Apollo Over the Moon: A View from Orbit


Many of the geological terms listed in this glossary, which were originally defined for terrestrial use, have been modified or shortened for this volume in keeping with their commonly accepted usage by lunar geologists.
Albedo- a measure of the reflectivity of a surface; using the Moon as an example, the ratio of sunlight reflected from the Moon to that reaching it.
Allochthonous- as used here, a part of the lunar crust that has moved from its original position by displacement along a fault.
Anorthosite- a light-colored igneous rock composed almost entirely of the mineral group plagioclase feldspar.
Asteroid- a subplanetary body within the solar system, synonymous with "planetoid."
Autointrusion (or autoinjection)- the movement of magma from the lower, still liquid, part of a flow into cracks in the hardened crust of the flow.
Avalanche- a mass of rock material that has slid or fallen rapidly under the influence of gravity; one form of mass wasting.
Basalt- a dark-colored igneous rock that most commonly solidifies on the surface in the form of lava flows. It is the dominant rock type in the lunar maria.
Base surge- a cloud of gas and suspended debris that moves radially outward across the surface at high velocity; may result from a violent volcanic explosion or from the explosion caused by a body traveling at high velocity when it impacts on the surface of a planetary body.
Basin- a large circular area on the Moon, typically 300 or more km in diameter, surrounded by one or more mountainous rings; may be occupied to varying extent by mare material, and may or may not be lower in elevation than the surrounding terrain. Basins are considered by most workers to be impact scars.
Bedrock- in situ solid rock.
Bistatic radar- a method of studying the electrical properties of the surface by reflected radio waves. In the lunar experiment the waves were emitted from the CSM and received on Earth both directly and after reflection by the lunar surface.
Breccia- a rock composed of fragments of preexisting rocks.
Cartography- the science or art of making maps.
Central peak (or central uplift)- a mountainous mass in the center of many impact craters more than 40 km in diameter; formed by the inward and upward movement of material originally below the level of the crater floor.
Cinder cone- a conical hill composed of volcanic cinders, ash, and larger fragments of ejecta.
Colluvium- a general term to include loose rock and soil material that accumulates at the base of a slope as the result of mass wasting processes. See talus.
Comet- seen as a light-giving body having a bright head and a luminous tail moving through space under the gravitational influence of the Sun. Mass-to-size ratio is low. Apparently composed of frozen gases, dust, and other cosmic debris.
Cosmic debris- material that originates anywhere in the universe beyond Earth's atmosphere; includes material believed to represent primordial condensation or sublimation products and debris resulting from collisions of meteorites, asteroids, and comets with each other, and with the planets and the Moon.
Crater- a hole or depression. Most are roughly circular or oval in outline, and, typically, depth is much less than diameter. On Earth most natural craters are of volcanic origin, whereas on the Moon most are of impact origin. Secondary craters are produced by the impact of material ejected from the parent or primary crater.
Creep- the slow, more or less continuous, permanent deformation and displacement of material under the influence of gravity; one form of mass wasting.
Crystalline rock- igneous rock consisting mainly or entirely of crystals.
Deceleration dunes- dunelike lobes of ejecta from impact craters formed as the velocity of the base surge cloud
decreases; most commonly formed on slopes facing toward the source of base-surge flow.
Degradation- the wearing down and general lowering of an area or a feature by any process of weathering and erosion.
Differentiation, magmatic- a general term for the various physicochemical processes that lead to the formation of two or more rock types from a common igneous melt.
Dike- a tabular body of intrusive rock that cuts across the planar structure of the surrounding (and older) rocks. See intrusion.
Diurnal (adj.)- recurring daily; in the case of the Moon, recurring every 28 Earth days.
Doppler tracking- a system for measuring the trajectory of spacecraft from Earth, using continuous radiowaves and the Doppler effect. Because of this effect, the frequency of the radiowaves received on Earth is changed slightly by the velocity of the spacecraft.
Drag fold- a subsidiary fold developed in response to movement along or within a larger structural feature.
Dune- a low mound or ridge of loose rock material. Most dunes on Earth are formed by wind action, whereas most of those on the Moon apparently are formed during the ejection of material from an impact crater.
Earthshine- sunlight reflected from Earth that illuminates the lunar surface.
Ejecta- rock material ejected during the process of impact (as from a meteorite impact crater) or by explosive volcanic action.
Erosion- a general term to include all processes whereby rock materials are disintegrated or dissolved and transported from one place to another, whether the agency be water, ice, wind, gravity, or impact cratering. In the case of the Moon, impact cratering is the dominant erosional process.
Extrusion- the process of emitting volcanic material (as liquid lava, particulate matter suspended in bases, or as solid fragments) onto the surface of a planetary body; also, the rock so formed.
Fault- a fracture along which rock masses have been displaced.
Fault scarp- a steep slope or cliff caused by displacement along a fault and, if unmodified by erosion, representing the exposed surface of the fault.
Flux- the rate of transfer of some quantity across a unit area. As used here it applies to the rate at which bodies impact the lunar surface.
