Europa Clipper Preparatory Photometry to Constrain Surface Properties

Abstract We infer the surface reflectance properties of Europa using multispectral data sets available from previous missions. We use 21 full-disk images of Europa from Voyager’s Imaging Science System, Galileo’s Solid State Imaging, and New Horizons’ Long Range Reconnaissance Imager at differing observation geometries (10°–128° (phase angle)) to compute disk-integrated surface-scattering properties over various geologic units. The derived empirical photometric models will serve the practical goals of data acquisition, aid in the calculation of instrument integration times, and facilitate quick-look data products for images acquired by Europa Clipper and other future missions to Europa. We use the Minneart and Lommel–Seeliger plus Lambert reflectance models to constrain the photometric parameters of Europa’s surface. We find that the surface albedo parameter, B 0, in the Minneart function gradually decreases with increasing phase angles. We also note that the photometric properties of Europa (geometric albedo at 0.47 μm is 0.72 on the leading side and 0.62 on the trailing side) require a significant “Lambert” term (A < 1) in the Lommel–Seeliger plus Lambert reflectance model. We also observe that the photometric parameters are not highly dependent on the geologic terrain type despite their visibly different albedos.