Earth Probably Had Two Moons

A tiny second moon may once have orbited Earth before catastrophically slamming into the other one, a titanic clash that could explain why the two sides of the surviving lunar satellite are so different from each other, a new study suggests.
The second moon around Earth would have been about 750 miles (1,200 kilometers) wide and could have formed from the same collision between the planet and a Mars-sized object that scientists suspect helped create the moon we see in the sky today, astronomers said.

The gravitational tug of war between the Earth and moon slowed the rate at which it whirls, such that it now always shows just one side to Earth. The far side of the moon remained a mystery for centuries until 1959, when the Soviet Luna 3 spacecraft first snapped photos of it. (The far side is sometimes erroneously called the dark side, even though it has days and nights just like the near side.)
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 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.
 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-sized celestial body early on in the history of the solar system. Spare rubble might also have coalesced into another companion moon, one about 750 miles wide, or one-third of our moon's diameter.

Eventually ( after 10 million to 100 million years) this moon's orbit would have destabilized once our moon's orbit expanded far enough away from Earth. The resulting collision would have been relatively slow at 4,500 to 6,700 miles per hour, leading its matter to splatter itself across our moon as a thick extra layer of solid crust tens of miles thick instead of forming a crater.  Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, told SPACE.com. Imagine "a ball of Gruyere colliding into a ball of cheddar."

The remains of this moon would make up the highlands now seen on our moon's far side. At the same time, the impact would have squished an underground ocean of magma toward the near side, explaining why phosphorus, rare-earth metals and radioactive potassium, uranium and thorium are concentrated in the crust there.