Laboratory measurements of light scattered by clouds and layers of solid particles using an imaging technique

Abstract

The Optical PRoperties of Astronomical and Atmospheric Grains (PROGRA2) experiment was developed in 1993 to study the light scattered by dust clouds, with an emphasis on its linear polarization. The instrument was progressively improved. A short description of the PROGRA2-vis instrument operating in the visible is given in this chapter. In the framework of the experiment, solid particles are lifted using two different means: either by reduced gravity conditions during parabolic flight campaigns, or by a nitrogen draught in a groundbased laboratory setting. The second instrument, PROGRA2-surf, is used to compare the measurements, obtained with the PROGRA2-vis, of the light scattered by the same grains but deposited in layers on the ground. The latter configuration allows one to study planetary regolith analogs. Various samples studied by each technique are presented and discussed. They are characterized by different types of constituent particles, including compact ones, either regularly or irregularly shaped, as well as aggregates of micrometer- and submicrometer-sized grains. We perform polarimetric comparisons between particle clouds and deposited layers and identify certain light-scattering tendencies versus particle size and absorption. Numerical simulations of the light-scattering processes allow us to cross-validate the experimental and numerical procedures. We compare our laboratory results to those obtained by remote or in situ observations in order to provide a better understanding of the physical properties of solid particles in the Earth's atmosphere and in the solar system (e.g., cometary comae). Examples of such applications are also given. © Springer Science+Business Media B.V. 2011.