A slow splat. A relatively slow impact by a companion body a third the size (and 4% the mass) of the moon may have created the moon's far side highlands as well as the near side's "Man in the Moon."

Martin Jutzi and Erik Asphaug

Did a Slo-Mo Crash Create the Two-Sided Moon?

Ever since the origin of Earth's moon was explained, the biggest question in lunar science has been accounting for the moon's two-sided geology: a near side that's relatively low-lying, flat, and marked by expanses of dark lava that define the "Man in the Moon," and a bright far side with an expanse of high-standing, mountainous terrain. Now, a pair of researchers is suggesting that a slow-motion impact by an ill-fated sister moon created this perplexing dichotomy.

"Impact modelers try to explain everything with collisions," says impact modeler Erik Asphaug of the University of California, Santa Cruz. Indeed, scientists have long agreed that the moon itself coalesced from the debris splashed off Earth when it was hit by a humongous planetesimal around 4.6 billion years ago. Asphaug and his fellow impact modeler Martin Jutzi of the University of Bern in Switzerland wondered whether the moon's dual face, too, could be explained through a celestial crash.

If so, the "impacter" had to build up the far side into mountainous terrain rather than blast out a crater. That means it had to be slow—and the slowest-moving impacter would be one in the same orbit as the moon. Earlier work by others had shown that smaller, secondary bodies would also coalesce from the debris of the collision with Earth that created the moon. Studies had also shown that only a secondary moon orbiting Earth at a gravitational balance point just ahead of or behind the moon would survive more than a few million years before slowly colliding with the moon.

As they report in this week's issue of Nature, Jutzi and Asphaug modeled the collision of a still-congealing moon—some tens of millions of years after it formed—and a companion a third the size of the moon and moving at little more than a tenth the speed of most planetary bodies on a collision course. They got more than they bargained for. A suitably slow mini-moon would indeed splatter onto the moon without forming a crater, their modeling showed, like mud thrown against a wall. But the impact would also squish magma from the magma "ocean" still lingering beneath the lunar crust over to the near side. That magma would be enriched in radioactive elements that could have generated the extra heat needed to flood large parts of the near side with the dark lavas of the lunar "maria," or seas. The dark maria form the hair and delineate the profile of the Man (or Woman) in the Moon.

Building up the far side by gently hitting it with another moon "is a bold idea, an interesting alternative," says impact specialist H. Jay Melosh of Purdue University in West Lafayette, Indiana. "It's not crazy." Certain aspects, such as how much of the sister moon's crust could be piled up on the far side, do need to be examined, he says. Another test of the idea will be possible once NASA's Gravity Recovery and Interior Laboratory mission, to be launched in September, probes the lunar interior by returning the most sensitive measurements of lunar gravity ever made.