Earth Had Two Moons

Earth may have once had two moons, but one was destroyed in a slow-motion collision that left our current lunar orb lumpier on one side than the other, scientists say.Astronomers have long been puzzled by the differences between the side of the moon that always faces Earth—the near side—and the side that always faces away, the far side. The topography of the near side is relatively low and flat, while that of the far side is high and mountainous with a much thicker crust.According to a new computer model, this discrepancy can be explained if a smaller "companion moon" collided with our moon's far side early in its history. Such a collision would have left the far side splattered with especially hard rocky material that now forms the current lunar highlands.

For the theory to work, the smaller moon must have crashed into the larger one at about 4,400 miles (7,081 kilometers) an hour.

At this relatively slow speed, the far-side collision wouldn’t have been energetic enough to melt rock or carve out a crater. But it would have been forceful enough to plaster material from the smaller moon onto the larger moon.

"This is the slowest possible collision the two massive bodies could have if they fell into each other’s gravity," explained study co-author Erik Asphaug, a planetary scientist at the University of California, Santa Cruz (UCSC).

The new theory, by Asphaug and UCSC postdoctoral researcher Martin Jutzi, is detailed in the current issue of the journal Nature.

"It's like a car crash, where you have crumpled bumpers but you don't melt the cars as they're colliding," Asphaug said. "This is the same kind of phenomenon."

According to their model, the two moons coexisted peacefully for about 80 million years, each in its own stable orbit. The moons were the same color and composition, but one was about three times larger than the other, Asphaug said.

This brief period of lunar harmony was shattered, according to the model, when natural gravitational interactions with Earth caused both moons to drift farther away from our planet. The sun's gravitational tug then destabilized the smaller moon’s orbit and caused it to fall into its larger sibling.

"Our moon looked like a big dinner plate in the sky ... and when it set, there was this other moon trailing it by about 60 degrees," he said.

Though not very energetic, the collision would have ejected trillions of tons of lunar debris into space, obscuring both moons for several days.

"When the dust cleared, you had one moon that might have looked similar to our moon today," Asphaug said.

For up to a million years after the event, Earth would have been bombarded by moon bits of various sizes, the biggest of which could have been as much as 62 miles (100 kilometers) across.

"By definition, a big collision occurs only on one side," he says, "and unless it globally melts the planet, it creates an asymmetry."The moon's far side is very different than its near side.

"You'd have meteors raining down all over the sky for a long period of time," Asphaug said, though there probably would have been no life yet on Earth to witness the spectacular sky shows.

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

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.