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We present a thermal observation of Callisto's leading hemisphere obtainedusing the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.87 mm (343GHz). The angular resolution achieved for this observation was$\sim$$0.16^{\prime\prime}$, which for Callisto at the time of this observation($D\sim 1.05^{\prime\prime}$) was equivalent to $\sim$6 elements across thesurface. Our disk-integrated brightness temperature of 116 $\pm$ 5 K (8.03$\pm$ 0.40 Jy) is consistent with prior disk-integrated observations. Globalsurface properties were derived from the observation using a thermophysicalmodel (de Kleer et al. 2021) constrained by spacecraft data. We find thatmodels parameterized by two thermal inertia components more accurately fit thedata than single thermal inertia models. Our best-fit global parameters adopt alower thermal inertia of 15-50$\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$ and a higher thermalinertia component of 1200-2000$\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$, with retrievedmillimeter emissivities of 0.89-0.91. We identify several thermally anomalousregions, including spots $\sim$3 K colder than model predictions co-locatedwith the Valhalla impact basin and a complex of craters in the southernhemisphere; this indicates the presence of materials possessing either a higherthermal inertia or a lower emissivity. A warm region confined to themid-latitudes in these leading hemisphere data may be indicative of regolithproperty changes due to exogenic sculpting.

Thermal properties of the leading hemisphere of Callisto inferred from ALMA observations