The giant impact caused a difference between the hemispheres of the moon

The blatant difference between the dark side of the heavily cratered Moon and the lower open basins facing the Earth has left scientists perplexed for decades.

New evidence regarding the Moon's crust suggests that the differences were caused by a lost dwarf planet colliding with the Moon at the beginning of the solar system's history. A report on the new research was published in AGU & # 39; s Journal of Geophysical Research: planets.

The mystery of both sides of the Moon began in the Apollo era when the first views of its hidden face revealed surprising differences. Measurements made by the GRAIL (Gravity Recovery and Interior Recovery) mission in 2012 helped to better define the structure of the Moon, including the thickness of its crust and the inclusion of a extra layer of material on its hidden side.

A number of ideas have been used to try to explain the asymmetry of the Moon. The first is that there were once two moons in orbit around the Earth and they merged at the very beginning of the formation of the Moon. Another idea is that a large body, perhaps a young dwarf planet, has been found in an orbit around the Sun that has collided with the Moon. This idea of ​​giant impact would have occurred a bit later than in a moon-melting scenario and after the moon formed a solid crust, said Meng Hua Zhu of the Institute of Space Sciences of the Macao University of Science and Technology and lead author of the new study. Signs of such impact should be visible in lunar crustal structure today.

"The detailed gravimetric data obtained by GRAIL has provided a better understanding of the structure of the lunar crust below the surface," Zhu said.

The new GRAIL discoveries provided the Zhu research team with a clearer goal using computer simulations used to test different early-moon impact scenarios. The authors of the study conducted 360 computer simulations of giant impacts with the Moon to determine if such an event, millions of years ago, could replicate the lunar crust of the moon. Today, as detected by GRAIL.

They found that the best solution for today 's asymmetrical moon is a large body, about 780 kilometers (480 km) in diameter, projecting into the near – moon part. at 22,500 kilometers at the hour (14,000 miles at the hour). This would be the equivalent of an object a little smaller than the dwarf planet Ceres moving at a speed about four times slower than meteors, pebbles and grains of sand that burn like "shooting stars" in Earth's atmosphere. The team also found another appropriate fit for the impact suits: a slightly smaller diameter, 450 km (720 km), an object reaching a speed slightly above 15,000 km / h (24,500 km / h) .

In both scenarios, the model shows that the impact would have projected large amounts of material that would fall on the surface of the Moon, burying the primordial crust on the hidden face in 5 to 10 kilometers of debris. This is the added crust layer detected on the hidden side by GRAIL, according to Zhu.

The new study suggests that the impactor was probably not a second moon at the beginning of the Earth. Regardless of the impactor – an asteroid or a dwarf planet – he was probably on his own orbit around the Sun when he met the Moon, Zhu said.

The giant-impact model also provides a good explanation for the unexplained differences in potassium, phosphorus and rare earth element isotopes such as tungsten-182 between the Earth's and the Moon's surfaces, the researchers explained. These elements could come from the giant impact, which would have added this material to the Moon after its formation, according to the study's authors.

"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.

The new study not only suggests an answer to current questions about the moon, but can also provide insight into the structure of other asymmetrical worlds in our solar system, as Mars wrote, the researchers.

"It's a very provocative article," said Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University and editor-in-chief of JGR: Planets. "Understanding the origin of the differences between the near and the opposite side of the moon is a fundamental problem in lunar science, because several planets have hemispherical dichotomies, but for the moon, we have a lot of data to test models and hypotheses with, so the implications of the work could probably be broader than just the moon. "