Massive impact likely shaped up Moon's two faces

Collision with a wayward dwarf planet may have caused the stark difference between the Moon's heavily-cratered farside and the lower-lying open basins of the Earth-facing nearside, according to a study.

The mystery of the Moon's two faces began in the Apollo era when the first views of its farside revealed the surprising differences, according to the study published in Journal of Geophysical Research: Planets. 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, researchers said. 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, from Macau University of Science and Technology.

Signs of such an impact should be visible in the structure of the lunar crust today. "The detailed gravity data obtained by GRAIL has given new insight into the structure of the lunar crust underneath the surface," Zhu said. The new findings from GRAIL gave Zhu's team of researchers a clearer target to aim for with the computer simulations they used to test different early-Moon impact scenarios.