Formation
Several mechanisms have been suggested for the Moon's formation. The formation of the Moon is hypothesized to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System.[50]
- Fission hypothesis
- Early speculation proposed that the Moon broke off from the Earth's crust because of centrifugal forces, leaving a basin – presumed to be the Pacific Ocean – behind as a scar.[51] This idea, however, would require too great an initial spin of the Earth and also would have resulted in the Moon's orbit following Earth's equatorial plane rather than its current path.[52]
- Capture hypothesis
- Other speculation has centered on the Moon being formed elsewhere and subsequently being captured by Earth's gravity.[53] However, the conditions conjectured necessary for such a mechanism to work, such as an extended atmosphere of the Earth in order to dissipate the energy of the passing Moon, are improbable.[52]
- Co-formation hypothesis
- The co-formation hypothesis proposes that the Earth and the Moon formed together at the same time and place from the primordial accretion disk. The Moon would have formed from material surrounding the proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon.[52]
- A major deficiency in all these hypotheses is that they cannot readily account for the high angular momentum of the Earth–Moon system.[54]
- Giant impact hypothesis
- The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact. A Mars-sized body (labelled "Theia") is hypothesized to have hit the proto-Earth, blasting sufficient material into orbit around the proto-Earth to form the Moon through accretion.[4] As accretion is the process by which all planetary bodies are therorized to have formed, giant impacts are thought to have affected most if not all planets. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system, as well as the small size of the lunar core.[55] Unresolved questions regarding this theory concern the determination of the relative sizes of the proto-Earth and Theia and of how much material from these two bodies formed the Moon. Recent oxygen isotope composition analysis of the Moon shows its oxygen isotope composition is more similar to the Earth's than this hypothesis would suggest.[56]
Lunar magma ocean
As a result of the large amount of energy converted during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly hypothesized that a large portion of the Moon was once initially molten. The molten outer portion of the Moon at this time is referred to as a magma ocean, and estimates for its depth range from about 500 km to the entire radius of the Moon.[14]
As the magma ocean cooled, it fractionally crystallised and differentiated, giving rise to a geochemically distinct crust and mantle. The mantle is inferred to have formed largely by the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallisation was complete, the mineral anorthite is inferred to have precipitated and floated to the surface because of its low density, forming the crust.[14]
The final liquids to crystallise from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym KREEP, for potassium (K), rare earth elements (REE), and phosphorus (P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of Oceanus Procellarum and Mare Imbrium on the near side of the Moon.[1]
Geologic evolution
A large portion of the Moon's post–magma-ocean geologic evolution was dominated by impact cratering. The lunar geologic timescale is largely divided in time on the basis of prominent basin-forming impact events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on the basis of stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.
The other major geologic process that affected the Moon's surface was mare volcanism. The enhancement of heat-producing elements within the Procellarum KREEP Terrane is thought to have caused the underlying mantle to heat up, and eventually, to partially melt. A portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon.[14] Most of the Moon's mare basalts erupted during the Imbrian period in this geologic province 3.0–3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years,[57] and the youngest eruptions, based on the method of crater counting, are hypothesized to have occurred only 1.2 billion years ago.
There has been controversy over whether features on the Moon's surface undergo changes over time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of outgassing does occasionally occur, and these events could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.[59][60]
Moon rocks
Moon rocks fall into two main categories, based on whether they underlie the lunar highlands (terrae) or the maria. The lunar highlands rocks are composed of three suites: the ferroan anorthosite suite, the magnesian suite, and the alkali suite (some consider the alkali suite to be a subset of the mg-suite). The ferroan anorthosite suite rocks are composed almost exclusively of the mineral anorthite (a calic plagioclase feldspar), and are hypothesized to represent plagioclase flotation cumulates of the lunar magma ocean. The ferroan anorthosites have been dated using radiometric methods to have formed about 4.4 billion years ago.[57][58]
The mg- and alkali-suite rocks are predominantly mafic plutonic rocks. Typical rocks are dunites, troctolites, gabbros, alkali anorthosites, and more rarely, granite. In contrast to the ferroan anorthosite suite, these rocks all have relatively high Mg/Fe ratios in their mafic minerals. In general, these rocks represent intrusions into the already-formed highlands crust (though a few rare samples appear to represent extrusive lavas), and they have been dated to have formed about 4.4–3.9 billion years ago. Many of these rocks have high abundances of, or are genetically related to, the geochemical component KREEP.
The lunar maria consist entirely of mare basalts. While similar to terrestrial basalts, they have much higher abundances of iron, are completely lacking in hydrous alteration products, and have a large range of titanium abundances.
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