Since the 1960s we've known that the far side of the Moon is strikingly different from the very familiar near side. But it’s not just about looks. Elevation, composition, and crustal thickness also differ between the two faces.
A new potential explanation for this asymmetry has been put forward by an international team of researchers. The scientists claim that the Moon experienced a dramatic impact on the near side, which threw plenty of material into circumlunar orbit before it rained back down on the far side. The hypothesis is reported in the Journal of Geophysical Research: Planets.
The team ran 360 impact simulations and selected the scenario that best explained the observations made by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission in 2012. The best scenario has the Moon colliding with a 780-kilometer (480-mile) object, slightly smaller than the biggest object in the asteroid belt, the dwarf planet Ceres.
This object hit our natural satellite at a speed of 6.25 kilometers per second (14,000 miles per hour), which is actually relatively slow. It is about a quarter of the speed of a shooting star entering the atmosphere. Another good fit is a slightly smaller object moving a bit faster.
The impact in both scenarios delivers 5 to 10 kilometers (3 to 6 miles) of material to the far side of the Moon. This could explain the added layer of crust detected by GRAIL, as well as the difference in composition between Earth's crust and the lunar crust.
Rather than being a second moon of Earth, the object was just one of the many larger objects that were abundant in the early Solar System. A Mars-sized planetoid is likely to have hit the primordial Earth and formed the Moon, and many other planets and moons are likely to have experienced impacts. Some of these could have led to asymmetric compositions like the Moon's.
“This is a paper that will be very provocative,” Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University and Editor-in-Chief of the journal, said in a statement. “Understanding the origin of the differences between the near side and the far side of the Moon is a fundamental issue in lunar science. Indeed, several planets have hemispherical dichotomies, yet for the Moon we have a lot of data to be able to test models and hypotheses with, so the implications of the work could likely be broader than just the Moon.”