科学美国人60秒:2-Face Moon Tells How It Got That Way

The dark side of the moon. It’s remote and mysterious. And not just because we can’t see it from Earth. When viewed from space, the moon’s back side looks totally different from its front. Now, researchers think they have a solution to the mystery, which they share in the Astrophysical Journal Letters. [Arpita Roy, Jason T. Wright, and Steinn Sigurðsson, Earthshine on a Young Moon: Explaining the Lunar Farside Highlands]

You may have wondered about the "man in the moon," that facelike image made by the large flat plains on the lunar surface that faces us. But scientists wonder why the far side doesn’t have comparable features.
 
According to the new analysis, this asymmetry has to do with how the moon was made. Not long after the Earth formed, a Mars-sized hunk of intergalactic debris smacked into our baby planet, flinging off material that then became the moon. The crash left both bodies boiling hot. But the smaller moon cooled down more quickly than the molten Earth… especially the part that faced the other way.
 
The minerals on the moon’s cooler side started to precipitate sooner. That head start gave the far side a thicker crust, which is more resistant to the weathering seen on the familiar side: weathering that gives a face character. Even on the moon.
 
—Karen Hopkin