Photoelectric photometry of Europa and Callisto 1976–1991

We have used a stable, dedicated telescope and photoelectric photometer to measure the brightnesses of Europa and Callisto near greatest elongation. The measurements, made regularly since 1976, delineate the phase functions of these objects from solar phase angles of about 12° to less than 0.5°. The phase functions of the two faces of Europa are essentially the same over this range, but Callisto's leading and trailing faces are quite different. The phase function of the leading face of Callisto is very similar to that of the Moon over the range of angles we observed. Europa exhibits a very strong opposition spike at phase angles of less than 1°. We compare these observed phase functions to those of some other atmosphereless bodies. These satellites exhibit a slight variation during the Jovian year which we attribute to the change in sub‐Earth latitude exposing alternate polar regions. If this variation and that due to rotation are adjusted for, the satellites are photometrically stable over the span of time covered by our observations.