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Emily LakdawallaMarch 3, 2009

A new moon for Saturn within the G ring

UPDATE: This new moon is now named Aegaeon.EDIT: Check out a correction posted on March 4: Cassini definitely could and may well get images of this newly discovered moon.

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:

New moon of Saturn within the G ring: S/2008 S1


New moon of Saturn within the G ring: S/2008 S1
This series of images was captured by Cassini over a period of 10 minutes and follows the path of a newly discovered moonlet within Saturn's dusty G ring. The images were taken with long exposures, so background stars form streaks. At the same time, the ring was orbiting Saturn, so it is also streaked, but in the direction of orbital motion. Within the G ring in each image is a small streak of light: this is the reflected light of the moonlet S/2008 S1. The images focus on an extremely tiny portion of the G ring, and when enlarged, show it at 7 kilometers per pixel. The moonlet itself is only about 500 meters across, far smaller than a single pixel in these images.
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.
An arc in Saturn's G ring

NASA / JPL / Space Science Institute

An arc in Saturn's G ring
These three images of the tenuous G ring were taken by Cassini about 45 minutes apart on May 24, 2005. In the first image, a bright arc is visible at the bottom edge of the ring. In the middle image, the bright arc has rotated around to the ansa (left side) of the ring. In the right image, the arc has moved up and to the right. The origin of this arc was then unknown.
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.
Methone and Anthe with their ring arcs


Methone and Anthe with their ring arcs
Arcs of material share an orbit with the tiny moons Anthe (top left) and Methone (bottom right). The arcs are confined by resonances with Mimas; the Methone arc only spans about 10 degrees of longitude, and the Anthe arc about 20 degrees of longitude.
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.
Ring arcs associated with small moons of Saturn

from Matt Hedman's presentation to the 2008 DPS meeting

Ring arcs associated with small moons of Saturn
Methone, Anthe, and Pallene are very small moons that were discovered in Cassini images of the region between Mimas and Enceladus, in orbit around Saturn. Further investigation of these moons has yielded the discovery of rings associated with these moons -- ring arcs for Anthe and Methone, and a complete ring for Pallene.
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." However, the moonlet so so small that even Cassini's sharp-eyed and steady camera can't resolve it as a disk -- Cassini just can't get close enough to the G ring. To see a 500-meter-diameter object as a thing 5 pixels across (that is, to get an image at a resolution of 100 meters per pixel), Cassini would have to pass within 17,000 kilometers of it. And the G ring is dusty and extended, posing lots of hazards to a fragile spacecraft; Cassini's just never going to be allowed to get that close to the G ring anywhere, much less in the precise spot where S/2008 S1 orbits. See my March 4 post on how Cassini may image S/2008 S1.

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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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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