Abstract
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 Combined with past infrared results, as well as modeling presented here of a previously unpublished night-time infrared observation from Galileo’s photopolarimeter–radiometer instrument, the multiwavelength constraints are consistent with a compaction profile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 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.
Highlights
The largest of the Solar System’s satellites, Ganymede is a prototypical icy ocean world, hosting a liquid water ocean under a thick ice shell (Kivelson et al 2002)
The solid ice case corresponds to a thermal inertia of ∼2000, which is inconsistent with observations of any icy Solar System body at any wavelength and provides a poorer fit to our data than the porous case; the results presented here use the conductivity model for the porous medium case
Over the frequencies covered by our Atacama Large Millimeter Array (ALMA) dataset, the decrease in brightness temperature can be accounted for by the fact that lower frequencies are sensitive to emission from deeper in the surface, through the compounding effects of colder physical temperatures at depth, and of the higher thermal inertias of deeper layers resulting in lower brightness temperatures
Summary
The largest of the Solar System’s satellites, Ganymede is a prototypical icy ocean world, hosting a liquid water ocean under a thick ice shell (Kivelson et al 2002). Thermal emission, measured at infrared through radio wavelengths, is sensitive to material properties such as emissivity, thermal conductivity, and heat capacity, the latter two of which are often parameterized through the thermal inertia (Γ, units of J m−2 K−1 s−1/2 throughout) These properties are in turn determined by the composition, water ice abundance and grain size, surface roughness, and vertical compaction profile, i.e. the density/porosity as a function of depth. We conducted a campaign in 2016-2019 to image the three icy Galilean satellites with ALMA; data were obtained at three frequencies: 97.5, 233, and 343.5 GHz (observations centered at 3, 1.3, and 0.87 mm respectively) These measurements are sensitive to different depth ranges within the upper tens of cm of the surface.
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