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Geology of the Moon
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Geology of the Moon
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphere outside of a sparse layer of gas. Because of this, the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, along with having a lower gravity compared to Earth. Because of its small size, it cooled faster in the early days of its formation. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. Six locations were sampled directly during the crewed Apollo program landings from 1969 to 1972, which returned 382 kilograms (842 lb) of lunar rock and lunar soil to Earth. In addition, three robotic Soviet Luna spacecraft returned another 301 grams (10.6 oz) of samples, and the Chinese robotic Chang'e 5 returned a sample of 1,731 g (61.1 oz) in 2020.
The Moon is the only extraterrestrial body for which we have samples with a known geologic context. A handful of lunar meteorites have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.
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). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45% (by weight). Carbon (C) and nitrogen (N) appear to be present only in trace quantities from deposition by solar wind.
For a long period of time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from Earth, capture, and co-accretion. Today, the giant-impact hypothesis is widely accepted by the scientific community.
The geological history of the Moon has been analysed into six major epochs, called the lunar geologic timescale. Starting about 4.5 billion years ago, the newly-formed Moon was in a molten state, and was orbiting much closer to Earth, resulting in tidal forces. These tidal forces deformed the molten body into an ellipsoid, with the major axis pointed towards Earth.
The first important event in the geologic evolution of the Moon was the crystallization of the near global magma ocean. Its depth is not known with certainty, but several studies imply a depth of about 500 km or greater. The first minerals to form in this ocean were the iron and magnesium silicates olivine and pyroxene. Because these minerals were denser than the molten material around them, they sank. After crystallization was about 75% complete, less dense anorthositic plagioclase feldspar crystallized and floated, forming an anorthositic crust about 50 km in thickness. The majority of the magma ocean crystallized quickly (within about 100 million years or less), though the final remaining KREEP-rich magmas, which are highly enriched in incompatible and heat-producing elements, could have remained partially molten for several hundred million (or perhaps 1 billion) years. It appears that the final KREEP-rich magmas of the magma ocean eventually became concentrated within the region of Oceanus Procellarum and the Imbrium basin, a unique geologic province that is now known as the Procellarum KREEP Terrane.
Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the Mg-suite norites and troctolites began to form, although the depths at which this occurred are not known precisely. Recent theories suggest that Mg-suite plutonism was largely confined to the region of the Procellarum KREEP Terrane, and that these magmas are genetically related to KREEP in some manner, though their origin is still highly debated in the scientific community. The oldest of the Mg-suite rocks have crystallization ages of about 3.85 Ga. However, the last large impact that could have been excavated deep into the crust (the Imbrium basin) also occurred at 3.85 Ga before present. Thus, it seems probable that Mg-suite plutonic activity continued for a much longer time, and that younger plutonic rocks exist deep below the surface.
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Geology of the Moon
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphere outside of a sparse layer of gas. Because of this, the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, along with having a lower gravity compared to Earth. Because of its small size, it cooled faster in the early days of its formation. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. Six locations were sampled directly during the crewed Apollo program landings from 1969 to 1972, which returned 382 kilograms (842 lb) of lunar rock and lunar soil to Earth. In addition, three robotic Soviet Luna spacecraft returned another 301 grams (10.6 oz) of samples, and the Chinese robotic Chang'e 5 returned a sample of 1,731 g (61.1 oz) in 2020.
The Moon is the only extraterrestrial body for which we have samples with a known geologic context. A handful of lunar meteorites have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.
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). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45% (by weight). Carbon (C) and nitrogen (N) appear to be present only in trace quantities from deposition by solar wind.
For a long period of time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from Earth, capture, and co-accretion. Today, the giant-impact hypothesis is widely accepted by the scientific community.
The geological history of the Moon has been analysed into six major epochs, called the lunar geologic timescale. Starting about 4.5 billion years ago, the newly-formed Moon was in a molten state, and was orbiting much closer to Earth, resulting in tidal forces. These tidal forces deformed the molten body into an ellipsoid, with the major axis pointed towards Earth.
The first important event in the geologic evolution of the Moon was the crystallization of the near global magma ocean. Its depth is not known with certainty, but several studies imply a depth of about 500 km or greater. The first minerals to form in this ocean were the iron and magnesium silicates olivine and pyroxene. Because these minerals were denser than the molten material around them, they sank. After crystallization was about 75% complete, less dense anorthositic plagioclase feldspar crystallized and floated, forming an anorthositic crust about 50 km in thickness. The majority of the magma ocean crystallized quickly (within about 100 million years or less), though the final remaining KREEP-rich magmas, which are highly enriched in incompatible and heat-producing elements, could have remained partially molten for several hundred million (or perhaps 1 billion) years. It appears that the final KREEP-rich magmas of the magma ocean eventually became concentrated within the region of Oceanus Procellarum and the Imbrium basin, a unique geologic province that is now known as the Procellarum KREEP Terrane.
Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the Mg-suite norites and troctolites began to form, although the depths at which this occurred are not known precisely. Recent theories suggest that Mg-suite plutonism was largely confined to the region of the Procellarum KREEP Terrane, and that these magmas are genetically related to KREEP in some manner, though their origin is still highly debated in the scientific community. The oldest of the Mg-suite rocks have crystallization ages of about 3.85 Ga. However, the last large impact that could have been excavated deep into the crust (the Imbrium basin) also occurred at 3.85 Ga before present. Thus, it seems probable that Mg-suite plutonic activity continued for a much longer time, and that younger plutonic rocks exist deep below the surface.
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