Cassini Phase Curves of the Galilean Satellites and Implications for Direct-Imaging of Icy Exoplanets

Direct observation of the disk-integrated brightness of bodies in the Solar System, and the variation with illumination and wavelength, is essential for both planning imaging observations of exoplanets and interpreting the eventual datasets. In my previous work, I used the Imaging Science Subsystem cameras aboard Cassini to determine the disk-integrated and wavelength-dependent variations of Jupiter, which will serve to inform observations of gas-giant exoplanets. Here, I present the derived phase variations of the four Galilean satellites, which may be useful proxies for icy exoplanets with little or no atmosphere. The data span a range of wavelengths from 400 - 950 nm and predominantly phase angles from 0 - 20 degrees with some constraining observations near 120 degrees. For Europa and Io, the variations on the surface result in significant changes in the disk-integrated reflectivity with planetocentric longitude. This implies that future exoplanet observations could, in principle, exploit this effect both to deduce surface variations and to determine the period of rotation.

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