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Planetary News: Cassini-Huygens (2004)Cassini Glimpses New Ring and One, Maybe Two, New MoonsBy Emily LakdawallaSeptember 10, 2004
Just when you thought the neighborhood couldn't get any more crowded, scientists have spotted a new ring and one or possibly two new moons at Saturn. The ring and one of the moons were discovered by Cassini Imaging Team member Carl Murray of Queen Mary College, London. Murray is quick to emphasize, though, that the discoveries were a team effort. "It's the result of the work of the entire Cassini Imaging Team. I was lucky to be the first to spot them." The new features include:
The features were discovered in images captured as part of a sequence designed by Imaging Team Leader Carolyn Porco for the specific purpose of discovering new moons. "We have planned many images to search the region between the A and F rings for diffuse material and new moons, which we have long expected to be there on the basis of the peculiar behavior of the F ring," said Porco. "Now we have found something but, as is usual for the F ring, what we see is perplexing." Murray was studying the images in order to learn more about the dynamics of the F ring, which he describes as "notorious for throwing up temporary features. The ones that Voyager 1 spotted had all disappeared by the time that Voyager 2 arrived." Gravitational interactions between F ring particles and the ring's two shepherd moons are the presumed cause of complex, fugitive braids and clumps in the faint ring, which lies just outside Saturn's brightest A and B rings. The sequence of images that Murray was examining for F ring features "was designed by Carolyn Porco specifically to look for moons," Murray explains. "You're staring at the ansa [the outer edge of Saturn's rings] because your camera has the highest resolution there. The sequence lasted something like 16 hours, which means that everything [in and near the F ring] would pass at least once through the field of view. I enhanced the images and compared them successively. You're looking for something that moves and maintains its [radial] position around the region that you're interested in. If you see it only in one frame, you discount it, but if you see it again and again you can discover something." Even with such a complete sequence of images, new moons can be difficult to spot. "There's a lot of cosmic ray hits and stars in the raw images," Murray says. "We're deviled by these cosmic ray hits. Some of them are quite spectacular; you have to learn to ignore them." Once he noticed the telltale dot passing near the F ring, "we caught it in 19 separate images. It was one of those 'Wow!' moments. You think, 'gosh, what's that?' It's a privilege to be the first person to see it."
The object Murray found is very small, only about 5 kilometers (3 miles) across. As close as it is to the F ring, scientists cannot yet be certain that it is really a moon and not just a temporary clump of F ring material. There will be more confidence in the "moon" classification once the Imaging Team can nail down the object's orbit. But an attempt to do just that has only obfuscated the matter. Imaging Team member Joseph Spitale examined an ansa observation sequence conducted 5 hours later, and did see an object in the right longitudinal position -- but it was inside the F ring, not outside. Spitale said, "If this is the same object then it has an orbit that crosses the F ring, which makes it a strange object." Murray's personal opinion is that the two objects are not the same. He thinks that he first observed "looks like a moon" but that the object that Spitale observed "looks like a clump." But, he emphasizes, "we don't really know yet. We have to examine all the possibilities carefully, especially when we're dealing with the F ring." At the same time that he found the object outside the F ring, Murray observed
a faint ring of material co-orbital with the tiny moon Atlas, in the region
between the A ring and F ring. If this turns out to be a long-lived ring completely
encircling the planet, it would be the first distinct ring discovered since
the G ring was first spotted in Voyager 1 images in 1980. But it's premature
to give the "Atlas ring" the next letter in the alphabet. "We
first have to see if the ring goes all the way around the planet," Murray
says. "And we have to see what the role of Atlas is. One possibility
is that Atlas is the source of the material -- maybe you're just looking at
the effects of some recent impact event. Another possibility is that material
is drifting in from outside and getting captured into orbit." For the
present, the new ring has been designated R/2004 S1.
Fortunately for the Cassini team, there should be ample opportunity to perform repeat observations to answer all of the questions about the new moon, or moons, and ring. "It's not that I want to be greedy," Murray says, "but it's not enough just to get one set of images -- it's the bigger picture, it's seeing the whole system. This is the beauty of Cassini. Because we are in orbit we can retarget, and we can look out for these guys and improve the orbits and perhaps actually get close images of them later on in the mission." As for the rings, Murray hopes to wring everything he can out of Cassini's four-year tour. "There are opportunities throughout the mission to get really good data on the rings. We would like to get out of the ring plane, to be high altitude." From that point of view, the rings will be open to view all the way around Saturn, not just at the two ansae. "We know that the ring system changes on a variety of time scales. We need to look at the beginning, middle, and end. It's something we can do with an orbital mission that we can't do with a flyby mission." Murray enjoys the challenge of studying Saturn's F ring. "It's almost as if the F ring is always one step ahead of us. Things like this -- the ones that really stump you -- you look at it and think, 'What on Earth is going on?' But those are the things you learn the most from." Murray has high hopes that Cassini's investigations will produce the evidence he needs to answer his questions about the F ring and its attendant moons, predicting "I think we'll crack it by the end of four years!"
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