Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission

Abstract

We present thermal observations of Ganymede from the Atacama Large Millimeter Array (ALMA) in 2016–2019 at a spatial resolution of 300–900 km (0."1–0."2 angular resolution) and frequencies of 97.5, 233, and 343.5 GHz (wavelengths of 3, 1.3, and 0.87 mm); the observations collectively covered all Ganymede longitudes. We determine the global thermophysical properties using a thermal model that considers subsurface emission and depth- and temperature-dependent thermophysical and dielectric properties, in combination with a retrieval algorithm. The data are sensitive to emission from the upper ~0.5 m of the surface, and we find a millimeter emissivity of 0.75–0.78 and (sub)surface porosities of 10%–40%, corresponding to effective thermal inertias of 400–800 J m⁻² K⁻¹ s^(−1/2). Combined with past infrared results, as well as modeling presented here of a previously unpublished night-time infrared observation from Galileo's photopolarimeter–radiometer instrument, the multiwavelength constraints are consistent with a compaction profile whereby the porosity drops from ~85% at the surface to 10⁺³⁰₋₁₀% at depth over a compaction length scale of tens of centimeters. We present maps of temperature residuals from the best-fit global models, which indicate localized variations in thermal surface properties at some (but not all) dark terrains and at impact craters, which appear 5–8 K colder than the model. Equatorial regions are warmer than predicted by the model, in particular near the centers of the leading and trailing hemispheres, while the midlatitudes (~30°–60°) are generally colder than predicted; these trends are suggestive of an exogenic origin.

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