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After a few tens of millions of years, Moon Jr. would become unstable, almost certainly falling into Moon Sr. But Jutzi and Asphaug's computer simulations of how that would play out showed that the short-lived satellite would have fallen surprisingly gently, at only a few miles per second, making more of a splat than a bang. At a more typical impact speed, which would be at least 10 times faster, says Zuber, "you'd make a big hole and fling off ejecta" in short form, a massive crater.
In this case, you'd form just a pile of extra stuff on one side, as though you slapped a handful of mud onto a basketball. And if the minimoon were about 750 miles (1,200 km) across, with about a third as much mass as its big brother, it could account for most of the extra material we now see on the far side.
The scenario could also explain why the near side is paved over with so much lava. At the time of impact, the moon would have cooled from its original molten state to form a thin crust, with an ocean of magma underneath. But all the extra mass added to the far side could have squeezed most of the subsurface magma around to the side that faces us, providing an ample supply of molten rock for later eruptions. "Every once in a while I read a paper I really enjoy," says Zuber. "This is a genuinely new idea. That's what really struck me."
A new theory in science isn't worth much, however, unless you can test it somehow. The best way would be to look for the mineralogical signature of such an event in rocks brought back from the far side of the moon. Unfortunately, no such mission is in the works anytime soon. But the Lunar Reconnaissance Orbiter, or LRO, which is circling the moon even now, has detectors that can get at least a sense of the minerals below.
And in September, another moon probe will be on its way, with Zuber as principal investigator. It's called GRAIL, for Gravity Recovery and Interior Laboratory. Actually, it's a pair of probes that will orbit in tandem; changes in the distance between them will measure the local lunar gravity with extraordinary precision. That will give Zuber and her team a detailed look at the moon's geological (or technically, selenological) structure and history, and when combined with LRO's data, could make or break Jutzi and Asphaug's idea.
Science is indifferent to what the answer will turn out to be, of course. But scientists and ordinary folks alike can't be indifferent to the romantic idea that two moons once shone together in the evening sky. If nothing else, it's a simple matter of planetary pride.