (Welcome to the Planets, 1995).

Tesserae (Venus) Tesserae terrain, which was first detected by the Soviet Venera 15 and 16 spacecraft in the early 1980s, is characterized by complex intersecting ridges and grooves. This terrain may have been formed by large blocks of material sliding and collapsing down slopes, pulled by the force of gravity. In the extremely high-temperature Venus environment, rock can behave more like a fluid, unlike the rigid behavior of rocks found on Earth (Planetary Geology for Teachers, 1999).

Tetrahedral A crystal form in the isometric system, having four faces each with equal intercepts on all three axes (Bates and Jackson, 1984).

Tharsis uplift (Mars) The Tharsis region is at the center of a huge bulge in the surface that is over 4000 km across and 10 km high. Huge shield volcanoes dominate the Tharsis region, including Olympus Mons on the northwest flank, the largest shield volcano in the solar system. The Tharsis uplift was likely caused by upwelling mantle material and is evidence for tectonism on Mars (Planetary Geology for Teachers, 1999).

Theia Mons (Venus) A large shield volcano which lies in the southern part of the Dvana Chasam rift valley (Planetary Geology for Teachers, 1999).

Thermal energy Energy associated with or pertaining to the heat derived from planetary processes such as gravitational collapse, solar radiant energy, radioactive decay and impactor accretion (Beatty and Chaikin, 1990).

Thermokarst An irregular land surface containing cave-in lakes, bogs, caverns, pits and other small depressions formed in a permafrost region by the melting of ground ice, in exterior appearance, it resembles uneven karst topography formed by the solution of limestone (Bates and Jackson, 1984).

Thrust faults A fault with a dip of 45° or less over much of its extent, on which the hanging wall appears to have moved upward relative to the footwall. Horizontal compression rather than vertical displacement is its characteristic feature. (Bates and Jackson, 1984).

Tick (Venus) A type of volcano. About 65.6 kilometers (41 miles) across at the base, this volcano has a flat, concave summit 34.8 kilometers (22 miles) in diameter. The sides of the volcano are characterized by radiating ridges and valleys. The rim of the volcano to the west appears to have been breached by dark lava flows that emanated from a shallow summit pit 5.4 kilometers (3.4 miles) in diameter and traveled west along a channel. A series of coalescing, collapsed pits 2 to 10 kilometers (1.2 to 6.2 miles) in diameter is 10 kilometers (6.2 miles) west of the summit rim (Planetary Geology for Teachers, 1999).

Tidal heat The frictional heating of a satellite’s interior caused by repeated flexure induced by the gravitational field of its parent planet or other nearby more massive object (Planetary Geology for Teachers, 1999).

Transform (strike-slip) A special variety of strike/slip fault, along which the displacement suddenly stops or changes form. Many transform faults are associated with mid-oceanic ridges, where the actual slip is opposite from the apparent displacement across the fault. Also, a plate boundary that ideally shows pure strike/slip displacement (Bates and Jackson, 1984).

Transition zone (Earth) A region with in the upper mantle bordering the lower mantle at a depth of 410 to 1000 km, characterized by a rapid increase in density of about 20% and an increase in seismic wave velocities (Bates and Jackson, 1984).

Transport The movement of sediment by natural agents (such as flowing water, ice, wind or gravity) either as solid particles or in solution, from one place to another (Bates and Jackson, 1984).

Tritium An isotope of hydrogen with 2 neutrons and 1 proton in the nucleus, designated by the symbol ³H (Beatty and Chaikin, 1990).

Trough A long linear depression (Welcome to the Planets, 1995).

T-tauri stage The pre-main sequence stage in the development of certain stars characterized by rapid and erratic changes in brightness. Planetesimals and planets form during the T-tauri stage (Kuhn, 1998).

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Undifferentiated (meteorites) Resembling Earth rocks, the stones are primitive, i.e., they are made up of primordial Solar System material. Primitive stones are chondrites, the class that makes up most of the meteorites that fall to Earth (~87% of falls and ~52% of finds). Chondrites are so-called because of the presence of small round chondrules, which are typically ~ 1 mm diameter and appear to be material condensed from the primordial solar nebula (Beatty and Chaikin, 1990).

