The Moon has striking asymmetries between its nearside and farside in topography, crustal thickness, and composition. A new study, published in the Journal of Geophysical Research: Planets, suggests these asymmetries were caused by a 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 NASA’s 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. Signs of such an impact should be visible in the structure of the lunar crust today,” said study lead author Dr. Meng Hua Zhu, a researcher with the Space Science Institute at Macau University of Science and Technology.
The scientists ran 360 computer simulations of giant impacts with the Moon to find out whether such an event millions of years ago could reproduce the crust of today’s Moon as detected by GRAIL.
They found the best fit for today’s asymmetrical Moon is a large body, about 480 miles (780 km) in diameter, smacking into the nearside of the Moon at 14,000 mph (22,500 kmh).
Another good fit for the impact combinations they modeled is a slightly smaller, 450-mile (720-km) diameter, object hitting at a mildly faster 15,000 mph (24,500 kmh).
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 km) of debris. That is the added layer of crust detected on the farside by GRAIL.
“Our study suggests the impactor was not likely an early second moon of Earth’s,” Dr. Zhu said.
“Whatever the impactor was — an asteroid or a dwarf planet — it was probably on its own orbit around the Sun when it encountered the Moon.”
“The giant impact model also provides a good explanation for the unexplained differences in isotopes of potassium, phosphorus and rare-earth elements like tungsten-182 between the surfaces of the Earth and Moon,” the researchers said.
“These elements could have come from the giant impact, which would have added that material to the Moon after its formation.”
Meng-Hua Zhu et al. Are the Moon’s nearside-farside asymmetries the result of a giant impact? Journal of Geophysical Research: Planets, published online May 20, 2019; doi: 10.1029/2018JE005826