Science —

Cassini may be witnessing the birth (or death) of a moon of Saturn

Objects, the biggest named "Peggy," spotted at the outer edge of the A Ring.

Cassini may be witnessing the birth (or death) of a moon of Saturn

"Saturn’s rings are a conveniently located dynamical laboratory," says the opening sentence of a new paper. The convenient part may be debatable, but the dynamism isn't. The rings are filled with gaps and wiggles, created by interactions among their particles and a collection of small moons that act as shepherds, their gravity ushering the rings' particles into distinctive orbits.

Now researchers have identified a series of bright objects embedded in the outer edge of Saturn's A Ring. The largest of these, which has been nicknamed "Peggy," may be as much as a kilometer across. The objects may represent a moon that is disintegrating after contact with the outer edge of the A Ring. But it could also be one in the process of formation—a process that may have played out many times in Saturn's past.

The initial observation of Peggy, shown above, came in a photograph taken a year ago yesterday by the Cassini orbiter. That prompted a dive into the image archive. Prior to May of 2012, the orbiter didn't have a good perspective for imaging the rings for over a year, but Peggy was visible in over 100 detections between then and November of 2013.

By January of 2013, a second object became visible that orbited near Peggy. Later, these were joined by two additional sets of objects, with distinct orbits and visual properties.

These could represent objects forming through the sort of collisional growth that is thought allow dust disks to give rise to planets and moons. And that's exactly the angle NASA's press release on the paper took, leading most headlines to echo the "new moon" story line. However, the paper itself indicates that that the opposite is more likely: we're probably witnessing the disintegration of a small moon. "We believe that this is strong evidence," they argue, "for the breakup of the precursor object due to a collision or tidal disruption."

It's a bit difficult to tell, in part because the outer reaches of the A Ring is a rather complicated place. Its edge is defined by an orbital resonance with the moon Janus; the ring's material makes six orbits for every seven by Janus. This creates seven wobbles in the ring itself, which extend 20km (12.4 miles) in either direction from the average location of the outer edge. But Janus shares its orbit with a second moon, Epimetheus, and regularly swaps orbital momentum with it. As a result, the location of the wobbles tends to shift around.

So, we'll need further observations to get a better sense of what Peggy is up to. But in either case, it may give us a better indication of how small bodies interact with the ring material. Which may in turn tell us more about Saturn's moons in general.

Once enough material condenses into a body that's denser than ice, it will rapidly migrate outward through the ring system and exit. Saturn's moons are arranged such that largest are the furthest out, which has led to the idea that each of the moons formed within a large, dense ring system and then took part of the material out with them. This would ensure that the next moon to form would have less material to form from, neatly explaining the inside-out arrangement of the moons.

Icarus, 2014. DOI: 10.1016/j.icarus.2014.03.024  (About DOIs).

Channel Ars Technica