POLARIZATION PROPERTIES OF THE GALILEAN SATELLITES OF JUPITER - OBSERVATIONS AND PRELIMINARY-ANALYSIS

We present new, detailed polarimetric measurements of the Galilean sat ellites of Jupiter with U, B, V, and R filters at phase angles ranging from 12 degrees to nearly 0 degrees. The polarization phase curves of Io, Europa, and Ganymede in the B, V, and R filters clearly show the presence of the polarization opposition effect in the form of a sharp peak of negative polarization centered at a very small phase angle of 0.degrees 6-0.degrees 7 and superimposed on the regular negative polar ization branch. This phase angle is comparable to the width of the spi kelike photometric opposition effect observed for Europa, thus indicat ing that both opposition phenomena are likely to be produced by the co herent backscattering mechanism. The U filter values of \P-min\ for Io and Europa are close to 0.60% and 0.47%, respectively, and exceed the respective BVR values by a factor of almost 2. The BVR polarization f or the trailing hemispheres of Io, Europa, and, especially, Ganymede i s systematically stronger than for the respective leading hemispheres. For Callisto, the leading hemisphere polarization is significantly st ronger than for the trailing hemisphere. The inversion angles for Io, Europa, and Ganymede are nearly wavelength independent and close to 10 .degrees 0, 8.degrees 6, and 8.degrees 8, respectively. The inversion angle for the trailing hemisphere of Callisto is also wavelength indep endent and is in the range of 12 degrees-13 degrees.