Revealing Callisto's Near Subsurface Thermophysical Properties with ALMA Calibration Data

TL;DR

The paper investigates Callisto's near-subsurface thermophysical properties by combining ALMA submillimeter observations with Galileo PPR data. It applies a two-component thermophysical mixture model to derive depth-dependent emissivity and inertia maps, demonstrating how near-surface thermal emission constrains regolith properties. The results yield a disk-averaged brightness temperature of about 133±15 K and inertia ranges for the two components of roughly 15–40 and 1200–2000 J m^-2 K^-1 s^-1/2, aligning with longer-wavelength ALMA findings. By leveraging ALMA flux-calibrator data, the work extends frequency coverage and enhances spatial understanding of Callisto's shallow subsurface, illustrating the broader value of calibrator datasets for planetary science.

Abstract

Thermal images at different wavelengths probe varying subsurface depths of planetary bodies, and therefore can inform us about their compositions, thermophysical properties, and impact histories. We identified six archival observations of Callisto obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between 2012 July 17 and 2012 November 4 at wavelengths of 0.43-0.47 mm (701.9-641.5 GHz). These wavelengths are shorter than those of nearly all other Callisto ALMA data and are sensitive to subsurface emission at depths (the upper ~cm) between those sounded by millimeter and infrared observations. We estimate the disk-averaged brightness temperature as 133$\pm$15 K, and use a thermophysical mixture model to find that Callisto's thermal emission is best fit by a ~50-50% two-component thermal inertia mixture of $Γ_{\text{low}}$~15-40 and $Γ_{\text{high}}$~1200-2000 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$, consistent with recent ALMA observations of Callisto at longer wavelengths. Finally, we present several previously unpublished Galileo Photopolarimeter-Radiometer (PPR) observations of Callisto and derive thermal inertia and spectral emissivity maps using the same model. Altogether, these ALMA and PPR maps improve our understanding of the thermal properties and spatial distribution of Callisto's shallow subsurface regolith, and demonstrate the value of ALMA flux density calibrator data for extending frequency coverage of existing science data.