Born from fire. How the Moon was formed and why it looks nothing like Earth

The Moon is the best-studied body in the Solar System after the Earth. It has been explored by numerous automated vehicles and the Apollo missions, which brought back a generous collection of lunar rocks. The data they collected helped shed light on many lunar mysteries.

So, here is how the Earth’s satellite was born, when volcanoes stopped erupting there, and why its visible side is so different from the reverse.

Giant blow

We take the presence of the Moon for granted. But if you look at our neighbors in the Solar System, it becomes obvious that this approach is wrong. Mercury and Venus have no satellites at all, and Mars is endowed with two tiny companions – and one of them (Phobos) will collapse in just a few tens of millions of years.

So why does the Earth have the Moon? And why is it so big? In the entire Solar System, there is no other pair in which the satellite would have such a large size and mass compared to the planet around which it orbits.

The first hypotheses about the origin of the Moon date back to the 18th century. Scientists made various assumptions, from the joint formation of two bodies to the fact that Earth’s gravity captured the Moon. Everything changed with the beginning of the space age. The analysis of lunar soil samples brought to Earth prompted discarding of all the numerous versions of the Moon’s formation. They were replaced by a completely new hypothesis of a giant collision, first proposed in 1975.

According to the theory, about 4.5 billion years ago, the newborn Earth collided with a Mars-sized protoplanet (commonly called Theia). This impact was of crucial importance for the future of our planet. Most of Theia’s material and a significant portion of the Earth’s mantle were ejected into Earth’s orbit. These fragments later formed the Moon. The Earth received a sharp increase in rotation speed, and its axis tilted. This tilt causes the seasons to change on our planet, which is one of the key factors that ensure the existence of life.

The giant impact hypothesis logically explains many of the Moon’s characteristic features. It is supported by the fact that the Moon’s orbit has a similar orientation to the Earth’s rotation, both bodies have a very similar isotopic composition, and the entire surface of the Moon was molten in the past. Of course, the impact hypothesis has several weaknesses. However, most of them can be explained by the peculiarities of where Theia was formed and the impact scenario. That is why the impact hypothesis has dominated the scientific community for the last half century.

What kind of Moon did dinosaurs see?

So, immediately after its birth, the Moon was not at all like it is now. It was completely covered by a giant ocean of magma, which took millions or even tens of millions of years to solidify. In the process of crystallization of the magma ocean, potassium, rare earth elements, phosphorus, and radioactive substances gradually sank. As a result, they (the acronym KREEP is used to designate them) formed a layer of rocks between the lunar crust and mantle, which is a unique feature of the Earth’s satellite.

In addition, at that time, the Moon was much closer to the Earth, at a distance of about 60 thousand kilometers (now this distance is on average 384 thousand kilometers). Therefore, the influence of its gravity was felt more strongly on our planet, leading to gigantic tides, which could also contribute to the emergence of life. And the Moon probably had its magnetosphere. Analysis of soil samples brought to Earth by the Apollo expeditions showed that these rocks were formed about 4 billion years ago in the presence of a fairly powerful magnetic field.

Since, as mentioned above, in that era the Moon was much closer to the Earth than it is today, scientists suggest that their magnetospheres could have been connected. The lunar magnetic field likely acted as an additional shield to protect the Earth’s atmosphere from powerful solar flares. In addition, along the force lines, particles of the Earth’s atmosphere could have reached the Moon. This may explain the presence of nitrogen and some other gases in lunar regolith.

In general, in those days, the Solar System was much more lively than it is now. It was filled with a large amount of building material that remained unused during the formation of the planets. This debris regularly collided with celestial bodies. And the Moon demonstrates how powerful this bombardment was. According to estimates, there are now about seven thousand craters with a diameter of more than 20 km on its surface. Almost all of the largest lunar craters are quite old – they were formed between 3.8 and 4.1 billion years ago. That tumultuous period is sometimes called the Late Heavy Bombardment. It is believed that it could have been caused by a change in the orbits of Jupiter and Saturn, which destabilized the solar system and brought a barrage of asteroids and comets down on the inner planets.

