The lack of "seas" and the abundance of mountains on the back of the Moon may be due to the impact of another Earth satellite, American planetologists believe. Such a companion could probably have formed with the Moon as a result of a clash between a young Earth and a planet the size of Mars. Its slow decline to the moon led to one half of it being covered with an uneven rock layer, tens of kilometers thick.

For billions of years, tidal forces have compared the time that the Moon once turns around its axis and its time around the Earth. For this reason, the Moon is always turned towards the Earth from one side and we can say that until the beginning of the space flight era, humanity had only a one-sided view of our nearest celestial neighbor.

The first image of the back of the Moon was sent to Earth by the Soviet automatic station "Luna-3" in 1959. It already showed that the two hemispheres of the Moon are not completely similar. The surface of the invisible side is covered by a number of high mountains and craters, while the side towards the Earth has many more planar formations and fewer mountain massifs.

Visible (A) and invisible (B) side of the Moon. The character of their relief varies considerably -

on the back is much more high mountains and craters.

According to photographs: John D. Dix, Astronomy: Journey to the Cosmic Frontier

Together with the fundamental question of the origin of the Moon as such, the difference between the terrain of its hemisphere remains one of the unresolved problems of contemporary planet science.
It excites people's minds, and even creates a totally fantastic hypothesis, according to one of them, the Moon has been recently connected with the Earth and its asymmetry is caused by a "scar" after separation.
The most common contemporary theories about the origin of the Moon are the so-called "Big Splash Theory" or "Giant Impact". According to them, in the early stages of the Solar System, the young Earth collided with a body comparable to the size of Mars. This cosmic catastrophe brought many splinters in Earth's orbit, part of which formed the Moon, and part fell back to Earth.

Planetologists Martin Jutzi and Erik Asphaug of the University of California (Santa Cruz, USA) have proposed an idea that is theoretically able to elucidate the differences in the relief of the visible and the back of the Moon. In their opinion, some huge collision could have created not only the Moon itself, but also another satellite of smaller dimensions. It originally remained in the same orbit as the Moon, but eventually fell on its larger brother and covered with its rock one of its sides, which is formed by another layer of rocks several tens of kilometers thick. They published their work in the journal Nature. (http://www.nature.com/news/2011/110803/full/news.2011.456.html)

Such conclusions were reached on the basis of a computer simulation performed on the "Pleiades" supercomputer. Even before they modeled the collision itself, Erik Asphaug discovered that outside the Moon, from the same protolunar disk, another small companion with one-third dimensions and a mass of about one-thirtieth of the Moon could have formed. Although, in order to stay in orbit long enough, it should reach one of the so-called Trojan points in the lunar orbit, which are the points where the forces of gravity of the Earth and the Moon balance. This allows bodies to stay in them for tens of millions of years. During such a time, the Moon itself was able to cool and harden its surface.

Finally, due to the gradual moving of the Moon from Earth, the position of another satellite in the orbit proved unsustainable, and it slowly (of course, cosmic) at about 2,5 km / s met the moon. What has happened can not even be called a collision in the usual sense, so there was no crater at the collision, but the lunar rock spread out. A large part of the incident body just fell to the moon and covered one half of it with a new thick layer of rock.
The final appearance of the moon terrain that they received as a result of computer modeling was very similar to what the back of the Moon actually looks like today.
Clash of the Moon with a small companion, followed by its decay on the Moon's surface and the formation of a difference in the height of the rocks of its two hemispheres. (According to Martin Jutz and Erik Asphaugo's computer model)

In addition, a model of American scientists helps explain the chemical composition of the surface of the Moon's opposite side. The bark of this half of the satellite is relatively rich in potassium, rare earth elements and phosphorus. It is assumed that originally these components (like uranium and thorium) were part of a molten magma, now hardened under a thick layer of moon crust.

The Moon's slow collision with a smaller body, in fact, pushed out the rocks enriched with these elements on the side of the hemisphere opposite the collision. This led to the observed distribution of chemical elements on the surface of the hemisphere visible from Earth.
Of course, the study does not yet solve the problems of Moon origin or the asymmetry of the hemisphere of its surface. But it is a step forward in our understanding of possible ways of developing the young solar system and, above all, our planet.

"The Elegance of Erik Asphaug's work is that he proposes to solve both problems at the same time: it is possible that the giant collision that formed the Moon also created several smaller bodies, one of which then fell to the Moon and led to an observable dichotomy. so Professor Francis Nimmo, a planetologist from the same "University of California", commented on the work of his colleagues. Last year, he published a work in Science magazine, advocating another way to solve the same problem. According to Francis Nimmo, to create a dichotomy of lunar terrain, the tidal forces between the Earth and the Moon are rather responsible, rather than an event that has the character of collision.

"To date, we do not have enough information to be able to choose from the two solutions offered. Which of these two hypotheses will prove to be correct will be clear after what information other space missions and possibly rock samples will bring us ”- added Nimmo.