Geology of the Moon

This image of the Moon taken through a telescope enhances the colors present on the lunar surface, each color represents a different mineral; red/orange is due to the presence of iron oxide and blue to titanium

Knowledge of lunar geology has increased significantly since the 1960s with manned and automated missions. Despite all the data collected, there are still unanswered questions that will only be answered with the installation of future permanent bases and a broader study of the surface. Thanks to its proximity, the Moon is the only body —besides Earth— whose geology is known in detail and from which samples were obtained from different regions. The manned Apollo missions contributed to the collection of 382 kilograms of rocks and lunar soil samples, which continue to be useful study objects for understanding their formation and that of other celestial bodies. Some probes from the Soviet Union's Luna program also returned small samples of lunar soil to Earth: Luna 16 (101 grams), Luna 20 (55 grams) and Luna 24 (170 grams).

The geological map of the Moon at scale 1-2.5M of the Chinese Academy of Sciences.

The origin of the Moon

Mare Imbrium and Copernician crater

For a long time the fundamental problem concerning lunar history was that of its origin. The hypotheses that have been elaborated in this regard are as varied as they are different from one another. The most important hypotheses are:

• Lunar capture: The capture of a moon completely formed by the gravitational field of the Earth is unlikely, as a close encounter with the Earth would have produced a collision or an alteration of the trajectory of the body in question and would probably never meet Earth again. For this hypothesis to work it would require a large atmosphere spread around the primitive Earth, which could slow the motion of the Moon before it escaped. This hypothesis is seriously considered to explain the orbits of the irregular satellites of Jupiter and Saturn; however, it is very difficult for the Moon to function.
• Fission hypothesis: exposes the idea that a primitive Earth with an accelerated rotation expelled a piece of its mass, and was proposed by George Darwin, son of the famous biologist Charles Darwin. This hypothesis does not explain why the Earth was rotating once every 2.5 hours and why the Moon and Earth do not follow with an accelerated rotational movement today.
• Acreation hypothesis: with this hypothesis it is established that the Earth and the Moon were formed together, in a double system. The problem of this hypothesis is that the rotational period of the Earth and the Moon is not explained, in addition to giving an answer to the absence of material of this dual system orbiting the two bodies, a phenomenon that can only be explained if the movement of Earth rotation and the lunar revolution are taken into account through a physical property called angular moment.
• Theory of great impact: refers to the impact of a body of the size of Mars (half of the Earth's radio and a tenth of its mass) on Earth when it was 90% of its current size. This impact would have expelled vast quantities of hot material around the Earth orbit and the Moon would have formed through the accumulation of this material.

Big Impact Theory

It is the most accepted hypothesis. Although proposed in 1984, its origins date back to the mid-1970s. This theory does satisfy the orbital conditions of the Earth and the Moon and why the Earth has a larger metallic core than the Moon. Modern theories of how planets form through smaller bodies—which would have been formed by still smaller bodies—predict that, when Earth's formation was nearly complete, there could have been a protoplanet, or primitive planetary body, the size of Mars (Tea), with about a tenth of the mass of the Earth, in the vicinity of the Earth's orbit, in such a way that it collided. For all this, the theory of the great impact, according to which the Moon originated with the remains of a great collision between planets, is a plausible event, and even that it could have been inevitable.

Image obtained in the mission Apollo 15

The energy involved in this collision is staggering: billions of trillions of tons of material would have evaporated and melted. In some places on Earth the temperature would have reached 10,000 °C. This would explain the unusual size of the Earth's metallic core; the Mars-sized body would have merged with Earth, incorporating its material into our planet's interior. If this event had never happened, not only would the Earth have no moon, but also the days would be longer and their durations would be close to a year.

The first major event in moon formation was the crystallization of oceanic magma. It is not known with certainty what its depth was, but according to different studies, the magma ocean was located at a depth of about 500 km. The first minerals to form in this crystallizing ocean were iron and magnesium silicates, olivine, and pyroxene. Because these minerals were denser than the confining material, they sank. The subsequent formation of plagioclase feldspar, of lower density than magma, was located in the upper part of the magma ocean, forming the anorthosite mountains, giving rise to the first lunar crust. The magma ocean stage ended about 4.4 billion years ago.

