Geology of the Moon

The geology of the Moon, also called selenology, studies the structure and composition of the Moon, which is quite different from Earth. Unlike Earth, the Moon lacks a true atmosphere and has very little water, so its surface changes mainly because of micrometeorites hitting it and 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.

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 382 kilograms of 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 contains many elements, including oxygen, silicon, iron, magnesium, calcium, aluminium, manganese, and titanium. Oxygen, iron, and silicon are among 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 carried there by the solar wind.

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 wondered how the Moon came to be. They had several ideas, such as the Moon breaking off from Earth, being captured by Earth's gravity, or forming together with Earth. Today, most scientists believe the Moon formed from a huge collision between Earth and another planet-sized body, known as the giant-impact hypothesis.

Geologic history

The Moon's geologic history is divided into six main epochs. About 4.5 billion years ago, the Moon was newly formed and in a molten state, orbiting closer to Earth. Tidal forces shaped it into an ellipsoid.

The first big event was the crystallization of a global magma ocean. Dense minerals sank, while lighter ones floated, forming a crust. Later, volcanic activity created the dark areas called lunar maria. These basaltic eruptions happened mostly between 3 and 3.5 billion years ago, though some might have occurred much more recently. Impacts from meteorites and comets continue to shape the Moon's surface today.

Lunar landscape

Main article: Topography of the Moon

The Moon's surface is covered with many craters from space rocks hitting it, along with some 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, while the maria are younger and look like dark seas. These maria were formed by ancient volcanic activity that poured lava over the surface. Some of these dark areas might still be active, but only in the very distant past.

The Moon's craters are mostly made when asteroids or comets crash into its surface. These impacts create craters of all sizes, from tiny pits to huge basins. The pattern of these craters helps scientists understand how old different parts of the Moon are.

Lunar magma ocean

Main article: Lunar magma ocean

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 molten, like a giant ocean of melted rock. As this melted rock cooled, different minerals formed and settled, creating the Moon's layers.

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

Lunar rocks

Main article: Moon rock

The Apollo program collected samples of rocks from the Moon, helping scientists understand its history. These rocks are made from minerals also found on Earth, such as olivine, pyroxene, and plagioclase feldspar. Some rocks contain a mineral called ilmenite, and a special mineral named armalcolite was found, named after the Apollo 11 astronauts.

The dark areas of the Moon, called maria, are made mostly of basalt, which has more iron and less plagioclase than the lighter highlands. These basalts vary in how much titanium they contain, with some having high amounts and others very little.

Internal structure

Main article: Internal structure of the Moon

The Moon's interior is made of solid rock, with its temperature and pressure increasing as you go deeper. Scientists learned about the Moon's inside mostly by using seismometers left behind during the Apollo missions, along with studying the Moon's gravity and rotation.

The Moon has a small, molten iron core that is less than 450 kilometers wide. Its crust, the outer layer, is about 50 kilometers thick on average, but it is a bit thicker on the side far from Earth. Studies near the Apollo 12 and Apollo 14 landing sites helped scientists understand how thick the crust is.