October 9, 2007—The first ever high-resolution images of Saturn's moon Iapetus have helped unlock the mystery of the satellite's oddly mottled surface.

The walnut-shaped moon has long been known to have dark areas and bright ones. But the detailed new images—captured by NASA's Cassini orbiter in September and released yesterday—reveal that Iapetus's two-tone color scheme is seen even at the crater level. That is, one side of a crater might be dark, while the other is white.

Furthermore, the dark is very dark, while the white is frosty bright.

"There is no grey," John Spencer of the Southwest Research Institute in Boulder, Colorado, said yesterday at a meeting of planetary scientists in Orlando, Florida.

Spencer believes the stark difference is caused by uneven solar heating.

Iapetus's high-noon temperature only reaches -230°F (-146°C). But that's warm enough for water in the ice contaminated with the dark material—believed to be space dust—to escape as vapor.

This concentrates the dust, making the dark areas darker yet. Bright areas remain cooler and retain their ice. They also collect frost as water vapor from nearby dark areas condenses.

—Richard A. Lovett

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