Geology of the Moon

The geology of the Moon, also called selenology, studies the structure and composition of the Moon. It is different from Earth. The Moon lacks a true atmosphere and has very little water. Its surface changes mainly because of micrometeorites hitting it, not because of weather. The Moon does not have plate tectonics like Earth and has much lower gravity. Because it is smaller than Earth, it cooled faster when it first formed.

Geologic map of the Moon, with general features colored in by age, except in the case of maria (in blue), KREEP (red) and other special features. Oldest to youngest: Aitkenian (pink), Nectarian (brown), Imbrian (greens/turquoise), Eratosthenian (light orange) and Copernican (yellow).

The Moon's surface has been shaped by impacts from space rocks and old volcanism, which may have stopped less than 50 million years ago. It is a differentiated body, meaning it has layers including a crust, mantle, and core. Scientists study the Moon using telescope observations from Earth, data from orbiting spacecraft, and actual rocks and soil brought back by missions.

Six missions from the Apollo program brought back lunar rock and lunar soil from 1969 to 1972. Other missions, like three from the Soviet Luna spacecraft and China's Chang'e 5, also returned smaller samples. The Moon is the only object outside Earth from which we have rocks with known locations. Some lunar meteorites have been found on Earth, but we do not know exactly where on the Moon they came from. Much of the Moon's surface has still not been explored, and scientists have many questions left to answer.

Elemental composition

The Moon's surface has many elements, including oxygen, silicon, iron, magnesium, calcium, aluminium, manganese, and titanium. Oxygen, iron, and silicon are the most common. Oxygen makes up about 45% of the Moon's surface by weight. Carbon and nitrogen are found only in very small amounts, likely brought there by the solar wind.

Lunar surface chemical composition
Compound		Composition
Name	Formula	Maria	Highlands
silica	SiO₂	45.4%	45.5%
alumina	Al₂O₃	14.9%	24.0%
lime	CaO	11.8%	15.9%
iron(II) oxide	FeO	14.1%	5.9%
magnesia	MgO	9.2%	7.5%
titanium dioxide	TiO₂	3.9%	0.6%
sodium oxide	Na₂O	0.6%	0.6%
Total (with rounding error)		99.9%	100.0%

**Neutron spectrometry data from *Lunar Prospector* indicate the presence of hydrogen (H) concentrated at the poles.**
	Relative concentration (in weight %) of various elements on lunar highlands, lunar lowlands, and Earth

Formation

Main article: Origin of the Moon

For a long time, scientists were not sure how the Moon was made. They had a few ideas, like the Moon breaking away from Earth, or Earth catching it with its gravity. Now, most scientists think the Moon was created when Earth crashed into a big object, called the giant-impact hypothesis.

Geologic history

The Moon's geologic history is divided into six main parts. About 4.5 billion years ago, the Moon was newly formed and was very hot, orbiting closer to Earth. Forces from Earth shaped it into an ellipsoid.

The first big event was the crystallization of a global ocean of melted rock. Heavy minerals sank, while lighter ones floated, forming a crust. Later, volcanic activity created the dark areas called lunar maria. These eruptions of dark rock happened mostly between 3 and 3.5 billion years ago. Impacts from meteorites and comets still shape the Moon's surface today.

Lunar landscape

Main article: Topography of the Moon

Mare Imbrium and the crater Copernicus

The Moon's surface has many craters from space rocks hitting it. It also has old volcanoes, hills, and dark areas made of hardened lava called maria.

The Moon has bright areas called highlands and darker areas called maria. The highlands are older and have many craters. The maria are younger and look like dark seas. These maria were formed by ancient volcanic activity that poured lava over the surface.

The Moon's craters are made when asteroids or comets crash into its surface. These impacts create craters of all sizes. The pattern of these craters helps scientists learn how old different parts of the Moon are.

Lunar magma ocean

Main article: Lunar magma ocean

A map of the Moon coloured in terms of crust thickness and its types of regions marked: the KREEP rich magma terrane is hatch-marked and labeled on the left map hemisphere, the near side of the Moon, in its most prominent part as the Mare Procellarum KREEP Terrane (PKT); on the right (the far side), the gray circle marks the South Pole-Aitken Terrane (SPAT); on each hemispheres the 12 crater basins with crustal thinng larger than 200 km (120 mi) are marked with black circles; the rest is Felspathic Highlands Terrane (FHT), with the region of high-thickness (in red and white) being the inner FHT.

The rocks brought back by the Apollo 11 mission showed that part of the Moon was once melted. Scientists think that after the Moon formed, a big part of it was melted, like a giant ocean of rock.

As this melted rock cooled, different minerals formed and settled. This created the Moon's layers.

This process helped form the Moon's crust and mantle. Important minerals sank to the middle, while lighter minerals like plagioclase floated to the top. This helped create the different rocks we see on the Moon today.

Lunar rocks

Main article: Moon rock

The Apollo program gathered rocks from the Moon. These help scientists learn about the Moon’s history. The rocks are made from minerals found on Earth, like olivine, pyroxene, and plagioclase feldspar. Some rocks have a mineral called ilmenite. A special mineral, armalcolite, was named after the Apollo 11 astronauts.

The dark parts of the Moon, called maria, are mostly basalt. This basalt has more iron and less plagioclase than the lighter areas. The basalt can have different amounts of titanium, from high to very low.

Internal structure

Main article: Internal structure of the Moon

The Moon's interior is made of solid rock. As you go deeper, it gets hotter and the pressure increases. Scientists learned about the Moon's inside using tools left during the Apollo missions. They also studied the Moon's gravity and how it turns.

The Moon has a small, molten iron core. It is less than 450 kilometers wide. The crust, the outer layer, is about 50 kilometers thick on average. It is a little thicker on the side far from Earth. Studies near the Apollo 12 and Apollo 14 landing sites helped scientists learn about the crust's thickness.