Fresh eyes on an old moon: New ALMA images of Callisto

The surface of Callisto is one of the best records of long-term impact damage our Solar System has to offer. For the past 4.5 Gyr, Callisto has seemingly done little more than collect and degrade impact features ranging from small simple craters to enormous multi-ring impact basins (e.g, Valhalla). At present, the surface properties of the other Galilean moons have been probed at IR and radio wavelengths, but Callisto remains relatively understudied. Here, we will present new 345 GHz images of Callisto obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). These images are part of a suite of observations we acquired to make 1) the first global thermal map of Callisto, and 2) the highest-resolution millimeter maps of its surface to date. Importantly, ALMA observations at 100-345 GHz can probe the physical properties of a surface (e.g, thermal inertia, emissivity) at depths of ~0.6-6 cm, bracketing the depths of diurnal temperature variations and sensing below the most heavily processed near-surface layers. Understanding the distribution of such physical properties across a planetary surface can illuminate both the interior and evolution of a planetary body. To interpret these images, we use a thermal model (de Kleer+ 2021) constrained by spacecraft data to model our observations. From these model images, we will provide global surface properties and will discuss both thermal anomalies that might correspond to known geologic features (e.g., large impact basins) and systematic patterns of thermal anomalies unaccounted for by the model.