The laboratory PROGRA2 database to interpret the linear polarization and brightness phase curves of light scattered by solid particles in clouds and layers

Abstract

Laboratory measurements of the brightness and linear polarization of the light scattered by clouds of particles are necessary to interpret remote sensing measurements of dust in space and in planetary atmospheres. For 30 years, such laboratory measurements were conducted by the PROGRA2 project for particles with different natures, sizes and shapes. The resulting database contains the brightness and linear polarization curves for 170 samples and at 4 wavelengths in the visible and near infra-red domains. The samples cover most of the particles that we expect to find in space and in planetary atmospheres. The particles can be compact, porous or aggregated, with sizes up to hundreds of µm; the monomers of aggregates can have sizes as small as 10 nm. The measurements were obtained for clouds of randomly oriented particles levitated on the ground conditions by an air draught technique, and under microgravity conditions during parabolic flights. Complementary measurements were obtained for deposited particles in layers to compare the main characteristics of the phase curves using the same sample. After a description of the instruments and of the samples under study, we present the database and the main results already obtained with PROGRA2. The phase curves often differ for deposited and levitating particles. Such phase curves are related to different physical properties of the particles, and can be compared to modelling calculations. In case of agregated particles, we discuss the influence of the monomers on the phase curves. Finally, the main results obtained for dust analogs of differents object of the solar system are compared to observations.