As fast as the lunar crust formed, and even while it was still forming, other types of magmas that would form norites and troctolites in the highlands began to form deep on the Moon, but have not yet been formed. knows how deep Magmas rose through the surface infiltrating through the anorthosite crust, forming large boulders and even erupting on the surface. Some of these magmatic bodies chemically reacted with the remnants of the magma ocean (KREEP) and others may have dissolved anorthosites. This period of lunar history ended about 4 billion years ago.

During these early stages of lunar formation, various impact events continued to modify the surface down to a depth of a few kilometers (even as deep as 20 km). Although not conclusively proven, the impact rate appears to have declined between 4.5 and 4.0 billion years ago, but then grew dramatically producing most of the visible basins on the Moon. This bombardment would have occurred in a period between 4,000 and 3,850 million years ago.

Once the rate of impacts subsided, the seas had time to form. Basalts formed more than 3.85 billion years ago. However, between 3.7 and about 2.5 billion years ago (the last number is very uncertain), lavas flowed on the lunar surface, forming the seas and other typical features. Along with the basalts came pyroclastic eruptions spewing remnants of molten basalt hundreds of kilometers away. Since volcanism ceased, the only geologic force on the Moon has been meteorite impacts.

Some of the largest craters on the Moon are Copernicus, with a diameter of 93 km and a depth of 3.76 km, and Tycho with a diameter of 85 km. Both craters ejected a large amount of material. The Apollo 17 mission landed in the Taurus-Littrow valley, an area where material from Tycho crater had been distributed; the study of rocks led to the conclusion that the impact would have occurred about 110 million years ago.

Moonscape

The lunar landscape is characterized by the presence of impact craters, the material ejected from them, some volcanoes, depressions filled by the magma ocean, hills and the marks left by lava flows.

The High and Low Plains

The most distinctive aspect of the Moon is the contrast of light and dark areas. The light areas are the highlands and are called terrae (from Latin earth; singular form: terra) and the darker plains are called maria (from Latin mares; singular form: mare), names coined by Johannes Kepler.

Highlands and Craters

Formation of an impact crater

The highlands have the largest number of impact craters from a diameter of about a meter to 1,000 kilometers. Before any robotic mission could reach the Moon, scientists thought that the origin of some of these craters was volcanic, an idea that changed radically with the return of lunar soil and rock samples with the Apollo missions clearly showing the important role of the impact process in soil formation.

Impacts occur at speeds approaching 20 km/s (70,000 km/h). In each impact, high pressure waves bounce off the projectile and the impacted body, a process in which the projectile (a meteorite) is destroyed by the passage of the shock wave, causing it to evaporate almost entirely. The impacted body material is strongly compressed and briefly decompressed afterwards. A portion of this material is vaporized and another part is melted, but most of it (a mass 10,000 times that of the meteorite) is ejected out of the crater, forming the ring that surrounds it. The central part of the crater is a more depressed area than the rest of the terrain.

The difference with volcanic calderas or cinder cones is that they do not have rings of accumulated material and their tops are above the surface level.

A small part of the impacted body is ejected great distances, giving rise to figures that resemble straight lines called radii.

The Seas

The maria (maria) cover about 16% of the lunar surface and were formed by lava flows that mainly filled the huge impact basins. Although it is thought that the Moon currently does not have any volcanic activity, it did in the past. According to studies, the volcanic activity of the Moon took place after the highlands were formed and after most of the cratering process had happened, for this reason, the lunar maria are younger than the highlands.

Before being confirmed by the Apollo missions, scientists already believed that the lunar maria were lava plains, since they had particular characteristics: patterns of lava flows and collapses attributed to tubes of lava. Material collected during lunar missions in the 1960s and 1970s confirmed the suspicion: the basins are made of a type of volcanic rock called basalt.

The seas fill most of the impact basins on the near side. In the 1960s, some scientists suggested that this demonstrated cause and effect: the impacts not only caused the formation of large craters but also caused the lunar interior to melt, triggering the volcanic process. However, closer examination of the seas shows that they must have been younger than the basins in which they reside.

