AGU: Giant Impact Caused Difference Between Moon’s Hemispheres

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AGU: Giant Impact Caused Difference Between Moon’s Hemispheres

Post by bystander » Mon May 20, 2019 4:42 pm

Giant Impact Caused Difference Between Moon’s Hemispheres
American Geophysical Union | 2019 May 20
The stark difference between the Moon’s heavily-cratered farside and the lower-lying open basins of the Earth-facing nearside has puzzled scientists for decades.

Now, new evidence about the Moon’s crust suggests the differences were caused by a wayward dwarf planet colliding with the Moon in the early history of the solar system. ...

The mystery of the Moon’s two faces began in the Apollo era when the first views of its farside revealed the surprising differences. Measurements made by the Gravity Recovery and Interior Laboratory (GRAIL) mission in 2012 filled in more details about the structure of the Moon — including how its crust is thicker and includes an extra layer of material on its farside.

There are a number of ideas that have been used to try and explain the Moon’s asymmetry. One is that there were once two moons orbiting Earth and they merged in the very early days of the Moon’s formation. Another idea is that a large body, perhaps a young dwarf planet, found itself in an orbit around the Sun that put it on a collision course with the Moon. This latter giant impact idea would have happened somewhat later than a merging-moons scenario and after the Moon had formed a solid crust, said Meng Hua Zhu ... Signs of such an impact should be visible in the structure of the lunar crust today. ...

the best fit for today's asymmetrical Moon is a large body, about 480 miles (780 kilometers) in diameter, smacking into the nearside of the Moon at 14,000 miles per hour (22,500 kilometers per hour). That would be the equivalent of an object a bit smaller than the dwarf planet Ceres moving at a speed about one-quarter as fast as the meteor pebbles and sand grains that burn up as "shooting stars" in Earth's atmosphere. Another good fit for the impact combinations the team modeled is a slightly smaller, 450-mile (720-kilometer) diameter, object hitting at a mildly faster 15,000 miles per hour (24,500 kilometers per hour).

Under both these scenarios, the model shows the impact would have thrown up vast amounts of material that would fall back on the Moon's surface, burying the primordial crust on the farside in 3 to 6 miles (5 to 10 kilometers) of debris. That is the added layer of crust detected on the farside by GRAIL, according to Zhu. ...

Are the Moon's nearside‐farside asymmetries the result of a giant impact? ~ Meng-Hua Zhu et al
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