Lunar geology

The Moon formed early in Solar System history, probably after a giant impact involving the young Earth, and its oldest rocks record crystallization of a global magma ocean. The leading impact model explains the Moon’s relative depletion in volatile elements and its close chemical relationship to Earth, although details of the impact and subsequent mixing remain active research questions.

Its differentiated interior consists broadly of a crust, silicate mantle, and small metallic core. Lunar crust is dominated by anorthositic rocks enriched in plagioclase, while the mantle contains magnesium- and iron-rich minerals. Seismic experiments from Apollo, together with measurements of lunar rotation and gravity, revealed a layered interior and helped constrain the size and physical state of the core.² The crust is substantially thicker on the far side than beneath many nearside regions, reflecting uneven early evolution.

The Moon’s visible surface records three broad stages: ancient crustal formation, extensive impact bombardment, and later volcanic and tectonic modification. Bright, heavily cratered highlands are older than the dark maria, which are basaltic plains produced when mantle-derived magma flooded large impact basins between roughly 3.9 and 3.0 billion years ago.¹

Impact cratering is the dominant surface process because the Moon lacks a substantial atmosphere, liquid water, and active plate tectonics. Craters range from microscopic pits to enormous basins such as South Pole–Aitken, and their ejecta provide natural probes of deeper crustal material. Apollo samples established absolute ages for lunar rocks and calibrated crater-counting methods used to estimate the ages of surfaces elsewhere in the Solar System.

The lunar regolith is a fragmented, mixed layer created mainly by billions of years of impacts. It contains rock fragments, mineral grains, impact glass, and agglutinates welded by micrometeorite bombardment; its thickness varies with terrain and age. Solar-wind particles and cosmic radiation alter exposed grains, producing the optical darkening and reddening known as space weathering.

Water and hydroxyl occur in small, variable quantities across the surface, including within or near permanently shadowed polar regions. Orbital observations and impact experiments support the presence of water ice in some cold traps, but its distribution, purity, depth, and accessibility remain uncertain. The polar deposits are geologically significant because they may preserve material delivered by comets, asteroids, and the solar wind over long intervals.

Small-scale lunar geology includes transient dust movement, thermal cracking, shallow moonquakes, and young-looking impact scars that complicate the simple picture of an inactive world. Apollo seismometers detected both deep tidal events and shallow seismic activity; later analyses have connected some shallow quakes to faults formed as the Moon slowly contracts.²

The Moon also preserves geological information that is difficult to retain on Earth. Impact melts can rapidly record ancient bombardment, while permanently shadowed terrain may conserve volatile compounds and cold-trapped ice. Lunar swirls—bright, sinuous surface markings associated with localized magnetic anomalies—appear to involve interactions among regolith, solar wind, and remanent crustal magnetism rather than ordinary albedo variation alone. These features make the Moon a natural archive for studying planetary surfaces, space weathering, and the early Solar System.

Anorthosite

A coarse-grained igneous rock dominated by the mineral plagioclase; it forms much of the ancient lunar highlands.

Mare

A broad, dark lunar plain formed chiefly by basaltic lava that filled an impact basin; the plural is maria.

Regolith

The loose layer of fragmented rock, dust, and impact-produced material covering the solid lunar surface.

Space weathering

The alteration and optical maturation of airless planetary surfaces by solar-wind particles, micrometeorites, and radiation.