Accreting Moons in Saturn’s Rings
A new picture of Saturn’s rings is emerging, and it has to do with moons. Cassini observations presented by Carolyn Porco, head of the Cassini imaging team, at the AGU meeting today show that the small moons within and near the rings are shaped like their Roche lobes. The Roche lobes are not the ears of some strange aliens, but rather are the region of space around an object where its gravity is dominant over the gravity of the planet or star that the object orbits. For example, the Moon orbits the Earth because the Moon is within the Earth’s Roche lobe: the Earth’s gravity controls the motion of the Moon, not the gravity of the Sun. If the Moon were much further from the Earth and outside its Roche lobe, then it would go into an independent orbit around the Sun instead of orbiting the Earth.
So what does this have to do with Saturn’s rings? The particles in Saturn’s rings have Roche lobes that are only a little bit bigger than the particles themselves. This is because they are so close to Saturn, so its gravity can easily dominate the gravity of the small ring particles or moons. Ring particles running into a moonlet can stick to the moonlet if they are within the Roche lobe, but fall off otherwise. The Roche lobe is not spherical, so if accreting ring particles fill the Roche lobe of a moonlet, they will create a low density moon with a particular shape: that of the Roche lobe. And here’s what that looks like.
Image credit: NASA/JPL/Space Science Institute
In this image of the small moon Pan, within the Encke gap of the A ring, the moon is seen to be elongated toward Saturn, and has the shape of the Roche lobe for an object at that location. Measurements of the densities of Pan and other moons show that the moons closest to Saturn are filling their Roche lobes. Any new particles hitting these moons cannot stick because they are pulled away by Saturn’s gravity, leaving the moons with a characteristic shape and density signaling that even within the rings their can be accretion. This may be a clue to the apparent youthful nature of the rings: in a relatively short time, say 10-100 million years from now, Pan will be hit by a small cometary object that will break it apart, creating a new ringlet.
December 16th, 2006 at 9:07 am
I’m not sure that the work Carolyn presented suggests anything *new* about the age of the rings or Pan. Would the presence of a moon of a different density (factor of ~2) or shape in that location change the distription timescale significantly? Or did you mean that the short disruption timescale works well with the ease for re-accretion in that we can destroy and re-form small moons many times?
December 16th, 2006 at 1:17 pm
In answer to the first question: no. In answer to the second question: a qualified yes. It’s not that the moons’ densities themselves help provide reservoirs for moons, it’s that they indicate that moons are in fact forming in Saturn’s rings. The pre-Cassini view was that there was a one-way collisional cascade, and any moons in the rings were the leftover fragments from a disruption and there is no going back. We now see that the moons are rubble piles, and their growth may be fundamentally limited by external disruption rather than tidal forces (assuming that over long times there could be cold sintering and compaction of the individual pieces to form a more coherent and tidally resistant moonlet). Thus, the rings can be recycled rather than just the eroded fragments of one or two original precursor moons.