  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.
