A new moon for Saturn within the G ring
Posted by Emily Lakdawalla
03-03-2009 12:59 CST
It's been a while since the last discovery of a moon at Saturn by Cassini; today comes a long-awaited announcement, the discovery of the parent body (perhaps just one of several) to Saturn's dusty G ring. It's long been suspected that there must be something responsible for the presence of that ring, but until now no one's been able to find the responsible moonlet. There's a good reason it was hard to find: they are guessing that the tiny thing is only about half a kilometer across, the size of Itokawa. It's provisionally named S/2008 S1, and its existence was announced today in the International Astronomical Union Circular #9023. My subscription to the IAUCs seems to have lapsed, so I don't know what other details are in the citation, unfortunately. Here's a set of photos:It's a tall order to find a 500-meter-diameter object within the G ring, which orbits at 170,000 kilometers from the center of Saturn, so is more than a million kilometers in circumference. Fortunately, Cassini scientists did have a useful clue where in the G ring to search. Back in 2005, they did a series of observations of the G ring and discovered that it contained a single bright arc of material. More recently, three moons discovered by Cassini, Methone, Anthe, and Pallene, have been shown to orbit within arcs of dusty material. The material comes from the moons themselves, knocked off by impacts; they are confined to their positions by orbital resonances with the nearby medium-sized moon Mimas. The arc in the G ring and the arcs associated with Methone, Anthe, and Pallene strongly hinted that there must be some parent body hiding within that G ring arc. In this diagram that I clipped from Matt Hedman's presentation at last year's Division of Planetary Sciences meeting, you can see where the Methone and Anthe orbits are located (orange and yellow dashed lines); and where Mimas' orbit is located (red dashed line). The very next faint ring closer to Saturn than Mimas is the G ring, now known to contain its own moon, S/2008 S1. According to the press release, the moonlet was first discovered in images from August 15, 2008; based on that discovery they then went back to earlier images and confirmed its presence. The release goes on to say that "they have since seen the moonlet on multiple occasions, most recently on February 20, 2009."
S/2008 S1 may just be the easiest to find of a number of parent bodies that supply dusty material to the G ring. Methone, Anthe, and Pallene just have arcs, but the G ring extends all the way around the planet. So there could well be other objects, maybe just 100 meters or 50 meters across, orbiting in different positions within the G ring, all supplying the dust that keeps the G ring its dusty self. With this discovery, I think Cassini's beginning to venture into the murky terrain that separates "moons" from "ring particles."
What would S/2008 S1 look like up close? I don't know, but I can hazard some guesses. It's roughly the size of Itokawa, but I would think that it wouldn't look anything like Itokawa's bouldery and gravelly surface. S/2008 S1 orbits within an arc of dust of all sizes. The small moons of Saturn that orbit close to and within the rings that Cassini has photographed -- Atlas, for instance, or Pandora -- have odd football (American football, that is, or rugby ball, if you prefer) or saucer shapes, formed from the slow accretion and sliding of dust around their surfaces. I'd guess S/2008 S1 would look like that. Try to land on it, and your every motion would kick up puffs and clouds of dust, flying away on ballistic trajectories, to orbit Saturn in the "sky" around the little world. Weird.
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