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 so two-faced.

The side of the moon we see (left) looks much different than the far side, as shown in these photos from the Lunar Reconnaissance Orbiter. Images: NASA/LRO/ASU

Measurements over the years, including from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, added to the mystery, revealing that the crust is thicker on the far side of the moon compared to the near side.

The crust of moon’s near side (left) is mostly 12 to 24 miles thick, whereas much of the far side is 18 to 30 miles thick. Image: NASA/JPL/GRAIL/S. Miljkovic

The moon’s history is one of countless cosmic collision. It all started, according to the leading theory, when a Mars-sized object grazed a young Earth in the very early days of the solar system, cleaving off material that circled Earth and coalesced into the moon. (There are other theories.)