Gamma ray- highly penetrating rays emitted by radioactive substances. Gamma radiation from the lunar surface was measured by gamma-ray spectrometers aboard the Apollo 15 and 16 spacecraft.
Gardening- mechanical mixing of the unconsolidated surface debris that occupies most of the Moon's surface, the regolith, or "lunar soil," by various processes, including meteorite impact and mass wasting.
Geodesy- the science of determining the exact size and shape of bodies in the solar system, and of the distribution of mass within the bodies.
Geophysics- the study of the physical properties of Earth, the Moon, and planetary bodies. Basic divisions are solid-Earth, atmospheric, hydrospheric, and magnetospheric. Apollo lunar geophysical experiments included studies of gravity, magnetism, heat flow, radioactivity, seismology, space physics, geodesy, and meteorology.
Glass- a form of igneous rock lacking crystal structure, produced by the rapid cooling of a magma.
Graben- an elongate depression formed by the downward displacement of a block of crust along faults bordering its long sides.
Igneous (adj.)- pertaining to or describing a rock that has solid)fied from molten material (magma), or the processes and conditions related to the formation of such rocks.
Imbrian- a unit of geologic time that describes the interval of time between the formation of the Imbrium basin and the end of the accumulation deposition of the lavas that occupy most of the maria on the Moon's near side.
Imbrian sculpture- a system of scarps, ridges, and troughs radial to the center of the Imbrium basin and transecting much of the lunar surface. The features are a response to the event that formed the basin, and, because of their wide extent, are useful in determining the relative age of rock units far from the basin.
Impact- a forceful collision. For example, the impact of a meteoroid traveling at high velocity with the surface of Earth or the Moon.
Intrusion- the process of emplacing magma into preexisting rock; also, the rock so formed (for example, a dike).
Isostatic equilibrium- the adjustment of the crust to maintain equilibrium among blocks of different density; examples: blocks of dense material will "sink" more than less dense blocks; excess mass or density in the upper part of a block is compensated by a deficit of mass in the lower part.
Laccolith- an igneous intrusion: top, comical; bottom or floor, essentially flat; and outline (when viewed from above), roughly circular.
Laser altimeter- an instrument used to measure distance between two points by means of the traveltime of a pulse of light. In the lunar laser altimeter, light is transmitted from the CSM and reflected from the lunar surface back to the detector in the CSM. Knowing the position and elevation of the spacecraft from orbital data, differences in elevation of the lunar surface were measured along the ground track.
Lava- molten rock material (magma) that has reached the surface; also, the solid)fied rock.
Lava channel- a channel on the upper surface of a partly or completely solidified body of lava through which liquid lava has flowed. Its rims may be higher than surrounding
terrain, like the natural levees along some rivers on Earth.
Lava tube- a tube within a body of partly or completely solid)fied lava through which liquid lava has flowed. If near the surface, rocks above the tube may collapse, resulting in a channellike depression on the upper surface of the lava body.
Levee- on Earth, a raised embankment bordering a river channel; on the Moon, a raised embankment along a presumed lava channel.
Limb- as used here, the east or west edge of the Moon when viewed from the direction of Earth. This term generally applies to the outer edge of the apparent disk of any celestial body.
Lineament- a broad term used to include any visible linear trend. It is commonly, but not always, of regional extent. It may consist of a single, more or less continuous feature; an alined series of a particular type of feature; or an alined series of unlike features. It is commonly interpreted as marking points of major dislocations of the crust.
Lithology- the physical character of a rock.
Magma- molten rock material generated within the Earth or Moon that cools to form igneous rocks.
Mare- a dark, level, relatively smooth part of the lunar surface (so distinct from the lunar highlands or terrae that most large mare areas on the near side are visible from Earth with the unaided eye). Most geologists now agree that they are underlain by solid)fied (basaltic) lava flows. (plural = maria)
Mare ridge- a ridge on a mare surface. The morphology varies considerably, but typically length is much greater than width, and width is much greater than height. (Also called "wrinkle ridge.")
Maria- plural of mare.
Mascon- literally, mass concentration; an area of the lunar crust characterized by an excess of mass. Those detected to date coincide with the circular maria, indicating the presence of relatively dense materials (basaltic lava) at shallow depth.
Massif- as used here, a discrete mountain mass; typically is bright and composes part of the uplifted mountainous rings around circular basins.
Mass spectrometer- an instrument for determining chemical species in terms of isotopic mass and relative abundances of isotopes within a compound. On the Apollo 15 and 16 missions a mass spectrometer was used to measure composition and density of the lunar atmosphere from the CSM in orbit.
Mass wasting- a general term for the downslope movement of rock material solely under the influence of gravity; includes slow displacement such as creep and rapid displacements such as earth flows, rock slides, and avalanches.
Metamorphism- the mineralogic, textural, and structural adjustment of rocks to physical and chemical conditions different from those under which the rocks odginally formed. Metamorphism by impact-generated shock is the dominant type of metamorphism in lunar rocks.
Meteorite- a meteoroid that has arrived on the surface of a moon or planet from outer space. Composition ranges from silicate rock to nickel-iron metal; size ranges from that of a submicroscopic particle to that of a body approaching the size of an asteroid or planetesimal.
