See Inside December 2006

A Reason for a Moonless Venus

One of biggest mysteries in the solar system is why Venus has no moon. A new model proposes that our sister planet may in fact have had a moon but that it was destroyed. At an October meeting of the American Astronomical Society, Alex Alemi and David Stephenson of the California Institute of Technology suggested Venus underwent not one large impact, like the collision thought to have formed Earth's moon, but two. The first caused the planet to rotate counterclockwise and created a moon that began to drift away. But a second impact sent Venus spinning clockwise, canceling the effect of the first collision. This reversal changed the gravitational interactions between the moon and planet and caused Venus's moon to start moving inward, ultimately colliding with the planet. The second impact may or may not have created a moon, too. If it did, this moon would have been swept up by the first one on its inward plunge toward doom.

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