A primordial second moon may have smacked into our existing moon billions of years ago, its remains pancaking across its larger sibling and disrupting the bigger moon’s still-cooling surface. This new theory could explain why the moon’s far side looks so different from the one that perennially faces us.

Both moons would have coalesced from the remnants of debris ejected when a Mars-sized object whacked the Earth early in its formation, scientists say. Most moon-formation theories have suggested our sole satellite formed from that chaotic jumble. But a new paper published today in Nature says a tiny Trojan moon, about one-thirtieth the size of the one we have today, survived as well, inhabiting a Lagrange point 60 degrees in front or behind the moon.

The two moons would have existed in peaceful harmony for tens of millions of years, long enough for both moons to almost completely solidify, say Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, and Martin Jutzi, now of the University of Berne. Earth’s gravity would have gradually caused both moons to migrate outward, until the Sun’s gravity started to play a role, according to a writeup in Nature News. Then the Lagrange point equilibrium would have been disrupted, setting the small moon adrift.