Iapetus (pronounced eye-APP-eh-tuss) is one of Saturn's 31 known moons. One hemisphere is very dark, and the other is very light, owing to different surface compositions (what you see in this picture is not a shadow!). Scientists don't know what's going on. But Cassini aims to find out.

The strange moon has a leading hemisphere that appears to be coated with some sort of dark material, and a more reflective trailing hemisphere. The differences are revealed by reflected sunlight -- the same thing that makes all planets and moons visible.

Iapetus is about 892 miles (1,436 kilometers) wide, or about one-third the diameter of Earth's moon.

This image was taken in visible light from about 1.8 million miles (3 million kilometers miles) away. During Cassini's four-year tour of the Saturn system, the spacecraft will conduct two close encounters of Iapetus, including one at just 622 miles (1,000 kilometers) from the moon's surface.

-- Robert Roy Britt

Credit: NASA/JPL/Space Science Institute