Albedo — The percentage of the incoming sunlight that is reflected by a natural surface.
ALSEP — Acronym for the Apollo Lunar Surface Experiments Package. The precise makeup of the instruments for each ALSEP varied from mission to mission.
Basin — A large impact crater, usually with a diameter in excess of 100 kilometers. Most basins have been modified by degradation of the original basin relief through downslope movement of debris and flooding of the basin interior by lavas.
*Crater — A typically bowl-shaped or saucer-shaped pit or depression, generally of considerable size and with steep inner slopes, formed on a surface or in the ground by the explosive release of chemical or kinetic energy; e.g., an impact crater or an explosion crater.
Ejecta — The material thrown out of an impact crater by the shock pressures generated during the impact event. Ejecta generally covers the surface around an impact crater to a distance of at least one crater diameter, with individual streamers of material extending well beyond this distance (see rays). The ejecta blanket of a crater becomes less visible with increasing age of the crater.
EVA — Acronym for Extra-Vehicular Activity - any activity that takes an astronaut outside the spacecraft during the mission.
Gibbous moon — The phase of the Moon during which more than half, but less than all, the visible hemisphere of the Moon is illuminated by sunlight.
Highlands — The densely cratered portions of the Moon that are typically at higher elevations than the mare plains. The highlands contain a significant proportion of anorthosite, an igneous rock made up almost entirely of plagioclase feldspar.
Lava — A volcanic rock protruded by the eruption of molten material.
LVR — Acronym for Lunar Roving Vehicle or Rover.
Mare — The low albedo plains covering the floors of several large basins and spreading over adjacent areas. The mare material is comprised primarily of basaltic lava flows, in contrast to the anorthosites in the highlands.
*Massif — A massive topographic and structural feature, especially in an orogenic belt, commonly formed of rocks more rigid than those of its surroundings. These rocks may be protruding bodies of basement rocks, consolidated during earlier orogenies, or younger plutonic bodies. Examples are the crystalline massifs of the Helvetic Alps, whose rocks were deformed mainly during the Hercynian orogeny, long before the Alpine orogeny.
Phase angle — The angle between the incident sunlight and the viewing direction when looking at an illuminated surface. Low phase angles result in relatively few shadows being cast by the surface relief.
Ray — A streamer of ejecta associated with an impact crater. Rays are most often of higher albedo than their surroundings. The albedo contrast may result from either disruption of the l ocal surface by the ejecta or by emplacement of ejecta on the surroundings, or both.
*Rille — One of the several trenchlike or cracklike valleys, up to several hundred kilometers long and 1-2 km wide, commonly occurring on the Moon’s surface. Rilles may be extremely irregular with meandering courses (“sinuous rilles”), or they may be relatively straight (“normal rilles”); they have relatively steep walls and usually flat bottoms. Rilles are essentially youthful features and apparently represent fracture systems originating in brittle material.
Scarp — A change in topography along a linear to arcuate cliff. The cliff may be the result of one or more processes including tectonic, volcanic, impact-related, or degradational processes.
Secondary craters — Craters produced by the impact of debris thrown out by a large impact event. Many secondary craters occur in clusters or lines where groups of ejecta blocks impacted almost simultaneously.
*Definitions marked with an asterisk are from the Glossary of Geology, 2nd edition, American Geological Institute, Falls Church, Virginia, 1980.