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

Bonnefoy, Lea; Gall, Alice Le; Lellouch, E.; Leyrat, C.; Janssen, M. A.; Sultana, Robin

doi:10.1016/j.icarus.2020.113947

Cited by 11 publications

(8 citation statements)

References 55 publications

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“…Impact features on other Galilean moons observed to be thermally cold via disk-resolved observations include Tros and Osiris on Ganymede (de Kleer et al 2021a; Brown et al 2022) and Pwyll on Europa (Trumbo et al 2017(Trumbo et al , 2018. Anomalously cold craters have also been observed in the Saturnian system on Titan (Janssen et al 2016) and Rhea (Bonnefoy et al 2020). With the results for Valhalla presented here, this general trend is confirmed to extend to the largest class of impacts in the solar system.…”

Section: Valhalla Impact Basinsupporting

confidence: 75%

Thermal Properties of the Leading Hemisphere of Callisto Inferred from ALMA Observations

Camarca¹,

Kleer²,

Butler

et al. 2023

Planet. Sci. J.

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We present a thermal observation of Callisto's leading hemisphere obtained using the Atacama Large Millimeter/submillimeter Array at 0.87 mm (343 GHz). The angular resolution achieved for this observation was ∼0.″16, which for Callisto at the time of this observation (D ∼ 1.″05) was equivalent to ∼six elements across the surface. Our disk-integrated brightness temperature of 116 ± 5 K (8.03 ± 0.40 Jy) is consistent with prior disk-integrated observations. Global surface properties were derived from the observation using a thermophysical model constrained by spacecraft data. We find that models parameterized by two thermal inertia components more accurately fit the data than single thermal inertia models. Our best-fit global parameters adopt a lower thermal inertia of 15–50 J m−2 K−1 s−1/2 and a higher thermal inertia component of 1200–2000 J m−2 K−1 s−1/2, with retrieved millimeter emissivities of 0.89–0.91. We identify several thermally anomalous regions, including spots ∼3 K colder than model predictions colocated with the Valhalla impact basin and a complex of craters in the southern hemisphere; this indicates the presence of materials possessing either a higher thermal inertia or a lower emissivity. A warm region confined to the midlatitudes in these leading hemisphere data may be indicative of regolith property changes due to exogenic sculpting.

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Section: Valhalla Impact Basinsupporting

confidence: 75%

Thermal Properties of the Leading Hemisphere of Callisto Inferred from ALMA Observations

Camarca¹,

Kleer²,

Butler

et al. 2023

Planet. Sci. J.

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“…The same effect has also been seen on the saturnian satellites from a combination of infrared and centimeter-wavelength Cassini observations (e.g. Le Gall et al 2014;Bonnefoy et al 2020), although the thermal inertia is higher on the jovian than the saturnian satellites for a given frequency band.…”

Section: Thermal Inertia and Porositysupporting

confidence: 63%

“…Craters with anomalously low brightness temperature are commonly observed on other icy satellites, including Pwyll crater on Europa (Trumbo et al 2017); several craters on Titan (Janssen et al 2016); and Inktomi on Rhea (Bonnefoy et al 2020). The effect has been attributed to high thermal inertia in the case of Europa, and in the case of Rhea either reduced emissivity due to larger penetration depth (Bonnefoy et al 2020) or higher thermal inertia (Howett et al 2014).…”

Section: Bright Impact Cratersmentioning

confidence: 99%

Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission

Kleer¹,

Butler²,

Pater³

et al. 2021

Preprint

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We present thermal observations of Ganymede from the Atacama Large Millimeter Array (ALMA) in 2016-2019 at a spatial resolution of 300-900 km (0.1-0.2" angular resolution) and frequencies of 97.5, 233, and 343.5 GHz (wavelengths of 3, 1.3, and 0.87 mm); the observations collectively covered all Ganymede longitudes. We determine the global thermophysical properties using a thermal model that considers subsurface emission and depth-and temperature-dependent thermophysical and dielectric properties, in combination with a retrieval algorithm. The data are sensitive to emission from the upper ∼0.5 meter of the surface, and we find a millimeter emissivity of 0.75-0.78 and (sub)surface porosities of 10-40%, corresponding to effective thermal inertias of 400-800 J m −2 K −1 s −1/2 . Combined with past infrared results, as well as modeling presented here of a previously-unpublished Galileo PPR nighttime infrared observation, the multi-wavelength constraints are consistent with a compaction profile whereby the porosity drops from ∼85% at the surface to 10 +30 −10 % at depth over a compaction length scale of tens of cm. We present maps of temperature residuals from the best-fit global models which indicate localized variations in thermal surface properties at some (but not all) dark terrains and at impact craters, which appear 5-8 K colder than the model. Equatorial regions are warmer than predicted by the model, in particular near the centers of the leading and trailing hemispheres, while the mid-latitudes (∼30-60 • ) are generally colder than predicted; these trends are suggestive of an exogenic origin.

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“…Thus, the low dielectric constants derived in this manner expose the depolarizing properties of Titan's surface. The method used here is analogous to the one applied to Cassini passive microwave radiometry observations of Titan (Janssen et al 2009(Janssen et al , 2016 and Rhea (Bonnefoy et al 2020). Janssen et al (2016) derived a global map of Titan's effective dielectric constant by comparing the passive thermal microwave emission at two orthogonal polarization angles (measured by rotating the spacecraft 90°in between two scans of Titan) to the brightness temperatures predicted by the White & Cogdell (1973) model.…”

Section: Effective Dielectric Constants From the Quasi-specular And D...mentioning

confidence: 99%

Composition, Roughness, and Topography from Radar Backscatter at Selk Crater, the Dragonfly Landing Site

et al. 2022

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The Selk crater region is the future landing site of NASA’s Dragonfly mission to Titan. The region was imaged by the Cassini RADAR at incidence angles from 5° to 72° and at various polarization angles. Using this data set, we mapped six terrain units and assembled a backscatter curve for each, providing normalized backscatter cross section (σ 0) as a function of incidence angle. By fitting these backscatter curves with a sum of a quasi-specular and diffuse terms and evaluating three alternative formulations of the first and two for the second, we extracted the best-fit surface effective dielectric constant, rms slope, and scattering albedo. Although the parameters’ absolute values are model dependent, relative values between terrains indicate real variations in surface properties. The results are consistent with the impact exposing and fracturing a low-loss tangent material such as the water-ice bedrock, which is likely also present in the hummocky terrains and to a lesser degree in the plains and interdune regions. The dunes and dark terrains are composed of smooth, uniform material with low dielectric constant (1.5–2.3 median values for all models) compatible with organic sand. A diffuse single-scattering model enabled independent derivation of the dielectric constant from high-incidence observations, leading to low values (<2) over all terrains, indicating a depolarizing (sub)surface. Finally, radarclinometry revealed lateral variations in rim height, which remains below 300 m along the SARTopo profile but reaches up to 600 m at other locations, hinting at a rim less eroded than previously thought.

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

Cited by 11 publications

References 55 publications

Thermal Properties of the Leading Hemisphere of Callisto Inferred from ALMA Observations

Thermal Properties of the Leading Hemisphere of Callisto Inferred from ALMA Observations

Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission

Composition, Roughness, and Topography from Radar Backscatter at Selk Crater, the Dragonfly Landing Site

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