Our Moon sports a few oddities. One of them is the remarkable difference between the properties of the hemispheres facing toward and away from us: on this side the crust is thinner and smoother, displaying basins filled with dark basalt while the other side is more thick, a heavily cratered highland. The abundance of some elements (KREEP: potassium, rare-earth elements and phosphorus) is also different. There have been theories to explain this dichotomy already, like differences in tidal heating or the impact that formed the South Pole-Aitken Basin. A new theory is based on an impact, but instead of a stranger from far, it assumes an event between moon siblings.

There are striking differences between the two hemispheres. This elevation map shows that the near side is much smoother while the far side is peppered with craters. The South Pole-Aitken Basin is the large blue depression on the lover half of the far side.

In their Nature paper, Erik Asphaug and Martin Jutzi (University of California, Santa Cruz, USA) started from the most accepted theory of the formation of the Moon. The material that formed the Moon was excavated when a massive object hit the proto-Earth. That material then slowly accreted into today's Moon. But according to the simulations, configurations could have developed where three bodies (Earth and two moons) could have remained for extended periods of time, for about 100 million years. That amount of time was necessary to let the objects cool: the larger body, the proto-Moon was covered with a shallow (<100 km deep) magma ocean, the smaller impactor, the victim, with already solidified crust.

Four snapshots from the simulation. Materials are: proto-Moon crust (grey), impactor crust (light blue) and mantle (dark blue) and a layer from the proto-Moon's mantle (yellow).

Over tens of millions of years though even the orbit of a trojan moon (same orbit but moving 60 degrees apart) destabilizes and would impact the proto-Moon sooner or later. But that is no ordinary impact! The two bodies meet with a speed of 2-3 km/s, much less than a couple of tens that is usual in the Solar System. Instead of forming a huge crater and throwing a lot of dirt out, the material of the impactor piles onto the surface. The end result of the simulation is quite close to reality: the material covers one hemisphere, creating the highlands, while forcing the magma ocean to the other side. That would also explain the differences in the abundances of elements.

Post-impact cross-section of the Moon. The material of the impactor (light grey) piled onto one hemisphere, the magma ocean (yellow) was pushed to the other side. If the impactor had any core (not likely), it sunk into the Moon's own (red) and didn't change the outcome.

This theory could be easily confirmed or refuted with sample return missions. Since the crust of the impactor formed earlier, it's older than the crust of the Moon: we only have to find that ancient material. But it's mostly present on the far side, only tiny amounts could be on the Earth-facing hemisphere. There is actually a NASA New Frontiers mission proposal called MoonRise that would return samples from the South Pole-Aitken basin on the far side, but it wasn't selected in the last round and faces many fascinating proposals in the coming selection process too. Well, that sounds as a request for GLXP missions, doesn't it?

Image sources:

1.) 3.) 4.) Nature

2.) Mark A. Wieczorek / NASA