How did the Moon form? It seems we need to know the answer, because we have more than a ton of its kind and the results of numerous studies conducted both from orbit and on the surface of the Earth’s satellite. An international group of scientists analyzed the research data and concluded that supporters of the most popular hypothesis will not like it very much.

The Moon is a strange object by satellite standards. Its diameter is quite “normal”, but its mass is 1.2 percent of the Earth’s mass, making it the largest satellite in the Solar System relative to its planet. There is another question about the structure: the small metal core makes up only four percent of the mass of the entire celestial body. For comparison: the Earth’s core makes up more than 30% of its mass. Finally, a natural satellite orbits the planet very quickly. In general, the history of the Moon’s formation should explain all these anomalies.

The overall density of the Moon is similar to that of the Earth’s mantle; the isotopic composition of the dark lunar rocks is most similar to that of terrestrial volcanic rocks. These similarities have led to the hypothesis that the Moon was formed by the collision of the Earth with a larger object. Over time, a satellite was formed from the planet’s material that collided into orbit. Studies of the evolution of the motion of the Earth-Moon system have led to the same conclusion.

The main problem with the collision hypothesis is that with such a development of events the Moon was formed not from the Earth, as computer models show, but mainly from the material of an “alien” body. In an attempt to explain the same composition of the satellite and the planet, scientists put forward similar hypotheses, the basis of which, as before, was a collision. For example, according to one of them, the object flew away, throwing matter from the Earth, according to the other, two “half-Earths” collided, completely “mixed” and formed the Earth and the Moon.

An international group of scientists from the institutes of the Technical University of Zurich, Switzerland, and the University of Rochester (USA) combined the results of geochemical and geophysical studies of the Moon to understand which of the moon formation scenarios is the most probable. The study is publicly available on the website arXiv.

A new geophysical analysis has found that the Moon’s core, with a radius of 300 ± 20 km, makes up about 0.8 to 1.5 percent of the satellite’s mass. Such “core” parameters impose restrictions on the composition of the core and other layers. If the rest of the Moon is similar in composition to the Earth’s upper mantle, then the core must be extremely dense (about 7.8 thousand kilograms per cubic meter) and composed almost exclusively of an alloy of iron and nickel. If the Moon’s composition contains more iron oxide than the Earth’s mantle, then the core must not be as dense and may contain significant amounts of other elements.

To date, we do not have seismic data on the structure of the Moon at depths below 1,100 kilometers (the radius of the moon is 1,700 kilometers). Perhaps the necessary information will be collected by the sensitive Farside Seismic Package seismometers that could go to the moon’s surface as part of the Lunar Geophysical Network mission (not yet confirmed) or the CLPS program (NASA).

The authors of the new scientific paper also deduced the relative content of 70 elements in the mantle and crust of the Moon. According to these data, the Earth and the Moon were formed from the same material, that is, from the same part of the protoplanetary disk, without impurities from the hypothetical impactor. Our satellite was formed about 4.5 billion years ago, from material that at that time had almost no volatile matter left.

To clarify the history of the formation of the different layers of the Moon, it is necessary to analyze rocks (preferably the mantle) collected in other parts of the satellite, far from where the instruments have already landed. Such samples could be delivered by the Chang’e-6 mission (China) and the Endurance lunar rover (concept, NASA).

The problem of the absence of “impurities” is solved by scenarios of a very powerful collision and “mixing” of the impactor with the precursor matter of the Earth. As the authors of the new study note, the behavior of matter in such extreme conditions has not yet been fully studied and is not included in numerical models of similar phenomena. The ratio of elements and their isotopes on the Moon at most cannot be formed at high temperatures, which is inevitable in a powerful collision with the “mixture” of matter.

In general, the Moon shows no traces of the collision of the Earth with the hypothetical planet Thea, which was in the process of formation. Either this event occurred, or in the process, the matter of our planet was perfectly mixed with the “guest” matter even before the natural formation of the moon. As the scientists concluded, “The giant impact model is the worst model we have to date for the formation of the Moon, not counting other models that have been tried to be proven more than once.”