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Second moon's remains may cover lunar far side

FAMILY squabbles rarely result in cannibalism, but that may be just what happened in the moon's youth. It may have gobbled up a smaller sibling, making itself permanently lopsided.



New simulations now suggest such moon-on-moon violence could explain a long-standing puzzle about the moon's two-faced nature. The moon's crust is thicker on its far side (pictured) and differs in composition from rocks on the side facing Earth.

Previous explanations for the difference have created their own puzzles. For example, a giant, high-speed impact could have blasted away much of the crust from the near side. But this would probably have led to a global magma ocean that would have cooled and erased the initial thickness difference.

Now Martin Jutzi and Erik Asphaug at the University of California, Santa Cruz, have simulated the effect of an impact between the moon and a smaller sibling 1300 kilometres across, about one-third as wide (Nature, DOI: 10.1038/nature10289).

Crucially, such an impact would have happened at about 2 kilometres per second, which, although fast in everyday terms, is very low compared with the typical speeds of asteroids and comets that blast out craters on the moon. Like racing cars that swerve and hit each other on a circular track, the moons' speed relative to each other is low because they travel in nearly the same orbit.

Such a low-speed impact does not melt or vaporise rock like a high-speed crash. Nor does it form a crater - it actually adds material to the moon. "In a way you do make a crater, but you fill it all in with the impactor material," says Asphaug.

The smaller moon breaks up and spreads out from the collision point like a landslide, thickening the larger moon's crust on one side. "It basically looks like you are smearing the impactor across the face of the moon," says John Chambers of the Carnegie Institution for Science in Washington DC, who was not involved in the study.

Like squashing one end of a tube of toothpaste, the impact also pushes subsurface magma to the opposite side of the moon from the crash point. This could explain why rocks on the moon's near side are richer in potassium, phosphorus and other elements that suggest they were among the last rocks to solidify out of the moon's primordial magma ocean.

One way to test the idea is to get rock samples from the far side, says Maria Zuber of the Massachusetts Institute of Technology. The simulation suggests rocks on the far side come mostly from the smaller moon, which should have previously cooled and solidified faster than the main moon, making its rocks older.

Low-speed collisions have also been proposed to explain some comets' layered structures. If a smaller comet hit a larger one, its icy remains might be plastered on the surviving sibling.

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Room for one more? (Image: NASA/Goddard)

Room for one more? (Image: NASA/Goddard)


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