Earth’s closest celestial neighbor has a secret, violent past, judging from two reports published last August in the journal Nature. One finding may explain the moon’s peculiar two-faced quality. In the 1960s early space probes showed that the lunar farside—the half never seen from Earth—is covered with steep, scarred mountains and none of the broad, flat volcanic “seas” that mark the side that points toward us. Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, traces this split personality back to a time when Earth had not one moon but two.
Asphaug suggests that our moon collided with a smaller companion moon 4.4 billion years ago. Material from the companion piled up into an enormous rubble heap, forming the farside’s craggy surface, Asphaug says. The smashup would have occurred about 100 million years after the moon itself was born, when another, much larger body crashed into Earth.
At the same time, Lars Borg of the Lawrence Livermore National Laboratory reanalyzed a moon rock collected by Apollo 16 and found it to be 4.36 billion years old, 150 million years younger than previously believed. The result hints at new, unknown details about the moon’s formative collision.