Many scientists associate the appearance of masks, positive gravitational anomalies on the lunar surface, with this bombardment. Their influence has to be taken into account by the designers of vehicles launched to the Moon. According to one version, the masks were formed when the denser lunar mantle squeezed up the bottom of the impact pits, forming lenses beneath them that consisted of denser matter that rose. A certain part of the mantle then broke through in the form of lava to the surface and filled the part that remained in this geologic depression. This is how denser regions were formed in the lunar crust.

Subsequently, the frequency of celestial bodies falling on the Moon decreased significantly. However, space bombardment has not stopped completely. Over the past billion years, several new craters have appeared on the lunar surface. Of course, they are smaller than the huge craters of the Late Heavy Bombardment era. But due to their youth, they are visible when observing the Moon even with a small telescope or binoculars. These are the craters Copernicus (93 km), Aristarchus (40 km), and Tycho (80 km). The youngest of the three is Tycho, surrounded by a characteristic ray system. Its age is 108 million years. So, we can safely say that the dinosaurs of the Cretaceous period (among which the most famous are tyrannosaurs, triceratops, and velociraptor) saw the Moon the same way we see it today.

Lunar volcanism and atmosphere

There are many characteristic dark spots on the surface of the Moon. They are called seas, which is quite ironic. The fact is that once these seas were indeed liquid, but they were not filled with water at all. They were formed by numerous eruptions.

Volcanism is the second most important factor, after the merciless meteorite bombardment, that gave the Moon its modern appearance. Eruptions on its surface began 4.2 billion years ago. And they were not at all like the ones that are happening on our Earth today. Since there is no plate tectonics on the Moon, eruptions continued in the same places for hundreds of millions of years. It is believed that they peaked between 3 and 3.8 billion years ago, when violent lava flows covered vast areas of the lunar surface. When it hardened, it formed giant basalt plains that stand out from the rest of the lunar surface due to their darker color. These are what we call seas.

The lunar eruptions of that era were so intense that it is possible that, for some, albeit relatively short, period, the Moon had a certain semblance of an atmosphere. This is at least what the results of a study conducted in 2017 indicate. It showed that the gas emissions during the eruptions were enough to form an atmosphere around the Moon consisting of carbon monoxide, sulfur, and water. At its peak (approximately 3.5 billion years ago), it was even more powerful than the current Martian atmosphere. The lunar atmosphere was probably capable of weathering rocks and causing storms. Its presence may also explain the existence of massive deposits of water ice at the bottom of the Moon’s perpetually shadowed polar craters. The atmosphere could have become a source of water for them.

Scientists estimate that the gas shell around the Moon has existed for about 70 million years. As volcanic activity declined, so did the release of gases. After that, the lunar atmosphere quickly evaporated. Around the same time, the Moon lost its magnetic field, which is usually associated with the solidification of its core.

For a long time, it was believed that volcanic activity on the Moon stopped completely about a billion years ago. But recent research shows that eruptions could have lasted much longer than previously thought. Thus, the LRO spacecraft managed to identify many areas covered with fairly fresh (by geological standards) lava deposits. Some of them are estimated to be 50 million years old. These data are also supported by the results of the analysis of lunar soil delivered by the Chang’e-5 mission. They found beads of volcanic glass 123 million years old. So, our planet’s satellite still retains some geological activity. Perhaps someday we will see a lunar volcano erupt.

The mystery of the reverse side of the Moon

If you look at any photograph of the reverse side of the Moon, you will notice that it is completely different from the one facing the Earth. The key difference is the almost complete absence of seas. While they occupy approximately 31% of the visible side of the satellite, in the case of the reverse side, this figure is only 1%.

But these are not all the differences. The reverse side of the Moon is much more cratered than the visible side. It is there that most of the largest impact formations are located. Also, the Moon’s crust on its back side is about 10 km thicker than on the visible side.