Example: the impact that formed the great Imbrium basin of the Mare Imbrium (Sea of Rains) spewed material out of the basin forming the mountains that surround the Serenitatis basin, that is, of the Mare Serenitatis (Sea of Rains). Serenity). That is why the Sea of Serenity is older.

The most important visible feature about the relative youth of the seas relative to the surrounding terrain is that the seas have fewer craters, meaning they have been around for less time. In fact, with the data collected in the lunar missions, it is known that the seas can form even billions of years after the basins are formed.

Another type of deposit associated with the seas, but also covering highland areas, are dark mantle deposits. These deposits cannot be seen with the naked eye but with the help of telescopes or the proximity of spacecraft. Before the Apollo missions, scientists believed that these were deposits produced by pyroclastic eruptions. Some deposits appear to be associated with broad, dark cinder cones, reinforcing the idea of pyroclastic eruptions, later confirmed by the finding of glass beads like those found in pyroclastic eruptions on Earth.

Unanswered questions about the seas

There are still some mysteries about the seas:

• Why did the volcanoes disappear and only ash cones associated with dark mantle deposits can be seen?
• If the volcanoes did not exist, where was the lava erupted?

In some cases it is visible that the lava emanated from the huge impact basins, or perhaps along cracks concentric to the basin, although in most cases it cannot be seen where it erupted from.

Another of the curious characteristics of the Moon is that almost all the seas are present on the visible side of the Earth. Most scientists believe that this asymmetry is caused by the highland crust being thicker on the opposite side, making it difficult for basalt to rise to the surface; it has also been suggested that the difference between the two faces may be due to the collision of a second moon that could also have formed in the impact that is thought to have formed the Moon; According to this idea, that hypothetical second moon would have a diameter of approximately 1,200 kilometers (1/3 the size of our satellite) and it collided with the largest one at a relatively low speed with the result that, instead of forming a large basin Upon impact, the smaller moon disintegrated, its remains covering what is now the hidden face.

The Deep Chasms

The Japanese SELENE probe discovered three deep circular holes in the lunar surface, perhaps caused by the collapse or fall of pieces of the roof of various lava tubes or intumescences; on Earth they are also produced when the surface of a lava flow solidifies and the molten paste inside continues to flow, leaving a hollow tube of rock. Thus, extensive networks of galleries and large vaults or hollow intumescences are formed, which sometimes collapse, forming holes. Later, the Lunar Reconnaissance Orbiter probe was able to specify its dimensions. One hundred meters deep is in the Sea of Tranquility. Another hole, detected in the Mare Ingenii, is 70 meters deep and 120 meters wide. A third, smaller hole, located in the Marius Hills, drops 34 meters below the surface. These places are considered promising for future human settlements on the Moon, as they would allow great cost savings and protection against cosmic rays and extreme temperatures. On the other hand, a new scan of the data collected by the LRO probe has made it possible to locate with a new computer algorithm up to 200 more holes. These places can offer access to the interior of cavern systems usable as shelter or storage. The diameter of the holes ranges from 5 to 900 meters.

The lunar surface

The surface of the Moon is gray in color and presents a large amount of fine sediment from countless meteorite impacts. This dust is called lunar regolith, a term coined to describe the layers of sediment produced by mechanical effects on the surfaces of the planets. The thickness of the regolith varies from 2 meters in the youngest seas to about 20 meters in the oldest highland surfaces.

The regolith is formed by the rocky material of the region where it is found, but it also contains remains of material ejected by distant impacts, for which reason the regolith constitutes a rock of great scientific value.

Regolith contains rocks, fragments of minerals derived from the original bedrock, glassy particles formed by impacts. In most lunar regolith, half the particles are composed of mineral fragments that are held together by impact glasses; these objects are called agglutinates. The chemical composition of the regolith varies according to its location; regolith in the highlands, like its rocks, is rich in aluminium. The regolith in the seas is rich in iron and magnesium, like basaltic rocks.

