If life as we know it exists elsewhere in this solar system, it's not likely to be on Iapetus, the third-largest of Saturn's 62 moons. For one thing, Iapetus is made almost entirely of ice. But Iapetus is interesting to earthly scientists for other reasons, namely that it resembles the universe's largest walnut.
For years, scientists who make it their business to think about these things have wondered what happened to give Iapetus its unique appearance. Now William McKinnon, a professor of earth and planetary sciences at Washington University, says he has a pretty good idea: an enormous and dramatic interlunar crash between Iapetus and another, smaller moon.
Once upon a time, McKinnon and his fellow researcher Andrew Dombard, a professor at the University of Illinois at Chicago, postulate, Iapetus had a moon of its own. Over time, Iapetus's stronger tidal forces destroyed the other moon's orbit and, eventually, the moon itself. The little moon broke into thousands of smaller particles that settled in a ring around Iapetus's equator. (Imagine Saturn orbited by another miniature Saturn.)
Eventually, the tidal forces that held the ring in place grew even weaker and the particles slammed into Iapetus.
McKinnon adds that Earth is in no danger of suffering a similar shower because the moon is actually moving farther away. "But if we were to bring our Moon inside what is called geosynchronous altitude, that special altitude where TV broadcast satellites (and other objects) are able to hover over one spot on Earth as they orbit, then the Moon would actually spiral in toward Earth," he explained. "Eventually our Moon would break into a ring of particles as it got very close and was torn apart by tides, and then those particles would enter the atmosphere and bombard the Earth at the equator."
"There are three critical observations that any model for the formation of the ridge has to satisfy," Dombard added. "They are: Why the feature is sitting on the equator; why only on the equator; and why only on Iapetus. I think we have something here that explains all those observations."
So far, McKinnon and Dombard have only done a lot of math to arrive at their theory. The next step will be to build a model that simulates the great crash. This may also help explain the giant dark stain that covers part of Iapetus. Or maybe it's just a sign of the impending forces of the Dark Side.
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