Abstract : The light scattered by particles present in cometary comæ can be used to provide an estimate of the size distribution, structure and composition of the particles.
Experimental simulations I participated in the definition and calibration of the ICAPS-PE (LSU) experiment selected by ESA for the ISS. It will produce fractal aggregates and analyze the light they scatter at several wavelengths and phase angles. I validated the concept of the instrument and developed a new concept to measure the Stokes parameters with liquid crystal variable retarders. I analyzed the light scattering measurements performed with the PROGRA2 experiment for coated spherical particles. The results give an estimate of the width and complex optical indices of absorbing mantles.
I developed a model of cometary coma to analyze polarimetric observations in terms of physical properties of solid particles. The model consists of aggregates (up to 256 spheroidal grains) and oblate spheroidal particles. A size distribution with a power law of coefficient -3 between radii values of 0.1 and 20 microns and with around 50% in mass of organics and silicates can reproduce the phase angle and wavelength variations of the polarization of C/1995 O1 (Hale-Bopp). The same model was used to analyze the interplanetary dust cloud. A size distribution between radii of 0.1 and 100 microns can reproduce the linear polarization phase curve. The decrease of the linear polarization at 90 degrees with decreasing heliocentric distance can
be explained by the degradation of organics material from 50% to 0% in mass between 1.5 and 0.5 AU.