We’ve known since the early years of the Space Age that the side of the Moon facing us is very different from its far side. While telescope users on Earth are familiar with wide, low-lying plains covered with lava flows (the so-called maria), the hidden half is more heavily cratered and mountainous, with a thicker crust.
Over the years many ideas have surfaced to explain this striking difference. Among the proposals have been a big pileup of material excavated from the far side’s enormous South Pole–Aitken basin, uneven tidal heating, and lopsided churning of the lunar interior.
Now a new study suggests that a single big act of violence could have produced the Moon’s two-faced appearance. According to a report published in today’s issue of Nature, a smaller satellite of Earth likely smashed into the Moon’s face early in its life. The low-velocity impact reduced the lesser orb to a thick layer of rubble that piled up mostly on one side of the Moon. Gravity then readjusted the whole thing back to being nearly round.
This model, constructed by Martin Jutzi (University of Bern) and Erik Asphaug (University of California, Santa Cruz) builds on the popular theory that a Mars-sized object collided with the young Earth 4.5 billion years ago, ejecting the matter that would become Moon — and, importantly, smaller short-lived satellites of Earth. The infant Moon swept up the smaller objects as it gradually spiraled out to a wider orbit. But, as a 2009 analysis by other researchers pointed out, one or more satellites could have shared the Moon’s orbit — objects called Trojans — and lingered for tens of million years before being swept up.
The Trojan that produced the thickly crusted far side is thought to have been the last and the largest to smack into the lunar surface. In Jutzi and Asphaug’s simulations, this body was about a third the young Moon’s diameter and added 4% to its mass — enough rock to thicken the entire far-side crust by 30 miles (50 km).
The impact didn’t necessarily happen on the hemisphere facing away from Earth. “The collision could have happened anywhere on the Moon,” says Jutzi. However, the one-sided veneer would have caused the Moon to wobble and assume its present-day orientation due to tidal forces from Earth’s gravity, with its heaviest side constantly facing us.
Because the Trojans struck the Moon relatively slowly, at an assumed 1.5 miles (2.4 km) per second, Asphaug points out that collisions would not have produced the deep craters and widely scattered debris that result when asteroids and comets hit the Moon.
The simulations also suggest that the far-side splat drove the molten rock beneath the Moon’s crust, its magma ocean, to its Earth-facing side. This would help explain why samples brought back by the Apollo and Luna missions contain abundant potassium, phosphorus, and some rare earth elements (together known as KREEP) from the mantle on the near side.
Jutzi previously worked on a model that attributed the lunar far-side highlands to tidal forces alone. Francis Nimmo (University of California, Santa Cruz), who co-authored that paper, says that the origin of the far side was as much a mystery as the formation of the Moon itself. “One of the elegant aspects of Erik’s article is that it links these two puzzles together.”
There’s no sure way to know how the Moon got its lopsided nature, so for now the “Trojan splat” remains an intriguing speculation. But NASA’s upcoming GRAIL mission to map the Moon's internal densities may settle which theory is correct. Also, Asphaug thinks that samples from the far side of the Moon would help, because the added Trojan material should be slightly older than the rest of the lunar crust.
He also urges readers to help study the lunar farside in closeup using Google Moon. “We need to do the patient work of comparing the model to the lunar data,” he says. Could this lead to another citizen-science project on the Internet? There’s potential if the right people get involved, Asphaug says. “This research appears to have touched more people than any of the science I have done, because of what the Moon represents to each person on Earth, whether you are a scientist or an artist or a mystic. That there were once two Moons, and that we see the second Moon on the far side today, is a rather powerful concept to think about!”