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

TL;DR

This study utilizes ALMA and PPR data to analyze Callisto's subsurface thermal properties, revealing a complex mixture of thermal inertias and improving understanding of its regolith's spatial distribution.

Contribution

The paper introduces a novel analysis of Callisto's subsurface thermophysical properties using high-frequency ALMA data and PPR observations, enhancing existing models.

Findings

Callisto's thermal emission best fits a mixed thermal inertia model.

Derived thermal inertia values range from 15 to 2000 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$.

ALMA data extends frequency coverage for planetary thermal studies.

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 $\Gamma_{\text{low}}$~15-40 and $\Gamma_{\text{high}}$~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 Finally, we present several previously unpublished Galileo Photopolarimeter-Radiometer (PPR) observations of Callisto and derive thermal inertia and spectral emissivity maps using the same model.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.