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
a measure of the reflectivity of a surface; using the Moon as an
example, the ratio of sunlight reflected from the Moon to that
- Allochthonous- as used here, a part of the lunar crust that has
moved from its original position by displacement along a
- 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
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.
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.
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.
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.
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
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
- 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.
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
(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
- 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
- Earthshine- sunlight reflected from Earth that illuminates the
rock material ejected during the process of impact (as from a
meteorite impact crater) or by explosive volcanic action.
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
- 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.
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
- 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
- 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
the science of determining the exact size and shape of bodies in
the solar system, and of the distribution of mass within the
- 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
- Glass- a
form of igneous rock lacking crystal structure, produced by the
rapid cooling of a magma.
an elongate depression formed by the downward displacement of a
block of crust along faults bordering its long sides.
(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.
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
a forceful collision. For example, the impact of a meteoroid
traveling at high velocity with the surface of Earth or the
- 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),
- 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.
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
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.
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
plural of mare.
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
as used here, a discrete mountain mass; typically is bright and
composes part of the uplifted mountainous rings around circular
- 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
- 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
- Morphology- as used here, the external shape and arrangement
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
- 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.
the exposed part of a unit of bedrock; rock not covered by surface
debris or vegetation.
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
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
- 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.
loose fragmental rock debris derived from and mantling a slope.
(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
- 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
- Squeeze-up- a small extrusion of viscous lava on the
solidified surface of a lava flow.
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
- 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
- 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,
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
an older, lighter, more densely cratered area of the Moon;
encompasses all the lunar surface except the maria (plural =
- 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
- 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.
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.