Iapetus is one of the oddest moons in the solar system. One hemisphere of the large moon of Saturn is almost pure white, coated by ice. The other, glimpsed at right in this image from the Cassini spacecraft, is almost as dark as coal. In addition, a tall ridge encircles the moon's equator (visible at far right), while a large impact basin scars its southern hemisphere (lower left). Recent simulations hint at a violent past for the moon. [NASA/JPL/SSI]
In the book version of 2001: A Space Odyssey, the monolith builders leave earthlings a clear beacon in their search for answers: a two-toned moon of Saturn. When it's on the front side of Saturn, it's as white as snow. But as it begins to pass to the planet's farside, it disappears.
As far as we know, there are no monoliths on the dark side of Iapetus. And the two-toned appearance is the result of dark soot blasted off the surfaces of other moons settling on the hemisphere of Iapetus that faces forward as it orbits Saturn.
But the moon still offers an interesting mystery: Its equator is girdled by a ridge that makes Iapetus look like a walnut.
A team led by Harold Levison of the Southwest Research Institute says it may have solved the mystery.
The solution says that, long ago, another body slammed into Iapetus, blasting material into orbit around it. Some of the material formed a ring, while some formed a small moon of Iapetus.
The ring material settled onto the surface, forming the ridge. Interactions between Saturn, Iapetus, and the small moon made Iapetus spin slower. Saturn's gravity then forced the small moon to slam into Iapetus, creating a giant basin that's still visible today.
All of this created a moon like no other -- without any help from aliens.
Saturn stands to the upper right of our own Moon this evening. It looks like a golden star. The true star Spica is to Saturn's upper left. More about this lineup tomorrow.
Script by Damond Benningfield, Copyright 2011
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