A detailed study of the linear polarization of Callisto in the UBVR filters has been carried out using the results of the author's observations and all the data available. The variations of polarization with phase angle, longitude, and wavelength have been investigated. Systematic shifts of different sets of observations have been detected, and all data have been consequently reduced to a unified system by introducing a correction to the degree of polarization.
The separation of the phase-angle and orbital curves of polarization is of special interest, because the observed polarization depends on the solar phase angle as well as on the longitude of the central meridian. The amplitude of longitudinal variations of polarization depends also on the phase angle. A method of separating the solar and orbital curves of polarization has been proposed, and their analytical description is given. The major sources of uncertainty in the problem of extracting the phase-angle dependence of polarization and the longitude one from the Callisto observations have been analyzed. The results obtained show that the satellite surface is polarimetrically heterogeneous. There is a cluster of seven data points that have very small values of polarization for the leading hemisphere. In our opinion, the Valhalla ring system, which has a high-albedo palimpsest in the center (β600 km diameter), may cause this particularly low polarization.