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

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.