Cassini imaging search rules out rings around Rhea

TL;DR

This study used Cassini images to search for rings around Rhea and found no evidence, challenging previous claims of narrow rings and suggesting alternative magnetospheric explanations for observed electron absorption phenomena.

Contribution

The paper provides the first direct imaging search ruling out rings around Rhea, contradicting earlier inferences and proposing a new magnetospheric process explanation.

Findings

No rings or material detected around Rhea in Cassini images.

Previous electron absorption signals are unlikely caused by rings, implying a different magnetospheric process.

Rings, if any, must be composed of objects larger than 10 meters, which is inconsistent with erosion models.

Abstract

We have conducted an intensive search for any material that may orbit Rhea, using images obtained by the Cassini ISS narrow-angle camera. We find no evidence for any such material, contradicting an earlier and surprising inference that Rhea, the second-largest moon of Saturn, possesses a system of narrow rings embedded in a broad circum-satellite disk or cloud (Jones et al. 2008, Science). If one accepts the calculations of the previous authors, our results would require any narrow rings around Rhea to be composed of objects no smaller than 10 meters in radius, in order to be massive enough to account for the observed charged-particle absorptions but diffuse enough to evade detection in our images. Such a particle-size distribution is unrealistic given that erosion processes will break down 10-meter objects to smaller sizes, which would then have been seen in our images. Furthermore, Jones et al. (2008) assumed that the absorption of electrons by hypothetical circum-Rhea particles is proportional to the volume of the particles, even when they are much larger than the electron penetration depth (van Allen 1983, 1987, JGR). If we assume instead that large particles can only absorb electrons efficiently near their surfaces, then our results rule out any solid material orbiting Rhea as the explanation for the absorptions detected by Jones et al. (2008), regardless of particle size, by a margin of four (for narrow rings) or two (for a broad diffuse cloud) orders of magnitude. We conclude that the anomalous electron absorptions reported by Jones et al. (2008) are the signature of a new and hitherto unknown magnetospheric process.