Photopolarimetric sensing of planetary surfaces

Polarization measurements over the surfaces of the Moon, Mercury, Mars and Saturn's rings, and global data for the Galilean satellites, have been recorded with telescopes in France. A number of asteroids were measured by B. Zellner in USA. The curves of polarization are diagnostic of the micro-texture of the surface, and demonstrate that all the atmosphereless Solar System objects so far observed (except Callisto trailing hemisphere) have their surfaces covered with a regolith of fines, as for the Moon, which is produced by the cumulative effect of meteoroid impacts. For all the silicaceous objects down to a diameter of 700 km, namely Mars, Mercury, the Moon, Callisto (for the apex hemisphere), the mean grain sizes are no larger than 20um. The asteroids have coarser grained regoliths, apparently because of their smaller gravitational escape velocities. The C type asteroid surfaces, assumed to be carbon rich, appear finer grained than the silicaceous S types. The M astereroids assumed to be metallic, are also covered with small fragments, becuase metals loose their ductile properties at low temperature and behave at impact like brittle silicates. The trailing hemisphere of Callisto has a texture almost reminiscent of bare rocks. Orbital considerations to excluse significant impact effects, and a scenario for the past evolution of the satellite are implied.The planet Mars, with wind effects due to a tenuous atmosphere, several intense past volcanic episodes, a high tectonic activity and a permafrost underground has a more diversified surface regolith. A detailed analysis was achieved with photopolarimeters placed on board the soviet Mars Orbiter Spacecraft MARS-5.The Saturn's rings, anisotropic multiple scattering effects are observed and exhibit variations often in few days or weeks. Mutual interactions and gravitational forces are at work to produce organized structures, whereas disorganization forces occur and the competition produced ephemeral situations.