This is what happens when two big things in space collide. Perhaps a collision like this created the wildly different faces of the moon, according to new computer modeling. Image: NASA/JPL-Caltech

Why is the Moon So Two-Faced?

A newly proposed collision could explain a longstanding mystery.

The side of the moon we see is marked by vast, smooth, low-lying areas called maria, which is Latin for “seas.” But the other side, glimpsed for the first time in 1959 by the Soviet Luna 3 spacecraft, is covered by craters upon craters. For decades, scientists have been trying to figure out why the moon is…