Cassini CIRS Emissivity Variations across Dione and Rhea’s Polar Regions

Abstract:

Since spring equinox in 2009 Rhea and Dione’s northern hemisphere has been warming, potentially causing volatiles previously trapped on its surface to sublimate. Recently Cassini’s Ion Neutral Mass Spectrometer (INMS) discovered traces of molecular oxygen and carbon dioxide around Rhea and Dione (Teolis et al., 2010; Simon et al., 2011; Tokar et al, 2012). Follow up observations showed that the gas concentrations were higher over the northern hemispheres of both satellites (Teolis et al., 2012), possibly due to seasonal variability. A surface capable of trapping the quantity of volatiles needed to produce the observed density of the exosphere must be both porous and cold (< 50 K) (Teolis et al., 2010). Both of these requirements may be met in Dione and Rhea’s polar regions. The presence of volatiles on the surface of these satellites, depending on their particle size and composition, could introduce observable emissivity variations into the spectrum observed by Cassini’s Composite Infrared Spectrometer (CIRS). A previous study (Carvano et al., 2007) found no evidence of emissivity variations in the CIRS spectrum of Saturn, Phoebe, Iapetus, Enceladus, Tethys or Hyperion, but did not consider Rhea or localized emissivity variations, and since then many more CIRS specta have been obtained. This work focuses on CIRS focal plane 1 (10 to 600 cm-1) north polar stare observations of Dione and Rhea, to obtain the high signal-to-noise required to observe subtle emissivity variations. The implications of both observed emissivity variations and null results will be discussed.