This artist's rendering provided by Martin Jutzi and Erik Asphaug,...

This artist's rendering provided by Martin Jutzi and Erik Asphaug, University of California, Santa Cruz via Nature shows a simulation of a collision between the Moon and a companion moon, four percent of the lunar mass, about 4 billion years ago. Earth once had a second moon, until it made the fatal mistake of smacking its big sister, some astronomers now theorize. Credit: AP

Once upon a time, the sky above Earth may have held two moons — until they smashed into each other to create the lunar body we know today. Such a collision early in the solar system's history could explain why the moon is lopsided, and why its far side looks so different from the face we can see, according to a report in Thursday's edition of the journal Nature.

Round as it may seem from our vantage point, the moon in fact bulges on one side — the far side, which is packed with high, jagged mountains. That's a more severe surface than the smooth side we see, filled with basins of volcanic rock.

The moon was formed about 4.5 billion years ago, after a Mars-sized body slammed into the molten Earth and flung a ring of debris into space. Much of that debris coalesced into a rocky satellite, which was roughed up about half a billion years later by a barrage of asteroids and other interplanetary material, leaving scars in the form of craters.

But at what point the moon became lopsided, and how, is still unclear.

Some scientists speculated that the moon's internal dynamics pushed material around. Others wondered whether the mountains were created by debris ejected from a freshly smashed crater.

Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, had a different idea. Researchers knew that space debris could theoretically get stuck around one of the moon's Lagrangian points — places in space where the gravity of the moon and Earth cancel each other out. In those areas, the debris could coalesce to form another moon, but it would only remain stable for a short time before the sun's gravity threw it off course.

So Asphaug and colleague Martin Jutzi of the University of Bern in Switzerland designed a model in which a moonlet about one-third as wide as the moon maintained a stable orbit around the Earth for tens of millions of years, until it floated from its Lagrangian point into the moon's path.

The moonlet would have crashed head-on with the larger lunar body, but the collision would have happened at relatively slow speeds of between roughly 4,500 and 6,700 miles per hour, the scientists calculated. At that rate, the material in the moonlet would splat onto the moon rather like a ball of mud hitting a surface. The rocky peaks on the far side of the moon are remnants of that impact, Asphaug said.

The force of the collision would have pushed the moon's internal magma sea over to the near side, which could explain why the visible half is rich in potassium, uranium and other elements, Asphaug and Jutzi wrote in Nature.

"It's a really new idea that puts a completely different take on the origin of the lunar dichotomy," said Robin Canup, a planetary scientist with the Southwest Research Institute in Boulder, Colo., who was not involved in the study.

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