Thermal properties of Rhea's Poles: Evidence for a Meter-Deep Unconsolidated Subsurface Layer

Abstract

Cassini's Composite Infrared Spectrometer (CIRS) observed both of Rhea's polar regions during two flybys on 2013/03/09 and 2015/02/10. The results show Rhea's southern winter pole is one of the coldest places directly observed in our solar system: temperatures of 25.4+/-7.4 K and 24.7+/-6.8 K are inferred. The surface temperature of the northern summer pole is warmer: 66.6+/-0.6 K. Assuming the surface thermophysical properties of both polar regions are comparable then these temperatures can be considered a summer and winter seasonal temperature constraint for the polar region. These observations provide solar longitude coverage at 133 deg and 313 deg for the summer and winter poles respectively, with additional winter temperature constraint at 337 deg. Seasonal models with bolometric albedos of 0.70-0.74 and thermal inertias of 1-46 MKS can provide adequate fits to these temperature constraints. Both these albedo and thermal inertia values agree (within error) with those previously observed on both Rhea's leading and trailing hemispheres. Investigating the seasonal temperature change of Rhea's surface is particularly important, as the seasonal wave is sensitive to deeper surface temperatures (~10cm to m) than the more commonly reported diurnal wave (<1cm). The low thermal inertia derived here implies that Rhea's polar surfaces are highly porous even at great depths. Analysis of a CIRS 10 to 600 cm-1 stare observation, taken between 16:22:33 and 16:23:26 UT on 2013/03/09 centered on 71.7 W, 58.7 S provides the first analysis of a thermal emissivity spectrum on Rhea. The results show a flat emissivity spectrum with negligible emissivity features. A few possible explanations exist for this flat emissivity spectrum, but the most likely for Rhea is that the surface is both highly porous and composed of small particles (less than approximately 50 um).