Thermophysical property variations across Dione and Rhea

•Maps of both bolometric Bond albedo and thermal inertia have been produced for Rhea and Dione.•A weak thermally anomalous region has been discovered on Dione, analogous to the “Pac-Man” anomalies on Mimas and Tethys.•Rhea’s Inktomi crater and ejecta blanket are also shown to have a higher thermal i...

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Bibliographic Details
Published in:Icarus (New York, N.Y. 1962) N.Y. 1962), 2014-10, Vol.241, p.239-247
Main Authors: Howett, C.J.A., Spencer, J.R., Hurford, T., Verbiscer, A., Segura, M.
Format: Article
Language:English
Subjects:
Albedo
Craters
Dione
Hemispheres
Ices
Inertia
Infrared observations
Lunar And Planetary Science And Exploration
Mimas
Satellites, surfaces
Saturn, satellites
Tethys
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Summary:•Maps of both bolometric Bond albedo and thermal inertia have been produced for Rhea and Dione.•A weak thermally anomalous region has been discovered on Dione, analogous to the “Pac-Man” anomalies on Mimas and Tethys.•Rhea’s Inktomi crater and ejecta blanket are also shown to have a higher thermal inertia than their surroundings. Maps of the variation in bolometric Bond albedo and thermal inertia across Rhea and Dione have been produced using various day and nighttime observations, taken by Cassini’s Composite Infrared Spectrometer (CIRS). The albedo maps show the same trend that has been previously observed on these satellites from reflected sunlight and thermal observations: a higher albedo on their leading hemispheres. The thermal inertia maps show two previously unknown anomalous high thermal inertia regions: low latitudes on Dione’s leading hemisphere and the bright ejecta blanket of Rhea’s Inktomi crater. The thermal inertia on Dione increases modestly from a background value of 8Jm−2K−1s−1/2 to 11Jm−2K−1s−1/2 in a region preferentially bombarded by high-energy electrons. We believe that this region on Dione is probably analogous to the thermally anomalous regions recently discovered at equivalent locations on Mimas and Tethys, dubbed Pac-Man features. The smaller magnitude of Dione’s thermal anomaly, compared to that of Mimas and Tethys, provides additional evidence that surface alteration by high-energy electrons produces these anomalies, as the high-energy electron flux decreases with increasing distance from Saturn. However, unlike on Mimas and Tethys, the thermally anomalous region on Dione does not display a spatially correlated decrease in the IR/UV (0.930μm/0.338μm) color ratio, implying that the minimum electron energy threshold of the IR/UV ice surface darkening mechanism is not met on Dione. The average of the mapped bolometric Bond albedos on the leading and trailing hemispheres of Dione are 0.49±0.11 and 0.44±0.13 respectively. On Rhea the thermal inertia increases from a background value of 10Jm−2K−1s−1/2 to 19Jm−2K−1s−1/2 on the ejecta blanket of Inktomi crater, probably due to a mixture of high and low thermal inertia material in this region, such as a fine- and large-grain ice mixture. This is the first time a thermal inertia anomaly has been associated with an impact crater on an icy saturnian satellite. On Rhea the average of the mapped bolometric Bond albedos on the leading and trailing hemispheres are 0.59±0.11 and 0.56±0.13 resp
ISSN:0019-1035
1090-2643
DOI:10.1016/j.icarus.2014.05.047