Experimental simulations of optically thin clouds of dust by imaging polarimetry: application to comets and interplanetary dust.

We use remote light scattering observations to tentatively infer the physical properties of the particles in cometary and interplanetary dust clouds. To interpret the results, numerical and experimental models are necessary. Laboratory scattering measurements on levitating particles with the PROGRA2 experiment -in dedicated microgravity flights or on ground for low-density particles- offer an alternative to simulate the scattering properties of real particles, which can present large size distributions and a large variety of structures and materials. Previous systematic experiments, numerical models and laboratory analysis of cosmic particles (e.g. Stardust samples) are used to optimize samples' properties -such as particles structures, sizes, and silicate to organics ratios-. We present cometary observations showing the evolution of the structures and of the polarization in the internal regions of the coma, linked to the evolution of particles properties. Associated experimental simulations help us to interpret how particles evolve within different coma regions and at different solar distances. Observations of the light scattered by the interplanetary dust cloud (IDC) particles have shown polarisation changes with the solar distance. Such changes are interpreted using numerical models to be related to variations in the composition and physical properties of the particles through various processes including thermal degradation. This interpretation is now validated on real mixtures of particles corresponding to the composition of the IDC at different solar distances