Lunar dust characterization by polarimetric signature

Context. In support of NASA's exploration program and the return to the Moon, the polarimetric signature of dispersed individual Lunar regolith dust grains is studied to enable the characterization of the dust exopsheric environment by remote, in-situ, and standoff sensing. Aims. We explore the value of the negative polarization branch (NPB) as a signature for characterizing individual grains to determine if it can be used in the same way as for surfaces of planets and atmosphereless bodies. Methods. The linear polarization phase curve for single spheres of silicate and for aggregates of spherical silicate grains of different porosity are computed using the discrete dipole approximation (DDA) for a range of grain sizes. Features in these curves are identified and their evolution explored as a function of grain size and aggregate porosity. We focus particularly on the so-called negative polarization branch that has been historically used to characterize planetary surfaces. Results. Calculations show that polarization phase curves for spherical grains exhibit a sharp transition over a narrow range of size parameter between two distinct regimes, one typical of Rayleigh scattering and another dominated by a large NPB. The linear polarimetric signature observed for aggregates is a composite of a) the polarization induced by individual grains composing the aggregate and b) the polarization due to the aggregate as a whole dust grain. The weight of each component varies depending on the porosity of the aggregate. An NPB similar to the one observed for atmosphereless astronomical bodies is present for different ranges of the size parameter depending on the value of the porosity. It appears as a remnant of the negative branch exhibited by the single spherical grains. The sharper, narrow negative branch that is measured for some granular surfaces in the laboratory or seen in astronomical observations is not observed here. Conclusions. These results suggest that the wide negative branch is due to the scattering by individual grains and single aggregates, while the narrow negative branch is more likely due to coherent backscattering or shadowing effects in bulk material. The shape and evolution of the NPB could be used to characterize spherical grains and to differentiate between aggregates with the same porosity but different sizes, but does not appear to be a practical candidate for univocally differentiating between aggregates of different porosity