Lunar science: The Apollo Legacy

Abstract

A general review of lunar science is presented, utilizing two themes: a summary of fundamental problems relating to the composition, structure, and history of the moon and a discussion of some surprising, unanticipated results obtained from Apollo lunar science. (1) The moon has a crust of approximately 60‐km thickness, probably composed of feldspar‐rich rocks. Such rocks are exposed at the surface in the light‐colored lunar highlands. Many highlands rocks are complex impact breccias, perhaps produced by large basin‐forming impacts. Most highlands rocks have ages of ∼3.9 × 109 yr; the record of igneous activity at older times is obscured by the intense bombardment. The impact rate decreased sharply at 3.8–3.9 × 109 yr ago. The impact basins were filled by flows of Fe‐ and, locally, Ti‐rich volcanic rocks creating the dark mare regions and providing the strong visual color contrast of the moon, as viewed from earth. Crustal formation has produced enrichments in many elements, e.g., Ba, Sr, rare earths, and U, analogous to terrestrial crustal rocks. Compared with these elements, relatively volatile elements like Na, K, Rb, and Pb are highly depleted in the source regions for lunar surface rocks. These source regions were also separated from a metal phase, probably before being incorporated into the moon. The physical properties of the lunar mantle are compatible with mixtures of olvine and pyroxene, although Ca‐ and Al‐rich compositions cannot be ruled out. Deeper regions, below ∼1000 km, are probably partially molten. (2) Lunar rocks cooled in the presence of a magnetic field very much stronger than the one that exists today, owing either to dynamo action in an ancient molten core or to an external magnetization of the moon. Lunar soil properties cannot be explained strictly by broken‐up local rocks. Distant impacts throw in exotic material from other parts of the moon. About 1% of the soil appears to be of meteoritic origin. Vertical mixing by impacts is important; essentially all material sampled from lunar cores shows evidence of surface residence. The surface layers of lunar material exposed to space contain a chemical record of implanted solar material (rare gases, H) and constituents of a lunar atmosphere (40Ar, Pb). Large isotopic fractionation effects for O, Si, S, and K are present. Physical properties of the surface layers are dominated by radiation damage effects. Lunar rocks have impact craters (≤1 cm) produced by microgram‐sized interplanetary particles. The contemporary micrometeorite flux may be much higher than is indicated by the microcrater densities, indicating time variations in the flux. Particle track studies on the returned Surveyor camera filter first showed that the Fe nuclei were preferentially enhanced in solar flares.