Why the moon has two faces

Scientists have suggested that an old collision between the moon and a dwarf planet left half the far side of the cratered lunar surface.

The new research suggests that a collision between the moon and a "slightly smaller" object than a dwarf planet in the early stages of the solar system is the best explanation.

The difference between the visible face of the Moon facing the Earth and the hidden face of the healed Moon is the subject of debate between scientists since the Apollo era.

Previously, scientists had suggested that the Earth had merged two billion years ago, creating the uneven surface of the far side that exists today.

Measurements made by the GRAIL (Gravity Recovery and Interior Laboratory) mission in 2012 have provided scientists with additional information on the structure of the Moon.

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These included the fact that the dark side of the moon had a thicker crust than the near face and included an extra layer of material, while the near face was characterized by a thinner and smoother crust.

"The detailed gravimetric data obtained by GRAIL has helped to better understand the structure of the lunar crust beneath the surface," said Dr. Meng Hua Zhu, co-author of the study.

Dr. Hua Zhu and his team performed 360 computer simulations of different scenarios in which impacts with the moon could lead to the crust of the moon today.

They found the best solution is a big body, about 480 km (780 km) in diameter, to hit the near moon at 22,500 kilometers an hour.

The model shows that the impact would have projected large amounts of material that would fall on the surface of the moon, burying the crust on the other side in debris of 3 to 6 miles (5 to 10 kilometers).

The model also provides a good explanation for the unexplained differences in isotopes of potassium, phosphorus, and rare earth elements such as tungsten 182 between the Earth's and the Moon's surfaces.

"Our model can therefore explain this isotopic anomaly in the context of the giant impact scenario of the origin of the Moon." the researchers write.

"It's a very provocative article," said Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University.

"Understanding the origin of the differences between the near and the opposite side of the moon is a fundamental issue in lunar science.

"Indeed, several planets have hemispherical dichotomies, but for the Moon, we have plenty of data to test models and hypotheses, so the implications of this work may be broader than those of the Moon."

Scientists hope to add to their models data from lunar missions, including the Chinese mission Chang & # 39; e-4, which landed on the other side of the moon in January.

The analysis of the Chang-e-4 mission data revealed substances from inside the moon at the surface of the crater where the probe landed.

This provides further evidence that a huge collision occurred at least 3.9 billion years ago.

The research was published in the Journal of Geophysical Research: AGU Plants.