Our moon has two faces.  One is the familiar man-in-the-moon side that always faces Earth.  The other side is mountainous and heavily cratered, possessing a thicker crust with almost none of the large impact basins we see as dark maria on the Earth-facing side.  The giant impact theory for the origin of the moon – that a Mars-size object hit the Earth and the debris coalesced into our planetary companion – has been controversial since it was first proposed.  Will adding another impact help?  It all depends on what one means by “scientific progress.”

In Nature this week,1 Jutzi and Asphaug presented a new model for the origin of the lunar highlands on the far side of the moon.  They first proposed that the impact against Earth formed two moons, not one.  The bigger piece formed the moon; the other piece, caught in a stable orbital position called a Trojan point, hung around for a few tens of millions of years.  Being smaller, it crystallized faster.  After awhile, something nudged it toward the bigger piece, and with a gentle collision less than the speed of sound in rock, it merged into the moon.  Their computer models show it moving most of the magma to the near side of the moon and depositing material on the far side, forming the lunar highlands.

Maria Zuber considered this theory in the same issue of Nature.2  She said that since several alternatives can produce the lunar profile, “the current study demonstrates plausibility rather than proof.”  The BBC News and Live Science summarized the theory with one frame from the computer simulation.

Being tied to the giant impact theory, the two-moon theory will suffer from the same defects (02/19/2007, 07/14/2008, 01/25/2009, 07/10/2010), but seems to offer some explanatory value at the price of complicating the picture with another body which, like the initial impactor, must be finely tuned to reside at the Trojan point for a period of time and then impact the larger body at the right speed.  Future missions might be able to support the theory with better gravity maps and sample returns.  For now, it is little more than a conjecture.

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