It seems to me that Saturn’s moon Rhea is a leading candidate for Rodney Dangerfield of the solar system. I tell you, it doesn’t get much respect. Even in the Saturn system, where Rhea is the second-largest moon, it ranks pretty low. You hear about Titan, with its thick atmosphere and methane lakes. Enceladus is known for its liquid-water geysers and Iapetus for its strange black and white hemispheres. Even tiny Mimas, with its cute “Death Star” crater, gets more ink than Rhea.
Maybe all Rhea needs is a better publicity agent. If so, help may be on the way. At this week’s Lunar and Planetary Science Conference in Houston, Roland Wagner of the German Aerospace Center in Berlin spoke about the Cassini spacecraft’s latest exploration of Rhea’s geology.
Rhea’s complex geology shows up nicely in this image, taken by the Cassini spacecraft when it passed close to Saturn’s moon last August. NASA/JPL/SSI
Wagner’s team analyzed the moon’s crater distribution and found that the heavily cratered terrain on Rhea’s leading hemisphere has an age of 4.0 to 4.2 billion years. Rhea does have some young features, however. In particular, a 30-mile-wide (48 kilometer) crater with bright rays which may be barely several million years old. The rays and secondary crater chains extend hundreds of miles from the crater.
This youthful feature may be related to one of the most surprising solar system findings of the past decade. Just last week, Cassini scientists announced in the journal Science that Rhea has a ring, mimicking the most-storied feature of its host planet.
Geraint Jones of Mullard Space Science Laboratory at University College, London, led the team that discovered a ring embedded within a broad debris disk surrounding Rhea. The disk spans several thousand miles, and consists of objects that likely range in size from pebbles to boulders. The team used two Cassini instruments to detect drops in the number of electrons on both sides of Rhea. “Seeing almost the same signatures on either side of Rhea was the clincher,” Jones says.
Since the discovery, Cassini scientists have run numerical simulations showing that Rhea could maintain rings for a long time. That ties back to the bright ray crater on the moon’s surface. Perhaps the impact that formed this crater also ejected the ring material into orbit. “No one was expecting rings around a moon,” Jones says. Maybe this unique trait will finally earn Rhea the respect it so richly deserves.