GLOSSARY Geophysical, geological, petrological, and mineralogical terms used in planetary science, in particular concerning research in this book on the chronology and evo­ lution of the Mars and the Moon, are defined. 1. Mars, Moon, and Planetary Bodies, Their Structure and Geological Units Amazonian Period: youngest of the 3 Martian stratigraphic periods Noachian, Hesperian, Amazonian, defined by Tanaka (1986) in terms of crater density. Aquifer: water-carrying rock formation below a planet's surface. Asteroids: rocky, metallic, 1 cm - 1000 km-sized objects, orbiting the Sun, mostly in the Asteroid Main Belt between the orbits of Mars and Jupiter; consist of pristine solar system material; most likely bodies that never coalesced into a planet. Asthenosphere: plastic zone under the lithosphere. In the case of Earth, this is broadly equivalent to the upper mantle. Caldera: a volcanic depression usually rougher circular or oval, caused by collapse and often found on summits of volcanos. Canal: term introduced as "canali" by Schiaparelli; Lowell ('" 1890) charted these hypothetical linear features on Mars, which he believed to be constructed by Mar­ tians. Not confirmed by later studies but some are associated with wind streaks and canyons; not to be confused with "channel." Chaotic Terrain: distinctive fractured surface area on Mars usually at the head of large channels; collapsed to '" 1 - 2 km below the surrounding terrain presumably due to rapid release of melted subsurface ice. Channel: apparent dry riverbeds on Mars, discovered by Mariner 9 in 1972. Core: central portion of a differentiated planet chiefly consisting of Fe and Ni. Crater: bowl-shaped depression with a raised rim, formed by a meteoroid impact (impact or meteorite crater); not applied to volcanic features in modern usage. Crust: outer layer of a terrestrial planet. Dentritic Network: same as valley network. Dorsum: ridge (Latin). Dunes: mound, ridge, or hill of wind-blown sand. Duricrust: platy Martian soil, apparently cemented by evaporites. Endogenic: arising from the interior of the Earth (opposed to exogenic). Eskers: bouldery ridges of sediment left by melting glaciers at their margins; meandering sand lines and gravel deposited by streams beneath glaciers. Fissure: crack extending far into a planet through which magma erupts. Flow-Lobes: lobe-shaped feature in the flow field of volcanoes, generated by in­ flation of the lava crust as a result of magmatic overpressure in the associated lava core; larger lobate features are also found in some Martian craters' ejecta blankets, thought to originate by interaction with ground ice . .w, Space Science Reviews 96: 471-482, 2001. ..~ © 2001 Kluwer Academic Publishers. 472 THE EDITORS Fluvial Channel: the conduit through which a river flows. Fluvial Valley: linear depression containing many channels. Fossa: ditch (Latin); long, narrow, shallow depressions. Fretted Channel: channel infretted terrain with steep walls and a flat floor. Fretted Terrain: fractured Martian surface area; eroded by landslides, volcanic flooding, or ice-rich debris flowing off walls of faulted valleys. Frost Mound: area of water frost layer. Graben: elongate, depressed crustal unit, bounded by faults on its long sides. Hellas: one of the largest impact basins on Mars, dominating structure and flow channel orientation over a large part of the southern hemisphere. Hesperian Period: middle of the 3 Martian stratigraphic periods defined by Tanaka (1986) in terms of crater density. Highlands: oldest cratered lunar surface areas; chemically distinct from the mare. Ignimbrites: deposits of pyroclastic flows of ash, gas and larger particles. Lahar: Indonesian word referring to mudflows that are volcanic in origin. Lineated Valley Fill: material in steep-walled Martian valleys, comprising their floors which have lineations (ridges and grooves); resemble alpine glaciers. Lithosphere: rigid outer rind of a planet (crust and upper mantle), as distinct from the underlying, more fluid, asthenosphere. Longitudinal Grooves: due to differential shear and lateral spreading at high ve­ locities of landslides between mounds of interior layered deposits. Magma Ocean: magma layer, covering an early planet; term derived from evi­ dence for same on the Moon 4.5 Gyr ago. Mantle: portion of a planet between crust and core; ",2900 Ian thick on Earth. Maria: dark lunar surface area; originally thought to be seas by classical observers; composed of basaltic lava plains; chemically distinct from the highlands. Martian Meteorites: also called SNC meteorites; include the 3 meteorite classes named after the location of the fall of a prominent member of each class: Sher­ gottites, N akhlites, Chassignites; identified to originate from Mars because of their