Rhea, the second largest moon of Saturn, has an icy surface marked by bright craters and networks of light streaks. Rhea’s surface consists primarily of water ice. Measurements of Rhea’s density suggest that the satellite has a rocky core. Rhea’s diameter measures 949 miles (1,528 kilometers). The moon orbits Saturn every 4.52 days at an average distance of around 327,500 miles (527,000 kilometers).
Impact craters cover Rhea’s surface. Some of the craters appear bright white compared to the surrounding terrain. These craters probably look bright because the impacts that created them exposed fresh, relatively clean water ice underlying the dirtier surface. These craters feature crater rays, bright streaks of material thrown out from a crater’s center during the impact. The crater rays extend across the surrounding area.
Rhea’s surface also includes irregular light streaks often referred to as “wispy terrain.” The streaks actually consist of large, overlapping fractures in the ice with shiny, clifflike walls. The cracks may result from past tectonic activity—that is, a shifting of the crust driven by heat from the moon’s interior. Rhea shows no signs of recent geological activity.
The Italian-born French astronomer Giovanni Domenico Cassini discovered Rhea using a telescope in 1672. In the early 1980’s, the United States space probes Voyager 1 and Voyager 2 flew by Rhea. The probes took photographs showing the wispy terrain. Beginning in 2004, the U.S. Cassini spacecraft photographed Rhea. It captured images which revealed that the wispy terrain actually consists of surface fractures. During a flyby of the planet in 2010, Cassini detected an exosphere, an extremely thin atmosphere, around the moon. The probe detected both oxygen and carbon dioxide. The discovery makes Rhea only the second body in the solar system, along with Earth, to have oxygen in its atmosphere.
