Iapetus and its giant ridge

Here's a picture of Iapetus taken more than 2½ years ago. New ones are being shot today, from a point 75,000 miles closer than this one was!

In my office, tucked away on my bulletin board, is a slightly yellowed paper listing every Cassini encounter with a moon of Saturn. On that list, highlighted in neon colors, is September 10, 2007. Today the craft flies past Iapetus for only the second time, and at a distance of around 1,000 miles. The last flyby was back on New Year's Eve 2004, when Cassini was snapping pictures from 75 times farther away.

When the smoke clears, I'm placing my bets that Iapetus will be the most interesting moon that Cassini studies during its mission. Okay, maybe not as interesting as Titan, but I do think it's going to prove to be more fascinating than the ice-spewing Enceladus.

This Cassini image shows Iapetus's leading hemisphere, which is as dark as an asphalt parking lot. Scientists were amazed to find a giant ridge girdling at least one-third of the moon's circumference. The highest peaks along the ridge are about 20 kilometers (12 miles) high, which rivals Mars's giant volcanoes in height. The impact basin in the center is about 400 kilometers (250 miles) across.

NASA/JPL/Space Science Institute

Why such a bold claim? Iapetus has two incredible features that this flyby promises to shed light on. First, it sports a strange equatorial mountain ridge — the belly band for those who know it well. It would be as if the Rocky Mountains circled the Earth along the tropics. Second, the moon looks like an Othello piece — black on one side, bright on the other. It's the solar system's closest thing to an Oreo cookie. A bunch of hypotheses have attempted to explain why that is, but a closer look should help narrow the possibilities.

So prepare to be blown away by the amazing images NASA will be releasing soon. And if you are like me and can't wait for the press releases, troll though the raw images as they come in from the spacecraft. You'll have to sort through hundreds of shots to find the gems — and remember that these haven't been processed yet — but they should be enough to temper your hunger for a few days.





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