Polarimetry of solar system bodies

International audienceSolar light scattered on surfaces or within clouds of solar system bodies is partially linearly polarized, as already emphasized in the 1970s by the interpretation of the observed polarization of Venus through the presence of sulfuric acid droplets within its clouds. This talk is actually intended to summarize our present understanding of the polarization induced by solar light scattering on irregular dust particles. Such particles are found on regolitic surfaces formed by impacts over billion of years on some solar system bodies (planets, moons, asteroids, cometary nuclei, trans- Neptunian objects) or within clouds (atmospheric dust clouds, cometary comae and tails, interplanetary dust cloud). Polarimetric observations of dust in the solar system may be technically challenging, with some signals needing to be isolated from any contaminating source on the line-of-sight, or being limited to small phase angles, or corresponding to different phase angles along the line-of-sight. They are nevertheless of major interest, since they enable comparisons between different regions of a given object (e.g., lunar or Martian surface, cometary coma) and between different classes of objects of a given category (e.g. asteroids, comets, trans-Neptunian objects). Numerous remote Earth-based observations, as well as a few in situ observations, lead to typical polarimetric phase curves with a shallow negative branch in the backscattering region (with possibly an opposition effect) and a wide positive branch with a maximum in the 80°-110° region. The variation of the polarization with the wavelength, for a fixed phase angle, may also be monitored for some categories of objects. Recent results will be summarized, with emphasis on polarimetry of small solar system bodies (comets, asteroids, interplanetary dust cloud), and specifically on changes likely to be triggered by the gradual evolution of dust particles. Interpretation of the data provides information on the geometric albedo, as well as on some physical properties of the scattering medium (i.e., size distribution, morphology and porosity, complex refractive indices). It mostly stems from elaborate light-scattering numerical and experimental simulations, and of comparisons between their results. Recent progresses related to such approaches will tentatively be presented. Finally, promising future observations and simulations will be mentioned