The origin of the moon
Several hypotheses have been proposed to explain the formation of the Moon which, based on the isotopic dating of the lunar rock samples brought to Earth by astronauts, dates back to 4.527 ± 0.010 billion years ago, i.e. about 50 million years after the formation of the solar system.
Fission theory
Historically, the first theories suggested that the Moon originated from the Earth, detaching itself by fission from its crust due to the effect of centrifugal force and creating the basin of an ocean (presumably the Pacific Ocean). This theory, known as fission theory, would however require an initial value that is too high for the Earth's rotation; and it is not compatible with the relatively young age of the oceanic crust.
Capture theory
Another theory, called capture theory, hypothesizes instead that the Moon formed in another part of the solar system and that it was subsequently captured by the Earth's gravitational attraction, which however would require an enormous extension of the Earth's atmosphere to dissipate the kinetic energy of the satellite in transit.
Growth theory
The accretion hypothesis assumes that the Earth and the Moon formed together in the same period from the primordial accretion disk. In this theory, the Moon was formed from the materials surrounding the proto-Earth, similarly to how the planets formed around the Sun. However, this hypothesis does not satisfactorily explain the scarcity of metallic iron on the Moon.
None of these theories, however, can explain the high angular momentum of the Earth-Moon system.
Giant impact theory
The giant impact theory is currently the most accepted by the scientific community. It was proposed in 1975 by William K. Hartmann and Donald R. Davis who hypothesized the impact of a body the size of Mars (which is called Theia or Orpheus) with the Earth. From this impact enough material would have been generated, in the circumterrestrial orbit, to allow the formation of the Moon.
The Canadian astronomer Alastair G. W. Cameron was also a staunch supporter of this thesis. Planets are also thought to have formed through the turning of smaller bodies into larger objects and, nowadays, it is recognized that impacts like this may have occurred to some other planets as well. Indeed, it is hypothesized that at the time of the formation of the Solar System, the planets that made it up were tens if not hundreds, and as their orbits led them to impact up to the current (not definitive) equilibrium, many were destroyed or merged into larger bodies. For example, the presence of lithium (the third element of the periodic table) on the Sun, which has no stellar origin as it is not formed in the cycle of the stars, but dates back to the first moments after the Big Bang, is perhaps due to a gaseous planet larger than Jupiter which collided with our star in ancient times.
As for the origin of the Moon, the hypothetical body Theia would have formed in a Lagrange point relative to the Earth, that is, in a gravitationally stable position along the same orbit of our planet. Here Theia would have grown progressively by incorporating the planetesimals and debris that occupied in large numbers the inner regions of the solar system shortly after its formation. As Theia grew to the size of Mars, its mass became too high to remain stably at Lagrange's point. According to this theory, 34 million years after the formation of the Earth (about 4533 million years ago) this body struck the Earth at an oblique angle, destroying itself and projecting both its fragments and a significant portion of the Earth's mantle into space.
According to some calculations, two percent of Theia's mass formed a ring of debris, while about half of its mass merged to form the Moon, a process that could have been completed within a century. It is also possible that a part of the heavier core of Theia has sunk into the Earth itself and merged with the original core of our planet. Computer simulations of the impact are able to predict both the value of the angular momentum of the Earth-Moon system and the small size of the lunar core.
It is believed that such an impact would have completely sterilized the earth's surface, causing the evaporation of any primordial seas and the destruction of all kinds of complex molecules. If ever on Earth there were already processes of formation of organic molecules at work, the impact of Theia should have abruptly interrupted them.
It has also been suggested that as a result of the impact other objects of significant size (but still smaller than the Moon's) were formed that would continue to orbit the Earth, perhaps occupying one of the Lagrange points of the Earth-Moon system. In the span of a hundred million years at most, the gravitational actions of the other planets and the Sun would still have destabilized their orbits, causing them to escape from the system or collisions with the planet or the Moon. A study published in 2011 suggests that a collision between the Moon and one of these smaller bodies (one-thirtieth the size of the Moon) could have caused the notable differences in physical characteristics between the two faces of the Moon. The simulations conducted suggest that if the impact between the two satellites had occurred with a sufficiently low velocity, it would not have led to the formation of a crater, but the material of the minor body would have "smeared" on the Moon, adding a thick layer of the crust of the highlands to its surface (which today we see occupying the hidden face of the Moon, whose crust is about 50 km thicker than those of the visible face).
The clues that support this theory derive from the rocks collected during the landings of the Apollo missions, which showed compositions of oxygen isotopes almost equal to those of Earth. Furthermore, the presence of samples of KREEP-type rocks (i.e. containing K = potassium, REE = Rare Earth Elements, P = phosphorus) indicate that in an earlier period a large part of the Moon was in a fluid state and the giant impact theory easily explains the origin of the energy required to form such an ocean of lava. There is several evidence that the Moon has a ferrous core, albeit a small one. Specifically, the average density, moment of inertia, and magnetic induction suggest it must be about a quarter of the moon's radius. For comparison, the other terrestrial-type bodies have a core equal to half the radius. The Moon would therefore have formed mainly from material coming from the Earth's mantle and from the object it impacted, while the core of the latter would have joined the Earth, thus explaining the value of the angular momentum. Obviously, the fact that the Moon is geologically dead, that is, completely solidified, including the iron core, prevents it from having an appreciable magnetic field similar to that of the Earth.
The same can be said about Mars, even if it is not sure that the planet has completely cooled down, especially due to the fact that all the heavier elements are concentrated in the center of the planets, due to the action of gravity during their formation, not only the ferrous elements, but also the radioactive ones, from which a large part of the heat develops which prevents it from cooling prematurely.
The still open questions regarding the giant impact hypothesis are:
- Some volatile elements of the Moon have not depleted as predicted by the theory.
- The percentage of iron oxide (FeO) of the Moon implies that the proto-lunar would come from a small fraction of the Earth's mantle;
- If the proto-lunar material comes from the impacted body, the Moon should be rich in siderophilic elements, but minimal quantities have been detected.
Counter-evidence
A recent study (May 2011) conducted by NASA brings evidences that tend to disprove this hypothesis. The study, carried out on lunar volcanic samples solidified 3.7 billion years ago and collected by the Apollo 17 mission in 1972, made it possible to measure a concentration of water in the lunar magma 100 times higher than those previously estimated. Volcanic rocks tend to include in them, within crystalline microstructures, some volatile elements, including water, and with very sophisticated analyzes it is possible to derive the quantity of water present in the lunar soil. According to the impact theory, the water should have dissolved almost completely during the impact, while from the data obtained here the estimated amount of water is similar to that present in the earth's crust. However, if I can throw a spear in favor of this hypothesis, it must be remembered that the formation of water on Earth occurred thanks to countless impacts of comets and asteroids from the primordial Kuiper Belt, which during the formation of the Solar System was destabilized from the orbits of Neptune and Uranus that passed through it, before placing themselves in their current inverted positions at the edge of the system.
This has been hypothesized to occur due to the gravitational perturbations of Saturn and Jupiter towards the other two gas giants. All of this caused a bombardment of the inner rocky planets that supplied them with water in such quantities as to flood them in the true sense of the word, and the Earth today with its immense oceans is the proof. The fate of water on the Moon and Mars was then different, but this does not prevent us from assuming that, even if the impact of Theia dissolved the water present, new contributions following the lunar and terrestrial cooling by the asteroids overflowing the Solar System may have supplied the Earth and its satellite, including a gravitational recovery fallout of the same water removed in the form of gas and steam from the impact with Theia.
