  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 $\sim$0.5 meter 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modeling presented here of a previously-unpublished Galileo PPR nighttime
infrared observation, the multi-wavelength 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 cm. 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 mid-latitudes
(~30-60 degrees) are generally colder than predicted; these trends are
suggestive of an exogenic origin.
