The natural satellite of the Earth is a very unusual object for the solar system. Recently, scientists have put forward a new theory explaining how the Moon got to where it is today, making some adjustments to the modern theory of a giant collision. Lead author Professor Sarah Stewart of the University of California outlined the new theory on October 31 in an article in the journal Nature.
The Moon is relatively large compared to the planet it revolves around. In addition, in its chemical composition, it is almost identical to the Earth, with the exception of some volatile compounds that evaporated in the distant past. This is what sets the moon apart from any other large object in the solar system, explains Sarah Stewart. She emphasizes that every other body in the solar system has a different chemical composition.
Traditionally, the theory of the origin of the moon, which can be read in any classic textbook, is as follows. At the end of the solar system's formation period, the "giant collision" phase began, when hot planet-sized objects collided with each other. A Mars-sized object touched another space object, which later evolved into planet Earth. At the same time, part of the substance was thrown into outer space - from this piece the Moon was formed. During the collision, the Earth received a significant increase in the speed of rotation, as a result of which the planet made one revolution around its axis in just 5 hours. Over the millennia, the Moon moved away from the Earth, and the planet's rotation speed slowed down, as a result of which, by now, the day began to last for 24 hours.
This theory was deduced by scientists in the course of observations of the current orbit of the Moon, the relationship of the rotational moment of the Earth-Moon system and the tidal forces between these two objects.
However, this traditional theory is not devoid of controversy and open questions. One of them is the composition of the moon, surprisingly similar to the Earth. Another is that if the Moon was formed from matter revolving around the Earth's equator, then its orbit would have to rotate relative to the equator. However, the current orbit of the Moon is tilted five degrees relative to the equator, which means that some other energy not taken into account by this theory should affect the movement of the Moon.
Professor Stewart and her colleagues (Mathia Cook of the US SETI Institute, Douglas Hamilton of the University of Maryland, and Simon Locke of Harvard University) have developed an alternative model that explains these inconsistencies in traditional theory.
In 2012, Cook and Stewart proposed that part of the torque in the Earth-Moon system could be transferred to the Earth-Sun system. This provoked more vigorous collisions early in the planetary formation process.
According to the new model, high-energy collisions produced a large amount of vaporized and molten matter, from which the Earth and the Moon were formed. As a result of this process, the Earth rotated around its axis with a two-hour interval, and the axis of rotation was directed towards the Sun.
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Since the collision could be more energetic than conventional theory suggests, the material from the Earth and the object that collided with it could have mixed, and the Earth and the Moon were formed from the same material, hence their chemical composition was similar.
As the rate of rotation slowed down due to tidal forces, the Moon moved away from the Earth until it reached a point called the “transition to the Laplace plane”, when the force of the Earth's influence on the Moon became less than the gravitational forces of the Sun. This led to the fact that part of the torque of the Earth-Moon system was transferred to the Earth-Sun system. This did not significantly affect the Earth's orbit around the Sun, but turned the Earth vertically. At this point, as shown by the model built by Professor Stewart's team, the Moon revolved around the Earth at a large angle, relative to the equator.
Over the course of several tens of millions of years, the Moon continued to slowly move away from the Earth until it reached the second transition point, the Cassini transition, after which the angle of inclination of the Moon's orbit relative to the Earth's equator changed by about five degrees.
The new theory elegantly explains the Moon's orbital and chemical composition from a single giant collision at the start. No additional intermediate steps were required to push this process forward, says Professor Stewart.
