Rhea's subsurface probed by the Cassini radiometer: Insights into its thermal, structural, and compositional properties

• Published in: Icarus
• Main Authors: Bonnefoy, Léa, Le Gall, Alice, Lellouch, Emmanuel, Leyrat, Cedric, Janssen, M., Sultana, R.
• Other Authors: PLANETO - LATMOS, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), Institut universitaire de France (IUF), Ministère de l'Education nationale, de l’Enseignement supérieur et de la Recherche (M.E.N.E.S.R.), Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG )-Université Grenoble Alpes (UGA)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: [SDU]Sciences of the Universe [physics]; [PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph]; South pole
• Online Access: https://insu.hal.science/insu-02902236; https://insu.hal.science/insu-02902236/document; https://insu.hal.science/insu-02902236/file/Rhea_paper_final_postprint.pdf; https://doi.org/10.1016/j.icarus.2020.113947

International audience During its 13.5 years of operation around the Saturn system, the microwave radiometer incorporated in the Cassini RADAR observed Rhea at 2.2 cm during 9 flybys, with resolutions up to a tenth of Rhea's radius. We compare the antenna temperatures measured by this instrument to simulated data generated with the combination of thermal, radiative transfer, and emissivity models, in order to derive new constraints on the thermal, structural, and compositional properties of the near subsurface of different regions of Rhea. We find that the Cassini radiometer probes depths of 5 to 15 m on Rhea (that is 200–700 wavelengths), implying a weakly absorbing regolith and therefore little contamination by non-ice compounds. In the South polar region, local summer and fall observations constrains the maximum loss tangent to be 8.1 × 10−4, implying a contaminant volumetric fraction of <10% and a porosity >10% in the first ~10 m. The derived thermal inertias (>60 MKS) in the South pole are higher than the ones measured in the thermal infrared (1–46 MKS), consistent with increasing compaction with depth. Over all of Rhea, current models relating surface microwave emissivity and backscatter cannot explain both the emissivities and the high radar backscatter recorded by the Cassini Radar, suggesting the presence of especially efficient backscattering structures in the subsurface of Rhea. This interpretation is consistent with the very low derived dielectric constants (1.1–1.5), indicating that the subsurface structures are depolarizing. In particular, the ejecta blanket of the Inktomi crater has an emissivity about 20% lower than its surroundings and is very radar-bright: the impact that formed this young crater must have excavated fresh water ice from the subsurface, while also creating structures (such as cracks) reflecting centimetric wavelengths.