Dione is one of the natural satellites of the planet Saturn. It is the fifteenth largest moon in the solar system and is primarily composed of water ice and rocky material. Dione has a diameter of about 1,123 kilometers (698 miles) and features a mix of heavily cratered regions and smooth, bright areas, indicating geological activity in its past. Discovered in 1684 by the astronomer Giovanni Domenico Cassini, Dione is named after a figure from Greek mythology, the mother of Aphrodite. The moon has a thin atmosphere and is believed to have a subsurface ocean, which may harbor conditions suitable for life.