IAU circular 8773 reports the discovery of a 1000km-wide Saturn ring arc by Cassini at the orbital distance of 3 km-wide Methone (3.23 Saturn radii). It's been given the designation R/2006 S5, and appears to be close to, if not including, the position of Methone itself.

Rather than being found in images, the arc's presence was revealed by the MIMI instrument detecting a drop in electron fluxes during a perihelion pass in September. The dust particles that make up the ring absorb magnetospheric electrons, forming a "wake" that Cassini passed through both inbound and outbound.

That's how Pioneer 11 discovered Jupiter's rings but nobody actually LOOKED for them till Voyager took a "Might as well take 2 frames to be sure Jupiter doesn't have rings" pair of shots... oops.

The Vikings did a ring and new satellite search before that. Tom Duxbury had a short paper about it, probably in Icarus. It turned up nothing, of course.

Phil

This is unusual in that Neptune is the only other planet with true ring 'arcs'. This is the first one found at Saturn. It's also orbiting well inside the E ring; a ring arc inside a complete (though admittedly diffuse) ring - that's also a first. I wonder if its in some kind of resonant orbit with Methone; I guess it would have to be, if it's at the same distance from Saturn as Methone. It's hard to believe that a 3-km-across object could have such a huge influence, but I know that celestial mechanics can do some weird things, sometimes.

- John Sheff
Cambridge, MA

Doesn't the G ring also have a pronounced arc, in addition to a diffuse complete ring? This was a recent finding, IIRC.

As Methone is a trojan moon sitting at a Legrange point, might dust be expected to accumualte in an arc around the Legrange point? Might we expect dust arcs at other Legrange points?

Floyd

Methone (and Pallene) aren't trojans of larger moons; as far as we know they're the only sizeable objects at their orbital distances, between the orbits of Mimas and Enceladus.

The MIMI instrument did observe increased electron absorption at around 66 degrees in longitude from Enceladus, which suggests an increase in dust number density there. As I understand it, at the distance of Enceladus, no small grains can stay for very long at one of the Lagrange points of Enceladus, but larger particles, maybe mm-cm-sized and upwards, could be dynamically stable there.

ISS confirms the existence of the Methone arc; also reported in IAU Circular 8970.