Light scattering by dust particles with the PROGRA2 instrument — comparative measurements between clouds under microgravity and layers on the ground

Light scattering by solar system dust particles is a major tool to gain access to some of their physical properties. Parameters such as size and porosity/roughness can be estimated by measuring the degree of polarization of the scattered light. To address this issue experimentally, we proposed to use microgravity to study ensembles of particles and the influence of size distribution or multiple scattering on the observed polarization. PROGRA2 is a CNES-funded instrument which has participated in 10 flight campaigns since 1993 on board the “Zero G” Caravelle, NASA KC-135 and Airbus A300 aircraft, with a yield of 32 polarimetric phase curves for different documented samples of varying sizes and packing density. Significant differences between microgravity and ground measurements were obtained. Results for Boron carbide, two planetary soil simulants and two meteoritic samples are presented along with the corresponding ground-based measurements. Mechanical adjustments have recently enabled the retrieval of data points for phase angles as low as 8°, thus making possible the study of the so-called “negative branch” of the phase curve. Measurements were obtained in two wavelengths, red (632.8 nm) and green (543.5 nm).