JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

The Galileo Photopolarimeter Radiometer (PPR) measured the first set of complete phase curves (polarization vs. phase angle) for the Galilean satellites, including the first such data for any high-albedo icy body. Unique simultaneous photometry and polarimetry at three wavelengths from 410 to 945 nm, together with unprecedented mapping of Ganymede and Callisto offers a powerful data set for investigating the surface texture of these diverse bodies in a manner orthogonal to that provided by imaging and spectroscopy. Measurement of the Moon during the Galileo Earth encounters verifies the behavior of the PPR polarimetry. Modelling of the polarization curves will proceed using code by Mishchenko that allows realistic particle shapes, as well as a range of sizes and refractive indices. Such modelling has shown promise in reproducing atmospheric polarization behavior for the Earth. Goguen established that techniques applicable to atmospheres are applicable for high-albedo surfaces. Analysis of the PPR data for the Galilean satellites will lead to improved knowledge of the fine-scale surfaces of those bodies. Correlation of mapped intensity/polarization behavior with existing maps, imaging, and NIMS data will define local variations in the style of surface alteration. By providing new information about the surfaces of the Galilean satellites, this work will: 1. Strengthen the set of clues available about the character of the surfaces, and the mechanisms that produce them. 2. Stimulate foci for future NASA investigations by pointing out regions of anomalous behavior. 3. Assist in future NASA robotic exploration by providing information of engineering value, relevant to thermal, composition, and particle size environments. We will fully calibrate the data set and derive final geometric parameters, using the best available information, and archive with the PDS high-level products such as phase curves and mapped images . Recent relevant publications include: "A quantitative test of the applicability of independent scattering to high albedo planetary regoliths", Goguen, J. D., Icarus (submitted, 1998; see Appendix). "Observing the Icy Jovian Satellites with the Galileo Photopolarimeter Radiometer Instrument", L.K. Tamppari, J.R. Spencer, T.Z. Martin, JGR Planets, 100, p. 18973 (1995). "Galileo Photopolarimeter Radiometer Observations of Jupiter and the Galilean Satellites", G.S. Orton, J. Spencer, L. Travis, T.Z. Martin, L. Tamppari, Science, 274, p. 389 (1996). "Observation of Shoemaker-Levy Impacts by the Galileo Photopolarimeter Radiometer", T.Z. Martin, G.S. Orton, L. Tamppari, L. Travis, and I. Claypool, Science, 268, p. 1875 (1995).