Rings around Rhea

by Clifford Johnson     Department: Space

There's a lot we don't fully understand about our solar system, including details of the formation of the planets, the origin of various satellites (including our own moon), and the nature of rings. Now it is known by more or less everyone that Saturn has rings, and we've learned a lot about them from the Cassini-Huygens mission that has done such a great job of sending marvellous images for us all to see. What is less known is that the other gas giants (Neptune, Uranus and Jupiter) have ring systems too. (It is a major source of annoyance to me that you can still find exhibits in museums that have not had  their information updated after all these years, so that Saturn is still thought of as "the one with the rings". -  Why do all these planetary scientists bother doing their work if teachers are going to try their best to make the subject seem dead and unchanging? - link to the Pluto issue at this point.)  Learning about how these ring systems form, evolve, interact with the other satellites, and play a role in a planet's environment (in a broad sense) is of great interest to scientists.

What has come to light recently is the remarkable news that one of Saturn's moons, Rhea, may itself have a ring system! There's indirect evidence of this from the Cassini craft (from 2005 data whose analysis took some time to do), and a press release about the findings can be found here. Extract:

Rhea is roughly 1,500 kilometers (950 miles) in diameter. The apparent debris disk measures several thousand miles from end to end. The particles that make up the disk and any embedded rings probably range from the size of small pebbles to boulders. An additional dust cloud may extend up to 5,900 kilometers (3,000 miles) from the moon's center, almost eight times the radius of Rhea.

"Like finding planets around other stars, and moons around asteroids, these findings are opening a new field of rings around moons," said Norbert Krupp, a scientist with Cassini's Magnetospheric Imaging Instrument from the Max Planck Institute for Solar System Research.

Since the discovery, Cassini scientists have carried out numerical simulations to determine if Rhea can maintain rings. The models show that Rhea's gravity field, in combination with its orbit around Saturn, could allow rings that form to remain in place for a very long time.

The image above right (from JPL) is an artist's (exaggerated) impression of what this might look like. More on the image can be found here.

-cvj

Tags: cassini, rings around rhea, saturn