This image shows Saturn's moon Rhea floating above the planet's murky atmosphere. The black line above the moon is the shadow of Saturn's rings falling across the planet's cloud tops. Rhea is Saturn's second-largest moon, at 949 miles (1,528 kilometers) in diameter. Rhea is one of the most heavily cratered objects in the Solar System. This means that its surface is extremely old and stable. There is no geologic activity taking place that could reshape the moon's surface features. Rhea is composed mainly of water ice with some rocky material mixed in.