March 6, 2008
Narrator: Could there be rings around a moon? I'm Jane Platt with a podcast from JPL -- NASA's Jet Propulsion Laboratory in Pasadena, California.
Scientists have found something very intriguing around Saturn's second largest moon, Rhea -- and here's what it sounds like.
Natural sound of electrons at Rhea
Narrator: Here to tell us about it is Geraint Jones, a Cassini scientist with the University College in London. He did this research while he was with the Max Planck Institute in Germany. Let me start out by asking you, apparently six of the instruments on Cassini found something very interesting around Rhea. Tell us what that is.
Jones: Yes, this started in 2005 when Cassini made a close flyby of Rhea, only 500 kilometers away, and the instruments from Rhea sent really unique signatures, and so we've looked at several possibilities and the only one that seems to fit with the data we have is that there's a ring, a disk of debris surrounding Rhea, maybe containing rings as well.
Narrator: So there's a disk of stuff and the best you can figure out that is a ring or rings around the moon Rhea, just as Saturn has rings.
Jones: That's right, yes, but they clearly haven't been observed previously, so they're much more diffuse than the rings of Saturn itself.
Narrator: And before this flyby of Rhea, you didn't know there was material around it at all?
Jones: Well, there's always some dust around all these moons, there are small dust particles always striking the moons of the outer planets and kicking up small particles from the surface, so we always expect to see some dust there, but the signatures we saw were consistent with a lot more material being there than what we expected.
Narrator: And when you say the signatures, for the layperson, how would you explain what you mean by signatures?
Jones: OK, some of the instruments on Cassini don't take pictures. They sort of sense the environment around Cassini, and there are charged particles, a soup of particles, ions, protons and electrons that are trapped around Saturn and they sweep past these moons. So when we pass close to these moons, we usually see a wake when these particles strike the moon or we're in the shadow. It's a bit like when you're driving in heavy rain and you can hear the rain striking the roof of the car, when you go under a bridge it briefly stops and then comes back again. And looking at the electrons as we go past these moons, usually that's exactly what we see, we see a really sharp cutoff and then it comes back on again. But when we went past Rhea, the electrons started falling away much further away than we expected, well outside this sharp drop-off which we saw as well. So it suggested that there was something much more extensive blocking the electrons before they reached Cassini.
Narrator: You knew there was something there and based on your calculation, your analysis, you think it is multiple rings?
Jones: Possibly, there's more than one way of looking at it. There appears to be a broad obstacle stopping these electrons, but also we see very sharp dropouts on either side of Rhea itself, which suggests that there could be at least one ring there, sort of a narrow ring, as opposed to a broad debris disk, which also appears to be there.
Natural sound of electrons near Rhea.
Narrator: And that is the actual sound of electrons near Rhea. Gaps in the sounds led the scientists to propose that there are rings around Rhea that blocked some of the electrons flowing past the moon. Tell us a little bit about Rhea, which is Saturn's second largest moon.
Jones: It's about 1500 kilometers across --
Narrator: Which is 950 miles, right?
Jones: That's right. And it's mostly icy and covered in craters, saturated basically, there are no really other strong features on there, it's just been hit by meteorites, comets, etc., since its birth, so it just looks really battered. And before this, it could be argued that it may have been one of the least interesting looking moons of Saturn, but maybe it could be one of the most interesting after what we’ve seen.
Narrator: Well, thanks so much for your time.
Jones: Thank you for your interest.
Narrator: More information on this story and on other findings from the Cassini mission are online at www.nasa.gov/cassini .
Thanks for joining us for this podcast from NASA's Jet Propulsion Laboratory.