Thermal properties of Rheas Poles: Evidence for a Meter-Deep Unconsolidated Subsurface Layer

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).

💡 Research Summary

This paper presents the first seasonal thermal observations of Saturn’s icy moon Rhea’s polar regions using the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft. Two close fly‑bys—on 9 March 2013 and 10 February 2015—provided spectra of the southern winter pole and the northern summer pole, respectively. The southern pole was found to be extraordinarily cold, with brightness temperatures of 25.4 ± 7.4 K and 24.7 ± 6.8 K, making it one of the coldest directly measured surfaces in the Solar System. In contrast, the northern pole exhibited a much warmer temperature of 66.6 ± 0.6 K. Because the two observations correspond to solar longitudes of roughly 133° (summer pole) and 313° (winter pole), they can be interpreted as seasonal temperature extrema for the same region.

To translate these temperature constraints into physical surface properties, the authors employed a one‑dimensional seasonal thermal model that solves the heat diffusion equation for a semi‑infinite slab. By varying bolometric albedo and thermal inertia, they identified a family of solutions that reproduce the measured temperatures. Acceptable fits require albedos between 0.70 and 0.74 and thermal inertias ranging from 1 to 46 MKS (J m⁻² K⁻¹ s⁻½). These values are consistent, within uncertainties, with previous determinations for Rhea’s leading and trailing hemispheres, indicating that the polar regions share similar surface reflectance and bulk thermophysical characteristics with the rest of the moon.

The low thermal inertia is the most striking result. Thermal inertia controls how quickly a material responds to temperature changes; a low value implies that the material is highly insulating and retains little heat. The derived range (1–46 MKS) suggests that the polar regolith remains highly porous down to depths probed by the seasonal wave (≈10 cm to several meters), far deeper than the diurnal skin depth (<1 cm) that dominates daily temperature variations. In other words, even at meter‑scale depths the material is not compacted, but rather consists of loosely packed, low‑density ice grains. This high porosity explains why the winter pole can cool to such extreme temperatures despite continuous exposure to interplanetary radiation over the long polar night.

In addition to broadband temperature measurements, the study includes a detailed analysis of a CIRS stare spectrum taken on 9 March 2013 between 16:22:33 and 16:23:26 UT, covering the 10–600 cm⁻¹ (≈16–1000 µm) range and centered at 71.7° W, 58.7° S. This spectrum represents the first thermal emissivity measurement of Rhea. The emissivity curve is remarkably flat, showing no discernible absorption or emission features that would be expected from crystalline water ice, silicates, or other common constituents. The authors discuss several possible explanations for this featureless spectrum. The most plausible scenario is that the surface consists of a highly porous aggregate of sub‑50 µm particles. Such fine‑grained, low‑density material would scatter infrared radiation efficiently, suppressing spectral contrast and yielding a near‑blackbody emissivity. Alternative explanations—such as a surface covered by a thin, spectrally neutral coating or a highly conductive substrate—are considered less likely given the concurrent low thermal inertia and high albedo.

Overall, the paper provides compelling evidence that Rhea’s polar caps are covered by a meter‑deep, unconsolidated layer of highly porous, fine‑grained ice. This conclusion has several broader implications. First, it demonstrates that seasonal thermal waves can probe subsurface properties at depths inaccessible to diurnal studies, offering a valuable tool for characterizing the internal structure of icy bodies. Second, the persistence of high porosity at meter scales suggests that processes such as sintering, impact gardening, or thermal metamorphism have been inefficient at compacting the regolith, perhaps because of the low surface temperatures and the lack of significant endogenic activity. Finally, the flat emissivity spectrum, combined with the low thermal inertia, reinforces the view that Rhea’s surface is dominated by a fluffy, frost‑like mantle rather than a compacted icy crust. These insights improve our understanding of the thermal evolution of Saturn’s moons and provide a benchmark for interpreting future observations of other icy satellites, including those that will be targeted by upcoming missions such as Europa Clipper and JUICE.