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For terrestrial exoplanets with thin atmospheres or no atmospheres, thesurface contributes light to the reflected light signal of the planet.Measurement of the variety of disk-integrated brightnesses of bodies in theSolar System and the variation with illumination and wavelength is essentialfor both planning imaging observations of directly imaged exoplanets andinterpreting the eventual datasets. Here we measure the change in brightness ofthe Galilean satellites as a function of planetocentric longitude, illuminationphase angle, and wavelength. The data span a range of wavelengths from400-950nm and predominantly phase angles from 0-25 degrees, with someconstraining observations near 60-140 degrees. Despite the similarity in sizeand density between the moons, surface inhomogeneities result in significantchanges in the disk-integrated reflectivity with planetocentric longitude andphase angle. We find that these changes are sufficient to determine therotational periods of the moon. We also find that at low phase angles thesurface can produce reflectivity variations of 8-36% and the limited high phaseangle observations suggest variations will have proportionally largeramplitudes at higher phase angles. Additionally, all the Galilean satellitesare darker than predicted by an idealized Lambertian model at the phases mostlikely to be observed by direct-imaging missions. If Earth-size exoplanets havesurfaces similar to that of the Galilean moons, we find that future directimaging missions will need to achieve precisions of less than 0.1\,ppb. Shouldthe necessary precision be achieved, future exoplanet observations couldexploit similar observation schemes to deduce surface variations, determinerotation periods, and potentially infer surface composition.

Reflected Light Observations of the Galilean Satellites from Cassini: a testbed for cold terrestrial exoplanets