young ages, basaltic composition, and inclusion of Martian atmospheric gas. Meteoroids: small interplanetary debris, from asteroids and comets. Meteor: a meteoroid as it enters the atmosphere at speeds of 15 - 70 kmls. Meteorites: solid objects striking a planet's surface, categorized as stony, iron, and stony-iron; mainly of asteroidal origin, a few from the Mars and the Moon. Moberg Mountain: see Table Mountain. Noachian: oldest of the 3 Martian stratigraphic periods defined by Tanaka (1986) in terms of crater density. Nues Ardentes: pyroclastic flow that traveled into water, separating gravitationally into a lower part containing most of the solid fragmental mass. Obliquity: tilt of a planet's axis from perpendicular to the plane of the ecliptic. Outflow Channel: Martian channel eroded by a fluid, apparently water; often starts full size in chaotic terrain; few, if any, tributaries; some 10 Ian wide and up to GLOSSARY 473 2000 km long; distinct flow features such as eroded craters with teardrop-shaped tails, scour marks, "islands". Palsas: mound of peat resulting from the formation of a number ice lenses beneath the ground surface; 2 - 30 m wide and 1 - 10 m high; similar to a pingo. Patera: ancient Martian volcano; often with the semi-circular Caldera crater at the volcano summit ('"'-'80 km across). Periglacial Landforms: topographic features along the margins of glaciers. Pingo: large conical mound containing an ice core; up to 60 - 70 m high; form in regions of permafrost; similar to paisa. Planetesimals: 1 m to 100 km-sized bodies that mostly accreted to planets. Planitia: plain (Latin); smooth low area. Plate Tectonics: tectonic activity associated with the breakup of the lithosphere into moving pieces, or "plates". Polygon: lineations, arranged in a polygonal pattern that outline flat-lying areas, scale is up to 5 - 20 km across. Pressure Ridges: long, narrow wavelike folds in the surface of lava flows. Rampart Crater: crater in which the ejecta blanket is thick and appears to have formed from slurry due to impact into ice-rich material. Ray: streak of material blasted out and away from an impact crater. Ridged Plains: formed by crust welling up between separating plates. Rille: long channel in lunar maria formed by open or underground lava flow. Rock Glaciers: flowing frozen debris whose surface layer thaws in summer. Runoff Channel: flow of water from precipitation to stream channels when the infiltration capacity of an area's soil has been exceeded. Rupes: scarp (Latin). Scabland Topography: created by catastrophic glacial outburst floods. Sediment: solid rock or mineral fragments deposited by wind, water, gravity, or ice; precipitated by chemical reactions; accumulated as loose layers. SNC Meteorites: see Martian Meteorites. Soil: upper layers of sediment. Source: location where igneous matter (lava and gases) erupts onto the surface. Table or Moberg Mountain: special type of volcano (e.g. in Iceland) produced by subglacial eruptions; the discharge of meltwater is calledjokulhaup. Talik: island of thawed ground surrounded by permafrost. Terrain: area of the surface with a distinctive geological character. Tharsis: broad, domed volcanic plains of Mars dominating much of one hemi­ sphere; major volcanoes are Olympus, Arsia, Pavonis, Ascraeus Mons. Tidal Stress: differential gravitational force per unit area acting on a planetary body by the Sun, a moon, or another planet, resulting in periodic bulging. Thyas: subglacial volcanic deposits from eruptions beneath glacial ice. Valles Marineris: major Martian canyon complex; similar in scale to the Red Sea. Valley: valleys on Mars have not been filled by water to their rim and usually no river bed is observed inside them; may have formed by sapping processes. 474 THE EDITORS Valley Networks: valley systems, often with complex, multiply-branched patterns of tributaries; mostly in ancient upland areas of Mars. Vallis: sinuous valley (Latin). Vent: opening in a planet's surface ejecting lava, gases, and hot particles. Ventifact: individual pieces and pebbles etched and smoothed by aeolian erosion i.e. abrasion by wind-blown particles. Vesicle: bubble-shaped cavity in a volcanic rock formed by expanding gases. Volcano: mountain formed from the eruption of igneous matter. Yardang: aeolian erosional rock feature formed by deflation and abrasion; elon­ gated and aligned with the most effective wind direction. 2. Geological Processes Abrasion: process of wearing down or rubbing away by means of mechanical friction by particles in ice, water or wind. Aeolian Processes: wind processes; from Greek Aeolus. Creep, Frost Creep: slow downslope movement of particles on slopes covered with loose, weathered material; directly by gravitation or by frost heaving (inter­ stitial