For more than 300 years we thought Saturn was the only body in the solar system with rings. But by 1989, after ground-based measurements and flybys from Voyagers 1 and 2, we had discovered rings around the other three gas giants—Jupiter, Uranus, and Neptune. Now the Cassini spacecraft appears to have found a ring system around Saturn’s second-largest moon, Rhea. The discovery took astronomers by surprise. Just what is a ring doing around a moon, especially one that is significantly smaller than our own?
Normally a ring forms when two things are present: space debris, either from a collision with a small body or left over from the solar system’s formation, and sufficient gravity to attract the debris and hold it in place. While the gas giants all have what it takes to hold a ring system in place, “a ring system around a moon comes as a surprise because ...
