Thematic Articles

The lunar poles feature a microenvironment that is almost entirely unknown to planetary science. Because of the very small tilt of the Moon’s axis with respect to the Sun, craters and other depressions near the poles are permanently shaded from direct sunlight. As a consequence, these surfaces should have maintained extremely low temperatures, well under 100 K, for billions of years. There is some evidence that these surfaces act as cold traps, capturing and sequestering volatiles from the Moon and elsewhere. Most popular attention has focused on the possible presence of water ice that might be used by astronauts in the future, but the poles may offer a unique scientific resource. Possible sources for volatiles at the lunar poles range from the Sun to interstellar clouds, and if present, such volatile deposits may provide unique information about many aspects of planetary science.

Geophysical data obtained from orbit and surface stations show that the Moon is a differentiated body possessing a crust, mantle, and core. The crust is on average about 40 km thick, and impact events with asteroids and comets have excavated materials to great depths within the crust. Moonquakes that are correlated in time with Earth-raised tides occur about halfway to the center of the Moon and suggest that the deepest portion of the mantle might be partially molten. The lunar core is relatively small in comparison with the cores of the terrestrial planets, with a size less than one-quarter of the Moon’s radius.

The first rocks to be returned from the Moon by the Apollo 11 astronauts were basalts from the mare basins. Analysis of these rocks led to the hypothesis that the mare lavas were remelts of a lunar interior that had experienced an early, profound chemical differentiation event produced by crystallization of a planet-wide lunar magma ocean. As Apollo missions continued to explore and sample the lunar surface, an increasingly diverse suite of mare volcanic rocks was discovered. Mare magmatism is concentrated in the time interval of 3.8 to 3.0 billion years before present. Among the samples were tiny, glassy spheres of ultramafic composition that formed during volcanic fire-fountain eruptions into the cold lunar vacuum. The results of high-pressure and high-temperature laboratory melting experiments on lunar mare basalts and volcanic glasses, along with geochemical evidence and physical modeling, affirm that remelting of the solidified products of a deep magma ocean still provides the best explanation for lunar maria magmas. However, the initial depth of the lunar magma ocean, the physical processes that accompanied solidification, and the heat source for remelting cumulates to form these late basaltic outpourings remain incompletely understood and present challenging problems for current researchers.

The impact history of the Moon has significant implications that go far beyond simply excavating the surface of a dry and lifeless world. The age distribution of lunar impact breccias inspired the idea of a catastrophic influx of asteroids and comets about 4 billion years ago and motivated new models of planetary dynamics. A late bombardment may have regulated environmental conditions on the early Earth and Mars and influenced the course of biologic evolution. The cataclysm hypothesis is controversial, however, and far from proven. Lunar explorers face the difficult task of establishing absolute ages of ancient impact basins and the sources for the impactors.

Samples from the Apollo (USA) and Luna (Soviet) missions and from lunar meteorites, coupled with remote sensing data, reveal that the ancient highlands of the Moon are compositionally diverse. The average surface material contains 80 vol% plagioclase. A major suite of rocks, the ferroan anorthosites, averages 96 vol% plagioclase. The feldspathic composition reflects plagioclase flotation in a magma ocean. Late-stage REE-rich magma pooled in the Procellarum region of the lunar nearside. The concentration of heat-producing elements in this region triggered mantle melting and overturn of the cumulate pile, forming two more suites of chemically distinct highland rocks, the magnesian and alkali suites.

The Moon is a geochemically differentiated object. It has a feldspathic crust (highlands regions) composed of three petrological suites. Underlying that crust is a compositionally heterogeneous upper mantle from which ferrobasalts and picrites (mare regions) were generated. Lunar samples retain a memory of the time-dependent flux of meteorites and comets, which has implications for the origin of sustainable life on Earth and the orbital evolution of the outer planets. Permanently shadowed regions at the lunar poles may contain reservoirs of volatile ices, which would have important resource potential for scientific bases. Geophysical data show that the Moon has a thick, seismically active lithosphere, a partially molten zone beneath that lithosphere, and a small metallic core. The pace of scientific exploration has quickened since 2003 with the successes of spacecraft from Europe, Japan, the People’s Republic of China, and India. Upcoming launches of spacecraft from these same nations and the United States herald a new era of lunar discoveries.