Comparison of Optical and Thermal Opposition Surges in the C Ring: Unveiling the Nature of the Opposition Effect

The opposition effect (OE) is a non-linear increase of the brightness near zero phase angle. So far, the rings’ OE is observed in reflected light (as the Visible Opposition Effect or VOE) and temperature from thermal emission (as the Thermal Opposition Effect or TOE). The nature of the origin of rings VOE and TOE is still a matter of debate. Indeed, the most common hypothesis is that VOE and TOE are caused by SH (Shadow Hiding of regolith surface irregularities and/or ring particles), although VOE should be partly due to CB (Coherent backscatter, i.e. constructive interferences at the exact backscattering direction). The bulk of the debate lies in the predominant, partial or null contribution of CB in VOE. To help solve this puzzle, our approach is to study the discrepancies of OE morphology from reflected and emitted lights. We use data from the ISS (Image Science Subsystem) onboard Cassini spacecraft, as well as those from the three Focal Planes (FP1, FP3 and FP4) of CIRS (Composite InfraRed Spectrometer). Using the original and improved parametric model of Bobrov (1970, Dollfus Editions, pp376), our first comparisons of brightness VOE from Deau et al. (2012 Icarus, final review) and temperature TOE from Altobelli et al. (2007, Icarus vol191, pp691), show that both surges are different in the C ring plateaux. This supports the idea that either VOE and TOE could have different origin; or TOE is not appropriately compared to VOE. We then investigate TOE in radiance units (W.cm-^2.sr^-1/cm^-1) and derived the first CIRS spectrograms for the C ring. Results of comparison of TOE in radiance units and VOE in I/F will be presented. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2012 California Institute of Technology. Government sponsorship is acknowledged.