GLOSSARY Geophysical, Geological, Petrological, and Mineralogical
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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