New Polarimetric Data for the Galilean Satellites: Io and Ganymede Observations and Modeling

New high-precision disk-integrated measurements of the polarization of Io and Ganymede in the UBVRI bands are presented. The observations were obtained using polarimeters mounted on the Crimean Astrophysical Observatory and the Peak Terskol Observatory in 2019-2023. For Io, the negative polarization branch (NPB) reaches a minimum of P min approximate to -0.25 +/- 0.02% in the V band at a phase angle of alpha min = 2.degrees 1 +/- 0.degrees 5. The inversion angle is alpha inv = 26 degrees +/- 6 degrees in the V and R bands. The NPB for Ganymede is an asymmetric curve, with P min = -0.34 +/- 0.01% at alpha min = 0.degrees 52 +/- 0.degrees 06 and alpha inv = 8.degrees 5 +/- 0.degrees 2 in the V band. Although Io and Europa have similar geometric albedos (0.63 and 0.67, respectively), their NPB shapes differ. The NPB of Ganymede (albedo of 0.43) is morphologically similar to that of Europa, although it is described by different parameter values (P min, alpha min, and alpha inv). This discrepancy is likely due to the compositions of their surfaces: Europa's with H2O ice, Io's with sulfuric/silicate composition, and Ganymede's with H2O ice and silicates. Numerical computations using the radiative transfer coherent backscattering method demonstrated a match to the polarimetric observations and to the geometric albedos for Ganymede with the single-scattering albedo approximate to 0.943 and mean free path length kl = 2 pi l/lambda eff approximate to 150, where lambda eff is the wavelength. For Io's regolith, the single-scattering albedo was found to be approximate to 0.979 and kl approximate to 40.