Uplands and highlands (Venus) Uplands represent a transition of terrain from the lowland plains to the highlands. Doming of the lithosphere implies large upwelling thermal plumes from the mantle that are possibly responsible for creating these uplands. Doming resulted in extension of the crust above, and led to predominantly extensional tectonic features, such as fracture belts, troughs, grabens, and rifts. The best example of an upland dome is Beta Regio, located around 280° E. longitude and 30° N. latitude. This region is 2500 kilometers across and contains a huge shield volcano, numerous crisscrossing faults, and several circular coronae (Planetary Geology for Teachers, 1999).

Upper mantle (Earth) The part of the mantle which lies above a depth of about 1000 km and has a density of 3.40 g/cm³, in which P-wave velocity increases from about 8 to 11 km/sec with depth and S-wave velocity increases from about 4.5 to 6 km/sec with depth. It is presumed to be peridotitic in composition and is sometimes referred to as the asthenosphere (Bates and Jackson, 1984).

U-shaped valley A deep, steep-sided through leading down from a cirque, and excavated by an alpine glacier that has widened, deepened, and straightened a preglacial river valley; e.g. Yosemite Valley, California (Bates and Jackson, 1984).

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Vent An opening or fissure in a planet's surface through which volcanic material erupts (Welcome to the Planets, 1995).

Viscosity A measure of resistance to flow (Welcome to the Planets, 1995).

Volatile elements A chemical element that exists in a gaseous state at relatively low tempertures (Bates and Jackson, 1984).

Volcanic glass Natural glass produced by the cooling of molten lava, or some liquid fraction of it, too rapidly to permit crystallization. Examples are obsidian, pitchstone, tachylyte, and the glassy groundmass of many extrusive rocks (Bates and Jackson, 1984).

Volcanic plains (Mars) Basaltic lava plains are the most widespread and voluminous features on Mars, covering over 60 percent of its surface. The plains have been compared to the huge flood basalts on the Earth and the lunar Maria, but are much more extensive (Planetary Geology for Teachers, 1999).

Volcanic plains (Venus) Basaltic lava plains are the most widespread and voluminous features on Venus. The plains have been compared to the huge flood basalts on the Earth and the lunar Maria, but are much more extensive (Planetary Geology for Teachers, 1999).

Volcanic rocks (extrusive) Magma that erupts onto the surface is called lava. It cools very quickly because the temperature at the surface is relatively low, and crystals will probably cease growth because the liquid is quenched. Volcanic glass is a type of quenched lava, and typical volcanic rocks are rhyolite, basalt, and andesite (Planetary Geology for Teachers, 1999).

Volcanic shields A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava (Bates and Jackson, 1984).

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Weathering The destructive processes by which rocks are changed in response to exposure to atmospheric agents at or near the Earth’s surface, with little or no transport of the loosened or altered materials (Bates and Jackson, 1984).

Wind streaks Dark and light streaks thought to be caused by the removal of a thin veneer of dust, revealing a darker surface beneath (Planetary Geology for Teachers, 1999).

Wrinkle ridges (Moon) A sinuous, irregular, segmented, apparently smooth elevation occurring within the borders of a mare region of the Moon’s surface characterized by dike-like outcrops, crest-top craters and longitudinal rifts. Wrinkle ridges are up to 35 km wide and 100 m high, and may extend for hundreds of kilometers. The likely originate in fissure eruptions or from volcanic activity along fractures (Planetary Geology for Teachers, 1999).

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Yardangs A long, irregular, sharp-crested, undercut ridge between two round-bottomed troughs, carved on a plateau or sheltered plain in a desert region by wind erosion, and consisting of soft but coherent deposits in the direction of the dominant wind, and may be up to 6 meters high and 40 meters wide (Gray et al., 1974).

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Zones (Jupiter) Thick rotating clouds swirl around the planet, blowing in opposite directions in adjacent bands. The lighter colored bands are called zones (Planetary Geology for Teachers, 1999).

 

 

Bates, R.L. and Jackson, J.A., editors, 1984, Dictionary of geologic terms, (third edition), American Geologic Institute: New York, Anchor Books, 571 p.

Beatty, J.K. and Chaikin, A., editors, 1990, The new solar system, (third edition): Massachusetts, Sky Publishing Corp., 326 p.

Gray, M., McAfee Jr., R. And Wolf, C.L., editors, 1974, Glossary of geology: Virginia, American Geologic Institute, 805 p.

Kuhn, K.F., 1998, In quest of the universe (second edition): Massachusetts, Jones and Bartlett, 568 p.

Planetary Geology for Teachers, 1999, Idaho State University:

http://wapi.isu.edu/Geo_Pgt/index.htm

http://pds.jpl.nasa.gov/planets/welcome.htm.