When our solar system was very young it was quite a bit more violent than it is now.

Earth Had Two Moons That Crashed to Form One, Study Suggests

The moon has two faces

The moon's far side is very different than its near side.

For instance, widespread plains of volcanic rock called "maria" (Latin for seas) cover much of the near hemisphere, but only a few maria are seen on the far one. In addition, while the surface of the near side is mostly low and flat, the far side is often high and mountainous, with the lunar surface elevated 1.2 miles (1.9 km) higher on average on the far side.

Now computer simulations hint a second moon essentially pancaked itself against its larger companion, broadly explaining the differences seen between the near and far sides. [10 Coolest Moon Discoveries]

Their calculations suggest this second moon would have formed at the same time as our moon. Scientists have suggested that our moon was born from massive amounts of debris left over from a giant impact Earth suffered from a Mars-size body early on in the history of the solar system. Spare rubble might also have coalesced into another companion moon, one just 4 percent its mass and about 750 miles wide, or one-third of our moon's diameter.