The Moon was created when a Mars-sized body called Theia collided with Earth, according to the giant impact hypothesis. However, evidence of an Earth magma ocean predicted by the theory cannot be found. The Moon’s creation may have been caused by a wobbling mass at point L5, which eventually collided with Earth, ejecting material that formed the Moon. The Moon’s size and composition require a special explanation, given by the giant impact hypothesis.
The dominant theory explaining the creation of the Moon is the giant impact hypothesis, which holds that 4.533 billion years ago (only about 70 million years after the formation of the Earth itself), a Mars-sized body called Theia it catastrophically collided with the Earth, throwing trillions of tons of magma and rock into Earth’s orbit, which coalesced and cooled to become the Moon. However, this theory isn’t perfect: It predicts that the Earth had a magma ocean at one time, for which no evidence can be found.
In the early history of the solar system, when the planets were just forming, the solar system was a busier place. Orbits were still stabilizing, and many early asteroids degraded into unstable orbits that fell into the Sun and Jupiter or were ejected into the outer reaches of the solar system. A gravitationally stable point for space rock coalescence, point L5, had a special harmony with Earth. Located within Earth’s orbit but occupying a different position than Earth itself, L5 is the current home of the Trojan asteroids.
As the material aggregated at L5, it is thought to have eventually surpassed the mass of Mars, which made its path unstable, swinging along the length of Earth’s orbit rather than remaining stationary at L5. Soon enough, this wobble brought it directly into contact with the Earth, in an event called the big bang or big splash. Tons of material were ejected and after 1-100 years this material coalesced to become the Moon, which was initially covered in magma oceans. Evidence of these oceans was found by the Apollo astronauts.
The Moon is peculiar for orbiting the Earth due to its relative large size. Venus, an Earth-sized planet, has no moon despite its similar mass. Mars has only a few, very small ones, with radii of less than 20 km. So Earth’s astonishing moon, among the largest in the solar system, requires a special explanation, given by the giant impact hypothesis. The observation that the composition of the Moon is similar to that of the earth’s crust, but not our mantle, has led scientists to formulate the giant impact hypothesis.
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