The familiar lunar view that humans see when they look at the night reveals a face with dark "seas" and bright craters. The mysterious far side, which has not been revealed until humans have started sending probes and then people over the years, is much more heavily cratered, with some of the dark plains of the near side.
But what caused the two parts so different? It was a matter of debate.
Now, researchers led by Meng-Hua Zhu, of the Macau University of Science and Technology, say that a dwarf planet that hits the Moon some time after its initial formation could have caused the lopsided world we see today. They published their research on May 20th in Journal of Geophysical Research: Planets.
A series of planetary-sized collisions
The standard story of the formation of the Moon is that some time in the history of the Earth, a planetary-sized body has hit our planet, throwing material into space, where it eventually coalesced into the Moon we see today. But even this dramatic story cannot explain all the strangeness of the Moon of the Earth, like the difference between the near and far sides of the Moon.
A clue came in 2012, when NASA's GRAIL mission showed that the near-lunar side has a thinner crust than the opposite side.
So over the years, when both sides suffered numerous asteroid attacks, the near side opened. The effused lava filled the basins and hardened in the dark Maria, or plain, today we see. But the far side has a thicker crust, so that's just a lot of craterization. But the puzzle of why the two hemispheres have different thicknesses to start with is more difficult to explain.
One way to solve the problem is if a small world – the size of the dwarf planet Ceres, which resides in the asteroid belt – should hit the near side of the Moon after it had already formed and solidified. The impact would have lifted the material, and when the material has settled, it may have fallen mainly on the far side of the Moon, burying it under 3-6 miles of lunar regolith. Zhu and colleagues conducted a series of computer simulations to test this scenario, and found that an object that rammed the Moon in advance between 14,000 and 15,000 miles now recreated the Moon as we see it today. It may seem fast, but it's only about a quarter of the speed of most of the small meteors that hit the Earth.
The impact could also help clarify some of the long-standing questions about the materials that make up the Moon. They are both very similar to Earth in some way – a sign that the Moon actually formed from the Earth's debris – and dissimilar in other ways, indicating additions from an external world.
This would make our moon a compound of three different large bodies – a different kind of three-body problem.
. [tagToTranslate] What forms the moon