Meteoroid- one of the countless small solid bodies in the solar system.
Morphology- as used here, the external shape and arrangement of landforms.
Mosaic- a composite picture formed by assembling overlapping aerial or orbital spacecraft photographs taken from different camera positions, or from the same camera position but at different angles.
Orthophotograph- a photographic copy, normally of an aerial or orbital photograph, that has been processed to remove the effects of camera tilt and the distortion caused by perspective viewing so that all distances measured on the orthophotograph are proportional by the same factor to horizontal distances measured on the ground.
Outcrop- the exposed part of a unit of bedrock; rock not covered by surface debris or vegetation.
Plains- a general term to describe the relatively level areas of the lunar surface. They range from light to dark and may be smooth or rough. The maria are commonly included as one variety of plains.
Primordial (adj.)- as used here, the oldest lunar rocks- those created during the Moon's formative stages.
Projectile- specifically, in this volume, a body that strikes the lunar surface. A projectile may be a meteoroid or other object from outer space, rarely a spacecraft or spacecraft component, or, most commonly, a discrete rock fragment explosively ejected from a crater.
Ray- narrow light or dark streaks that extend radially outward from some lunar craters. They are a natural result of the impact process and form when ejected material covers or disturbs the preexisting surface.
Regolith- unconsolidated fragmental rock debris, regardless of origin, that almost everywhere forms the surface of the Moon; also called the "lunar soil."
Rille- a trenchlike valley on the Moon. Rilles vary widely in size, but width and depth are small compared to length. Viewed from above they may be sinuous, straight, or angular.
S-Band transponder- a device aboard the CSM that uses the traveltime of radiowaves transmitted from Earth and returned to it to aid in tracking the spacecraft. As an experiment on board Apollos 13 to 17, it measured small variations in the Moon's gravity under the ground track of the spacecraft.
Scarp- a relatively straight cliMike face or slope that separates terrain lying at different levels.
Scree- loose fragmental rock debris derived from and mantling a slope. See talus.
Seismic (adj.)- related to mechanical vibrations within Earth or the Moon. A common probable cause of seismic vibrations on the Moon is the impact of meteorites.
Shock metamorphism- the permanent changes (physical and chemical) produced in rocks by the passage of a transient high-pressure shock wave. The only known natural cause is by hypervelocity impact, thus the expression is essentially synonymous with impact metamorphisms.
Slickensides- the polished striations on a rock surface caused by friction generated by faulting.
Specific gravity- the ratio of the density of a substance to the density of another-commonly water. The average specific gravity of lunar basaltic rock samples is about 3.4, which means that a unit volume weighs about 3.4 times as much as the same volume of water.
Squeeze-up- a small extrusion of viscous lava on the solidified surface of a lava flow.
Stellar- of or pertaining to the stars.
Stereoscope- an optical device to facilitate obtaining a stereoscopic image. (See next definition.)
Stereoscopic view, image- the impression of the third dimension, normally depth, which can be obtained by viewing two photographs of the same area taken from slightly different points.
Structure- the general disposition (attitude, arrangement, or position) of the rock masses of a region or area. The term "structure" also is applied to individual structural features, such as that of a graben, fault, or basin.
Summit crater- a crater occupying the crest of a volcanic cone or dome.
Superposed- that condition wherein one stratified rock unit overlies, and hence is younger than, another such unit; also, a physical feature such as a crater located on, and younger than, another feature.
Talus- loose fragmental rock material derived from a cliff or slope and lying at its base.
Tectonic movement- the displacement of large masses of the crust, whether by uplift, subsidence, or large-scale folding and faulting. On the Moon it is considered to include the displacement caused by large-scale impact events.
Terminator- the line separating the illuminated and darkened areas of a nonluminous planetary body such as Earth or the Moon. In the absence of an atmosphere, as on the Moon, this line is very sharply defined.
Terra- an older, lighter, more densely cratered area of the Moon; encompasses all the lunar surface except the maria (plural = terrae).
Thrust fault- a relatively low-angle fracture along which one rock mass has moved upward and over another.
Topographic (adj.)- pertaining to the three-dimensional configuration of the solid surface of a planetary body and to its graphical description, usually on maps or charts.
Trajectory- the path of a moving body through space or the atmosphere.
Transient (adj.)- passing quickly into and out of existence; that is, of short duration.
Transverse fault- a fault that strikes obliquely or perpendicularly to the general structural trend.
Vesicle- a cavity in a lava formed by the entrapment of a gas bubble during solid)fication of the lava.
Viscosity- the property of a fluid that resists internal flow; its internal friction.
Volcanism- includes all the processes whereby magma and its associated fluids rise in the crust and are extruded onto the surface and ejected into the atmosphere.
Wrinkle ridge- synonymous with mare ridge.
X-ray fluorescence experiment- an experiment carried onboard the Apollo 15 and 16 spacecraft for determining the chemical composition of the lunar surface. It records the fluorescent X-rays that are emitted from the Moon's surface as a result of its bombardment by X-rays from the Sun.