Earth once had two moons, astronomers* now think. But the smaller one smashed* into the other in what is being called the “big splat.”

    The astronomers came up with an explanation why the moon’s far side is so much more hilly than the one that is always facing Earth.

    The theory, outlined last week in the journal Nature, comes complete with computer model runs showing how it might have happened and an illustration* that looks like the bigger moon getting a pie in the face*. Outside experts said the idea makes sense, but they aren’t completely sold yet.

    This supposedly happened about 4.4 billion years ago, long before there was any life on Earth. The moons themselves were young, formed about 100 million years earlier when a giant planet smashed into Earth. They both orbited* Earth and rose in the sky together, the smaller one trailing a few steps behind like a little sister in tow.

    The smaller one was a planetary lightweight. The other was three times wider and 25 times heavier, its gravity* so strong that the smaller one just couldn’t resist*, even though it was parked a good bit away.

    “They’re destined to collide. There’s no way out. This big splat is a low-velocity* collision,” said study co-author Erik Asphaug, a planetary scientist at the University of California, Santa Cruz.

    What Asphaug calls a slow crash is relative: It happened at more than 8,000 kmph, but that’s about as slow as possible when you are talking planetary smashups. It’s slow enough that the rocks didn’t melt.

    And because the smaller moon was more than 960 km wide, the crash took a while to finish.

    The rocks and crust from the smaller moon would have spread over and around the bigger moon without creating a crater*, as a faster crash would have done.

    About a day later, everything was settled and the near and far sides of the moon looked different.

    Co-author Martin Jutzi of the University of Bern in Switzerland said the study was an attempt to explain the odd crust* and mountainous terrain* of the moon’s far side. Asphaug noticed it looked as if something had been added to the surface, so the duo started running computer models of the crashes.

    Earth had always been an oddball in the solar system as the only planet with a single moon. While Venus and Mercury have no moons, Mars has two, while Saturn and Jupiter have more than 60 each.

       (SD-Agencies)