The moon could have arisen not as a result of a single collision of the Earth and a protoplanet the size of Mars, but in the course of a series of collisions of small planetary "embryos" with the Earth, which would explain the oddities in the isotopic composition of the Moon, according to an article published in the journal Nature Geoscience.
“If the Moon was formed in the course of such a series of collisions, then this would allow solving the problem with isotopes in a very original way. Its formation will be similar to what happens when an artist mixes paints in a palette - the more colors you mix, the less the final result changes and the more it will look like a dark brown paint,”says Gareth Collins of Imperial College London (UK).
Secrets of the Moon
For the past 30 years, it has been generally accepted that the Moon was formed as a result of the collision of Theia, a protoplanetary body, with the "embryo" of the Earth. The collision led to the release of the matter of Theia and the proto-Earth into space, from this matter the Moon was formed. The theory of the collision of the proto-Earth with a large celestial body explains well the mass of the Moon, the low iron content on it, and other parameters.
However, in such a collision, a significant part of the material that makes up the moon should have come from the hypothetical Theia. In its composition, it should have been different from the Earth, as most of the celestial bodies of the inner region of the solar system, which includes the terrestrial planets and asteroids, differ from it. But in fact, the composition of the Earth and the Moon is very similar, up to the same proportion of isotopes of many metals and other elements.
Moon formation diagram
Promotional video:
Rufu et al. / Nature Geoscience 2017
Relatively recently, planetary scientists have proposed a rather exotic solution to this problem - the so-called "yula planet" hypothesis. In accordance with it, the young Earth had to rotate very quickly and at the same time lie on its side, like Uranus, and the collision with Theia was supposed to slow it down and turn its axis. Such a scenario, in principle, has the right to life, but it is extremely unlikely, which makes planetary scientists look for other options for the birth of the Moon.
Hagai Perets of the Israel Institute of Technology in Haifa and his colleagues proposed an alternative explanation for the absence of differences in isotope fractions in the rocks of the Moon and Earth, suggesting that the companion of our planet could not have formed "in one sitting", but in parts.
Space billiards
If the Earth collided not with one Theia the size of Mars, but with a whole set of small protoplanets, then the speed of rotation of our planet and the consequences of the collision may not be as large as the classical scenario of the formation of the Moon requires. Each time such "mini-moons" hit the Earth, they threw some of its matter into orbit, where it merged with the pre-existing Moon, formed after the first collision.
Guided by this idea. Peretz and his colleagues calculated the consequences of a collision of the Earth with several such "embryos" and checked whether the Moon could grow to its present size and achieve the desired isotope ratio in this way.
As it turned out, such a scenario is indeed possible - the sequential fall and destruction of about 20 small protoplanetary bodies, whose mass is about 1-10% of the mass, will be enough to form the Moon in its present size and with its present composition. On the other hand, if even one "mini-moon" escapes from the Earth, then the formation of the moon will require a much larger number of planetary embryos.
Such a hypothesis may seem even less realistic than the idea of a "yula planet", but in the first epochs of the life of the solar system, it was inhabited by thousands of large and many small embryos of planets, many of which were "catapulted" into open space or set on a collision course with the Earth and other planets at the moment when Jupiter and Saturn began their movement towards the Sun. It is possible that some of them crashed into the proto-Earth and gave birth to the Moon.
