Cassini at Iapetus
 Iapetus, a moon of Saturn, looks like a walnut in this image from the Cassini spacecraft. A tall, sharp mountain ridge traces the moon's equator, providing the walnut appearance. One side of ice is as white as snow (at the top of the picture), while the other is almost as dark as charcoal. Cassini will fly close to Iapetus on September 10. [NASA/JPL/SSI]
Cassini is scheduled to give us an even better look at the ridge today and tomorrow, as it flies less than a thousand miles from Iapetus -- by far its closest approach to date.
Iapetus is best known for having two wildly different sides. One hemisphere is as bright as new-fallen snow. Most of the other hemisphere is darker than charcoal. The bright side is made of ice. The dark side is, too, but it's coated with something else -- perhaps dust that was blasted off one of Saturn's other moons.
The difference in the light and dark sides is so pronounced that it's visible from Earth, even though Saturn averages close to a billion miles away. But the ridge didn't show up until Cassini flew past Iapetus for the first time, almost three years ago.
The ridge stretches at least a third of the way around Iapetus. It's only about 12 miles wide, but up to eight miles tall. That gives it steep slopes, so the ridge might be a popular vacation spot for skiers of the distant future.
The ridge might have built up as two parts of the moon's crust smashed together. Or it might have built from within, as ice pushed up from below the surface. Cassini's encounter may help scientists explain the ridge of the "walnut" moon.
Script by Damond Benningfield, Copyright 2007
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