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The moon, as seen by Apollo astronauts.

The near side of the moon (right) is much smoother than the far side.

Photograph courtesy NASA

Ker Than

for National Geographic News

Published August 3, 2011

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.

(Take a moon mysteries and myths quiz.)

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.

"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).

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.

"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."

Moon Collision Created Meteor Shower

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

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.

"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.

(Related: "Trojan Asteroid Found Sharing Earth's Orbit—A First.")

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.

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.

"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.

Lunar Smashup Opens Up "Cool Problems"

Astronomer Jeffrey Taylor of the University of Hawaii said the new moon theory is very interesting and worth further investigation.

Asphaug and Jutzi's model not only accounts for the moon's asymmetry, Taylor said, but the findings also explain the fates of the smaller, companion satellites that another theory predicts should have formed alongside our moon.

One of the leading theories for how our moon formed is that it was born after a Mars-size planet crashed into Earth shortly after the solar system's formation about 4.5 billion years ago. (See "Earth-Asteroid Collision Formed Moon Later Than Thought.")

Scientists think the earlier smashup created a ring of molten rock debris around Earth, which eventually coalesced into several bodies, including our current moon. (Related: "The Moon Has Shrunk, and May Still Be Contracting.")

But "if that's the case, what happened [to the smaller moons]? This is one thing that could have happened to them," said Taylor, who was not involved in the study.

The new theory isn't without its problems, however. For example, it doesn't explain why the lumpy far side of our moon shows a high concentration of aluminum, Taylor said.

If the two moons had formed from the same material, as is assumed, the companion moon—and its splatterings—should have been low in aluminum, like our own moon's interior.

(Related: "Ancient Moon Had Earthlike Core?")

However, this problem could be resolved by future lunar studies, Taylor said, and it's not a serious enough reason to dismiss the theory.

"If anything," he said, "it just opens up more cool problems to work on."

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