  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).