What causes such a significant difference? According to the most popular explanation, it is due to the uneven distribution of the radioactive element-rich KREEP rock layer. Their main deposits are concentrated mainly under the Oceanus Procellarum and the Mare Imbrium on the visible side of the Moon. It is believed that the heat generated by KREEP rocks contributed to volcanic activity and supported colossal eruptions.

But what exactly led to this unevenness? According to one theory, it was caused by a huge collision that created the largest lunar crater, the South Pole-Aitken impact basin, with dimensions of 2400×2050 km. It could have launched a kind of “wave” in the KREEP layer, so its main part was concentrated on the visible side of the Moon.

However, there is also an alternative opinion, which, however, also refers to a large collision. According to it, after the collision of the Earth and Theia, not one but two satellites were formed: the “main” Moon and a smaller body. Subsequently, they merged, which explains the difference between the hemispheres.

The third hypothesis suggests that a large object fell on the visible side of the Moon. The collision resulted in the ejection of a huge amount of material, which then settled on the back side of the Moon, forming an additional layer of rocks.

And finally, according to another version, the lunar asymmetry is associated with the heat that our planet radiated at the dawn of the Solar System. The side of the Moon facing the Earth was subjected to severe heating and remained in a molten state for longer, which explains the increased volcanic activity.

Dangerous moon dust

Since the Moon has no atmosphere, a hypothesis emerged in the mid-twentieth century that over billions of years, the dust that settles from interplanetary space should have formed a layer on its surface so thick that a spacecraft would simply fall through and drown in it when trying to land. This, in particular, was the basis for the plot of Arthur C. Clarke’s novel Moon Dust.

Of course, the fears turned out to be false, as Kharkiv astronomer Mykola Barabashov first proved in the early 20th century. The Moon is covered by regolith, a layer of loose material consisting of rock fragments and minerals. They were formed during the bombardment of its surface for billions of years by micrometeorites and charged particles. This pulverized the lunar rocks into thinner and thinner material. The bulk of the particles are 0.03-1 mm in size. Astronauts and scientists who have studied regolith have noted that it has a characteristic gunpowder smell.

Although it is impossible to drown in regolith, it does pose a certain danger to human health. Since its particles have never been exposed to water, they are very sharp. Inhaling them can lead to lung damage and respiratory diseases. Lunar dust also has toxic properties. Some participants of Apollo expeditions have complained of coughing, throat irritation, watery eyes, and blurred vision. At least one of the astronauts developed a full-blown allergy.

In addition to being harmful to health, due to its abrasive nature, lunar dust can cause abrasion or wear on surfaces due to friction. This means damage to spacesuits, equipment, and solar panels. So, if humanity wants to stay on the Moon for a long time, it will have to find some way to solve the problem of moon dust.

Lunar “attractions”

In addition to craters and traces of ancient eruptions, there are some interesting “attractions” on the surface of our planet’s satellite. These include, in particular, lunar caves. Spacecraft have photographed many holes that probably lead to subsurface tunnels. They are believed to be lava tubes or tunnels.

Many engineers believe that such tubes will serve as one of the best places to create permanent settlements. Their vault can act as a good shield to protect against micrometeorites, radiation, and sudden temperature changes. And if a separate section of the tube is sealed and filled with air, the colonists will have at their disposal huge amounts of internal space.

Another region of the Moon that is worth paying close attention to is its poles. Automated stations have collected a wealth of data showing that many polar craters, whose bottoms are never illuminated by the Sun, contain deposits of water ice. Lunar colonists could use them to extract drinking water, oxygen, and rocket fuel components.

The lunar poles are also interesting for the presence of peaks of eternal light. This is the name given to elevated areas that are almost constantly illuminated by the Sun. This makes them an ideal location for solar power plants that could generate energy for lunar settlements. It is possible that in the future, such areas will become one of the most valuable resources on the Earth’s surface.

This article was published in #2 (191) 2024 of Universe Space Tech magazine. You can buy this issue in electronic or paper versions in our store.