  For terrestrial exoplanets with thin atmospheres or no atmospheres, the
surface contributes light to the reflected light signal of the planet.
Measurement of the variety of disk-integrated brightnesses of bodies in the
Solar System and the variation with illumination and wavelength is essential
for both planning imaging observations of directly imaged exoplanets and
interpreting the eventual datasets. Here we measure the change in brightness of
the Galilean satellites as a function of planetocentric longitude, illumination
phase angle, and wavelength. The data span a range of wavelengths from
400-950nm and predominantly phase angles from 0-25 degrees, with some
constraining observations near 60-140 degrees. Despite the similarity in size
and density between the moons, surface inhomogeneities result in significant
changes in the disk-integrated reflectivity with planetocentric longitude and
phase angle. We find that these changes are sufficient to determine the
rotational periods of the moon. We also find that at low phase angles the
surface can produce reflectivity variations of 8-36% and the limited high phase
angle observations suggest variations will have proportionally larger
amplitudes at higher phase angles. Additionally, all the Galilean satellites
are darker than predicted by an idealized Lambertian model at the phases most
likely to be observed by direct-imaging missions. If Earth-size exoplanets have
surfaces similar to that of the Galilean moons, we find that future direct
imaging missions will need to achieve precisions of less than 0.1\,ppb. Should
the necessary precision be achieved, future exoplanet observations could
exploit similar observation schemes to deduce surface variations, determine
rotation periods, and potentially infer surface composition.
