With a leading side dark as charcoal and trailing side bright white, the 914-mile-wide Iapetus is literally the yin-and-yang of Saturn’s family of moons.
The color variation on Iapetus is due to the fine coating of dark material that falls onto its leading hemisphere, possibly sent its way by smaller, distant Phoebe traveling within the recently-discovered giant diffuse ring. This dark coating of dust causes that half of Iapetus’ surface to warm up ever-so-slightly-more than the other, making the water ice evaporate and redepositing it on the other side. This in turn just reinforces the whole cycle…a positive feedback loop.
Both halves of Iapetus feature single prominent impact craters over 300 miles wide, and a several-mile-high mountainous ridge that circles its equator. See high-res color images of both sides of the moon here.
The image above was taken on February 25 by the Cassini spacecraft, at a distance of over a million miles. It’s been cropped and contrast enhanced from its original.
Image: NASA/JPL/Space Science Institute