The lunar regolith is also very important because it stores information about solar history. Particles that make up the solar wind, composed primarily of helium, neon, carbon, and nitrogen atoms, strike the lunar surface and embed themselves in mineral grains. By analyzing the composition of the regolith, especially its isotopic composition, it is possible to determine if the activity of the Sun has changed over time.

Gases from the solar wind could be useful for future lunar bases, since oxygen and hydrogen (water), carbon, and nitrogen are not only essential for life but also highly useful for making fuel. In this regard, the existence of large amounts of Helium-3 is especially important, which could be used as combustible energy material in nuclear fusion generators.

There is a large amount of oxygen stored in silicates, silicon dioxide (SiO₂), lunar rock minerals, oxides of calcium (CaO), iron (FeO) and magnesium (MgO). About 43% of the soil mass is oxygen and the solar wind provides the rest.

Moon Rocks

Highland Rocks and Lunar Magma Ocean

The first rocks collected by Apollo 11 corresponded to basalts. Although the Apollo 11 mission took place over the Sea of Tranquility, millimeter fragments of highland rock were also collected. These are mainly composed of the mineral plagioclase feldspar; some fragments only contained plagioclase. These rocks are called anorthosites.

The highlands are made mostly of plagioclase because this mineral accumulated on top of the magma ocean by flotation, giving rise to the hypothesis that the Moon was once covered by a magma ocean.

Formation of anortosite bark

The concept of the magma ocean was verified in 1994 with the US probe Clementine, which in its polar orbit for two months took pictures at different wavelengths. The scientists analyzed the iron content on the lunar surface through variations in the intensity of reflected light at different wavelengths. The magma ocean hypothesis predicts that the lunar highlands should have a low iron content, less than about 5% by weight (recorded as iron oxide FeO). According to measurements of the Clementine, the average presence in the highlands is less than 5% FeO by weight. These data were confirmed in 1998 when another US probe, the Lunar Prospector, orbited the Moon.

The highlands contain other types of igneous rocks: the most abundant are norites and troctolites, rocks formed by equal amounts of plagioclase and olivine or pyroxene (both of which are iron- and magnesium-containing silicate minerals). Radiometric dating of these rocks suggests that they are younger than anorthosites that were formed after the magma ocean had crystallized.
Highland rocks are also quite complex due to the cratering process. Most of these rocks are complex mixtures of others. The original rocks were melted, mixed, and impacted during the first 500 million years of the Moon. These rocks are called breccias. Some of these breccias are so mixed up that they contain breccias within breccias. Most of the anorthosites, norites, and troctolites are actually rock fragments within breccias.

The interesting thing about highland breccias, especially impact breccias (rocks partially melted by an impact event) is that most of them date from 3.85 to 4.0 billion years ago. years. This leads to the idea that the Moon experienced a very intense meteorite bombardment during that period, however, it must be taken into account that the sampling of rocks returned by the Apollo missions is very small and corresponds to a small region of the Moon.

Many breccias and some igneous rocks are enriched with a set of elements not common on Earth. These elements do not tend to be a fundamental part of the minerals present in rocks. Its presence originates when the magma crystallizes, and the part that is still liquid is progressively enriched with these special elements. The rocks that contain them are called KREEP, a name that represents the initials of potassium (chemical symbol K), rare elements of the Earth, from English Rare-Earth Elements (REE) and phosphorus (chemical symbol P). It is currently believed that the KREEPs represent the last remnants of ocean magma crystallization. Large impacts excavated the crust ejecting the lower material mixing it with other debris forming KREEP breccias.

Mineral abundances in lunar rocks

Moon minerals

Geological constitution of the seas

Distribution of lunar rocks

The main characteristic of basaltic rocks compared to rocks from the highlands is that basalts contain a greater amount of olivine and pyroxene and less plagioclase. Remarkably many of them also have an iron-titanium ore oxide called ilmenite. Because the first sampling of rocks had a high content of ilmenite (and other related minerals) they were named “high titanium” basalts in reference to the exceptional concentrations of this metal. Apollo 12 returned to Earth with basalts of lower concentrations and were called “low titanium” basalts. Subsequent missions and Soviet automated missions returned with even lower concentration basalts, the “very low titanium” basalts.

