Geology of the Moon

The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is quite different from that of Earth.

The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather.

[3] In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism,[4][5] which is now thought to have ended less than 50 million years ago.

[7] Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data.

Elements known to be present on the lunar surface include, among others, oxygen (O), silicon (Si), iron (Fe), magnesium (Mg), calcium (Ca), aluminium (Al), manganese (Mn) and titanium (Ti).

Starting about 4.5 billion years ago,[16] the newly formed Moon was in a molten state and was orbiting much closer to Earth resulting in tidal forces.

The surface has also experienced space weathering due to high energy particles, solar wind implantation, and micrometeorite impacts.

The major products of volcanic processes on the Moon are evident to Earth-bound observers in the form of the lunar maria.

The oldest radiometric ages are about 4.2 Ga (billion years), and ages of most of the youngest maria lavas have been determined from crater counting to be about 1 Ga. Due to better resolution of more recent imagery, about 70 small areas called irregular mare patches (each area only a few hundred meters or a few kilometers across) have been found in the maria that crater counting suggests were sites of volcanic activity in the geologically much more recent past (less than 50 million years).

Although variations in the crustal thickness might act to modulate the amount of magma that ultimately reaches the surface, this hypothesis does not explain why the farside South Pole-Aitken basin, whose crust is thinner than Oceanus Procellarum, was only modestly filled by volcanic products.

Many of the lunar basalts contain small holes called vesicles, which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface.

One of the most notable sinuous rilles is the Vallis Schröteri feature, located in the Aristarchus plateau along the eastern edge of Oceanus Procellarum.

An example of a sinuous rille exists at the Apollo 15 landing site, Rima Hadley, located on the rim of the Imbrium Basin.

The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint.

This realization allowed the impact history of the Moon to be gradually worked out by means of the geologic principle of superposition.

Adopting this approach in the late 1950s, Gene Shoemaker took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.

Small craters tend to form a bowl shape, whereas larger impacts can have a central peak with flat floors.

The impact process excavates high albedo materials that initially gives the crater, ejecta, and ray system a bright appearance.

This can occur when an area of darker basaltic material, such as that found on the maria, is later covered by lighter ejecta derived from more distant impacts in the highlands.

The largest impacts produced melt sheets of molten rock that covered portions of the surface that could be as thick as a kilometer.

The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials.

Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine-grained layer termed regolith.

The atoms that compose the solar wind – mostly hydrogen, helium, neon, carbon and nitrogen – hit the lunar surface and insert themselves into the mineral grains.

The gases of the solar wind could be useful for future lunar bases, because oxygen, hydrogen (water), carbon and nitrogen are not only essential to sustain life, but are also potentially very useful in the production of fuel.

[27] Any intact lava tube on the Moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultraviolet radiation and energetic particles, and extreme diurnal temperature variations.

The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean.

Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.

Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.

Additionally, zircon analysis of Apollo 14 samples suggests the lunar crust differentiated 4.51±0.01 billion years ago.

Alternatively, it is possible that transient magnetic fields could be generated during impact processes on airless bodies such as the Moon.