In 2007, the Cassini orbiter was programmed to whiz just 1,000 miles (1,600 kilometers) above Iapetus’ surface. The images it captured showed that the black-white motif continues even at small scales, inside tiny impact craters. Strangely enough, this moon has no gray places.
Iapetus has the slowest spin of any Saturnian moon, and its 79.3-day rotation creates lengthy, warm days and long, bitterly cold nights. The sunlit ice turns directly to water vapor (a process called sublimation), and this vapor then gets redeposited in the cold areas. This model adequately explains the stark blacks and whites.
Unfortunately for astronomers, Iapetus’ second bizarre feature has no credible explanation at all — a raised ridge higher than Mount Everest that runs perfectly around its equator. This strange feature causes the moon to resemble a poorly made model globe whose two halves were glued sloppily, resulting in a raised equatorial band.
This equatorial ridgeline — 12 miles (20 km) wide and 8 miles (13 km) high — was discovered by the Cassini spacecraft on the last day of 2004. It is most uniform as it runs through Cassini Regio, the dark section, and includes individual mountains and parallel ridges. As it passes through Iapetus’ bright region, it shows itself as a series of separate peaks. Craters have impacted this raised equatorial feature, showing that it is ancient and not some recent tectonic uplift. It gives the yin-yang Enceladus the topographical appearance of a walnut.
Astronomers have made a few guesses about what could have caused such a band perfectly coincident with the equator, but all of these models have serious problems, and it remains one of the solar system’s mysteries.
A final oddity of Iapetus is its orbit. Although it is a substantial moon, it does not orbit around Saturn’s equator and ring plane like the others do. Because of this inclination, Iapetus is the only large moon that would offer good views of the planet’s magnificent rings; from all the others, the rings appear forever edge-on and look like a thin straight line. From Iapetus, Saturn appears the same size as Earth as seen from our Moon.
Unfortunately, Iapetus’ odd huge orbit makes it hard for the Cassini spacecraft to reach it. After the single close flyby September 10, 2007, there have been no others, nor are any planned, so it will be a while before we crack this walnut’s mysteries.