The Clementine probe provided data showing a wide range of titanium content in basaltic rocks, with those with high content being the least abundant.

The shapes of the mineral grains in which they are present in the basalts of the seas indicate that these rocks were formed in lava flows, some thin (one meter thick) and others thicker (up to 30 meters). Many of the lunar basalts contain small holes called vesicles, which were formed by gas bubbles trapped when lava solidified. It is not known with certainty what gases escaped from these rocks. On Earth the vesicles are formed with the release of carbon dioxide, water vapor accompanied by some sulfur and chlorine. There are no signs of the existence of water on the Moon. It was probably carbon dioxide and carbon monoxide, with some sulfur.

Samples of pyroclastic glass are colored green, yellow, and red. The difference in color reflects the amount of titanium they have, thus, the green particles have the lowest concentrations (about 1%) and the red ones have the highest concentrations with 14%, much more than the basalts with the highest concentrations..

Experiments carried out on basaltic rocks and pyroclastic glass show that they formed when the interior of the Moon was partially melted. Rocks do not have a specific melting temperature, as they melt over a range of temperatures: basalts melt at temperatures between 1,000 and 1,200 °C. The experiments showed that melting on the Moon took place at a depth of between 100 to 500 km, and that the partially melted rocks contained mainly olivine and pyroxene with some ilmenite in the regions that formed the high-titanium basalts.

The lunar interior and moonquakes

Comparison of the interior of the Earth and the Moon.

The Moon does not have plate tectonics and therefore does not constantly renew itself like the Earth. Moonquakes, or moonquakes, are minimal and the largest (magnitude 5) only occur about once a year. The lunar interior is quite different from Earth's; the lunar crust is about 70 km thick on the near side to about 150 km on the far side. The seas are about 1 km thick (data derived from photogeological studies). Samples returned to Earth and probe data suggest that the lower crust contains less plagioclase than the upper crust. Beneath the crust is the lunar mantle, the longest layer of the Moon. There may be a difference in the constitution of the rocks above and below a depth of 500 km, representing the depth of the magma ocean. Beneath the mantle is the lunar core whose size is uncertain although estimates place it between 100 to 400 km.

Although the Moon does not have a magnetic field like Earth, it did in the past. Lunar rocks are magnetized, with the oldest being the ones with the greatest magnetism. This assumes that in the past the magnetic field was stronger. The reason for its weakening is uncertain, although it serves to theorize about the absence of a liquid iron core as in the terrestrial case, which in its internal movement produces the electrical currents necessary for the creation of the field. Another of the differences thus derived is that the average density of the moon is about 3.3 g/cm³, while the average density of the Earth is 5.5 g/cm³.

In some regions of the Moon the intensity of the gravitational field is more intense, this mystery was solved with the Lunar Prospector by associating them with large concentrations of masses (mascons) present in the seas of the basins.

About 80° from the south pole lie the remnants of the enormous Aitken Basin, the largest in the solar system, some 2,500 km in diameter. Most of this area, some 15,000 km², does not receive sunlight thanks to the elevated surfaces with which they are surrounded. Both the radar images from the Clementine probe and the neutron spectrometer data from the Lunar Prospector indicate that the region contains deposits of frozen water. Until then, the presence of a deposit of between 10 and 300 million tons was suspected. The Lunar Prospector also discovered that the north pole contains about twice as much ice as the south pole.

The study of lunar rocks

Most of the lunar rocks are stored in the Lunar Receiving Laboratory at the Lyndon B. Johnson Space Center in Houston, Texas. A small percentage is distributed in support facilities at Brooks Air Force Base, near San Antonio, Texas. Many lunar samples are in researchers' laboratories around the world. A small number of moon rocks are on public display in museums and only three pieces can be touched. These are the “touchable rocks” cut from basalt rocks from the Apollo 17 mission. One of these rocks is located at the Smithsonian Air and Space Museum in Washington, D.C. Another piece is at the Space Center Houston near the Johnson Space Center. A third rock that can be touched is in the Museum of Sciences at the National Autonomous University of Mexico.