The Moon: According to a recent study, it's two moons in one.
The Moon: According to a recent study, it's two moons in one.Courtesy Mark Ryan
A new study published in Nature proposes that our Moon once had a companion satellite that it eventually accreted in a celestial collision. Planetary scientists, Erik Asphaug, of the University of California, Santa Cruz, and Martin Jutzi of the University of Bern in Switzerland devised computer simulations that show how it could have happened.

According to present lunar origin theory, four and half billion years ago, while the Earth’s system was forming, gravitational forces attracted a Mars-sized object that collided with the early Earth. The collision - more of a glancing blow than a direct hit - tossed terrestrial material into space that coalesced into our Moon. But during the period of coalescence – perhaps for tens of millions of years - a smaller companion moon (about 1/3 the size of the larger moon) would have been visible in Earth’s primitive sky. Geologically speaking, the mini moon’s existence would have been short-lived. The system was unstable, and sooner or later the moonlet’s orbit would decay and it would be pulled either into Earth’s mass or into that of the larger satellite.

Computer simulations set up by Asphaug and Jutzi reconstruct the latter taking place. The researchers propose that the dominant moon was still in a semi-molten state when its smaller companion collided with it at a sub-sonic speed. Being smaller, the doomed moon would have cooled faster and would have been more solidified, but the collision was hardly devastating. It’s low impact speed made it more like a clump of mud being lobbed against a wall. There wasn’t enough force in the collision to punch through, but just enough to make it stick.

More evidence: lunar composition differences

During NASA’s Apollo lunar program in the late 60s and early 70s, astronauts collected several samples of rock from the near side landing sites. The rocks brought back proved rich in potassium (K), rare earth elements (REE) and phosphorus (P) – hence the acronym. These elements, which are scarcer on the Moon’s dark side, crystallize very slowly in cooling magma, and remain molten until the entire mass of magma solidifies. So according to the researchers, when the collision occurred, it was enough to push much of the still molten magma - and the KREEP along with it - to the near side, and leave a pile of mountainous terrain on the far side.

I find this all pretty fascinating. The hypothesis answers several questions that have been puzzling lunar scientists for several years, and fits well into what we observe now. Of course we only see the Moon’s near side. Gravitational forces keep much of the far side hidden from us except via photography and lunar probes (Why that is can be learned here).

Story in Scientific American
Story on Weather
Apollo Lunar Program

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