Thermal properties of Rheas 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).

💡 Analysis

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).

📄 Content

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

C.J.A. Howett1, J.R. Spencer1, T. Hurford2, A. Verbiscer3, M. Segura2.

1 - Southwest Research Institute, Colorado, USA. 2 - Goddard Space Flight Center, Maryland, USA. 3 – University of Virginia, Charlottesville, Virginia, USA.

Corresponding Author and their Contact Details: C.J.A. Howett Email: howett@boulder.swri.edu Telephone Number: +1 720 240 0120 Fax Number: +1 303-546-9687 Address: 1050 Walnut Street, Suite 300 Boulder, Colorado 80302 USA

Abstract

Cassini’s Composite Infrared Spectrometer (CIRS) observed both of Rhea’s polar regions during a close (2,000 km) flyby on 9th March 2013 during orbit 183. Rhea’s southern pole was again observed during a more distant (51,000 km) flyby on 10th February 2015 during orbit 212. The results show Rhea’s southern winter pole is one of the coldest places directly observed in our solar system: surface temperatures of 25.4±7.4 K and 24.7±6.8 K are inferred from orbit 183 and 212 data respectively. The surface temperature of the northern summer pole inferred from orbit 183 data is warmer: 66.6±0.6 K. Assuming the surface thermophysical properties of the two polar regions are comparable then these temperatures can be considered a summer and winter seasonal temperature constraint for the polar region. Orbit 183 will provide solar longitude (Ls) coverage at 133° and 313° for the summer and winter poles respectively, whilst orbit 212 provides an additional winter temperature constraint at Ls 337°. Seasonal models with bolometric albedo values between 0.70 and 0.74 and thermal inertia values between 1 and 46 J m-2 K-1 s-1/2 (otherwise known as MKS units) can provide adequate fits to these temperature constraints (assuming the winter temperature is an upper limit). Both these albedo and thermal inertia values agree within the uncertainties 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 (~tens of centimeters to meter depths) than the more commonly reported diurnal wave (typically less than a centimeter), the exact depth difference dependent upon the assumed surface properties. For example, if a surface porosity of 0.5 and thermal inertia of 25 MKS is assumed then the depth of the seasonal thermal wave is 76 cm, which is much deeper than the ~0.5 cm probed by diurnal studies of Rhea (Howett et al., 2010). The low thermal inertia derived here implies that Rhea’s polar surfaces are highly porous even at great depths. Analysis of a CIRS focal plane 1 (10 to 600 cm-1) stare observation, taken during the orbit 183 encounter between 16:22:33 and 16:23:26 UT 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 (<~50 µm).

1 Introduction On 9th March 2013 the Cassini spacecraft had a close (2,000 km) encounter with Saturn’s mid-sized icy satellite Rhea. The remote sensing instruments onboard Cassini viewed Rhea’s southern winter hemisphere on approach and Rhea’s northern summer hemisphere during departure at approximately nadir geometry. Thus, during one encounter high- spatial resolution observations were obtained of both of Rhea’s poles. Nearly two years later CIRS caught another glimpse of Rhea’s southern pole, this time from further away (51,000 km) and at a high emission angle (80° to 90°). To date these data sets provide the best coverage of Rhea’s polar regions by CIRS, as they were taken at high-spatial resolution and at mostly low emission angles.

The Composite Infrared Spectrometer (CIRS) is one of Cassini’s remote sensing instruments and was taking data during both of these Rhea encounters. It is from these data that the surface temperatures of Rhea’s polar regions can be inferred, where a polar region is loosely defined as lying between the pole and 60° N/S. CIRS has three focal planes covering 10 to 1400 cm-1 (c.f. Flasar et al., 2004). Focal plane 1 (FP1) covers 10 to 600 cm-1 (16.7 to 1000 µm), enabling the temperatures of even very cold surfaces to be determined (<40 K). However, FP1’s drawback is that it’s made from a single circular detector, which has the lowest spatial resolution of CIRS’ three focal planes (3.9 mrad/pixel). The other focal planes (focal planes 3 and 4, known as FP3 and FP4) cover 600 to 1100 cm-1 (9.1 to 16.7 µm) and 1100 to 1400 cm-1 (7.1 to 9.1 µm) respectively, and are both 1x10 arrays of 0.273 mrad/pixel detectors. These wavelength ranges make FP3 and FP4 s

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