(199d) Properties of Lunar and Other Regoliths, from Comparisons between Light Scattering Observations and Numerical or Experimental Simulations
AIChE Spring Meeting and Global Congress on Process Safety
2006
2006 Spring Meeting & 2nd Global Congress on Process Safety
Fifth World Congress on Particle Technology
Extrapolatory Scaling in Particle Processing
Wednesday, April 26, 2006 - 2:00pm to 2:20pm
Extraterrestrial regoliths are relatively inaccessible media formed by successive impacts on the surface on atmosphere-less solar system bodies, such as the Moon, the asteroids, and possibly comets nuclei and Kuiper Belt Objects. Understanding their physical properties (size, size distribution, shape and porosity of the particles) is mandatory to assess the main processes that have shaped their evolution. As far as the Lunar regolith is concerned, a precise study of the changes of physical properties with different terrains is also required to prepare efficiently future human exploration.
Unique information about physical properties is provided by remote light scattering observations in the visible domain, together with thermal emission observations, and mapping. Solar light scattered by regoliths is actually partially linearly polarized, with the linear polarization only depending upon the physical properties of the scattering particles and upon the geometry and wavelength of the observations. Lunar (as well as Martian and asteroidal) polarization phase curves are typical of irregular particles with a size greater than the wavelength; the value of the maximum in polarization usually increases with decreasing values of the albedo. Some authors [see e.g. 1] have already used spatially resolved observations to point out some anomalies and to correlate them to specific terrains.
We are developing numerical and experimental simulations to better infer the properties of such media. As far as numerical simulations are concerned, we are systematically studying the light scattering properties (brightness and linear polarization) of compact mono-disperse and core-mantle grains, and of aggregates thereof. Their dependence upon the size of the grains, the size of the aggregates, the size distribution, the complex refractive index (related to the composition), the shape and the porosity are derived [2]. Monte-Carlo techniques have been developed by an other group [3] to take into account multiple scattering for spherical mono-disperse particles; the results are to be compared with the measurements we perform [4] on large porous aggregates of silica spheres.
Experimental simulations indeed provide a complementary approach: instead of requiring computations of multiple scattering by irregular particles, they allow studies of the differences between deposited particles (with multiple interactions) and levitating particles. Since 1994, we have been measuring such properties with the PROGRA2 instrument, for about 150 samples including meteoritic dust and NASA JSC Lunar and Martian soil analogues [5-7]. The weak negative branch characteristics depend upon the material and the packing density of the particles [8,9]. Besides, the maximum in polarization increases with the average size for compact particles larger than the wavelength (up to a level that depends upon the characteristics of the sample), and the dependence on the maximum in polarization versus the albedo follows, for light material, a trend opposite the one found for absorbing material.
Finally, we will present future experimental projects, with the PROGRA2 experiment in reduced gravity (on samples of interest for Lunar exploration), and with the ESA ICAPS/IMPACT multi-user research laboratory on board the ISS. The latter should allow us to analyze, through an innovative Light Scattering Unit [8], the optical properties of samples of dust aggregates and regolitic-type fragments.
Acknowledgements. This work is supported by CNES and ESA.
References [1] Shkuratov, Y., Ovcharenko, A., Zubko, E., Miloslavskaya, O., Muinonen, K., Piironen, J., Nelson, R., Smythe, W., Rosenbush, V., Helfenstein, P., Icarus 99, 468, 1992 [2] Lasue J. and Levasseur-Regourd, A.C., J. Quant. Spectros. Radiat. Transfer (submitted), 2005 [3] Muinonen, K., Waves in random media, 14, 365, 2004 [4] Hadamcik, E., Renard, J.B., Levasseur-Regourd, A.C., Worms, J.C., Adv. Space Res. In press, 2005 [5] Worms, J.C., Renard, J.B., Hadamcik, E., Brun-Huret, N., Levasseur-Regourd, A.C., Planet. Space Sci., 48, 493, 2000 [6] Worms, J.C., Renard, J.B., Hadamcik, E., Levasseur-Regourd, A.C., Gayet, J.F., Icarus 142, 281, 1999 [7] Renard, J.B, Worms, J.C., Lemaire, T., Hadamcik, E., Brun-Huret, N., Appl. Opt., 41, 609, 2002 [8] Shkuratov, Y., Ovcharenko, A., Zubko, E., Miloslavskaya, O., Muinonen, K., Piironen, J., Nelson, R., Smythe, W., Rosenbush, V., Helfenstein, P., Icarus 159, 396-416, 2002 [9] Hadamcik, E., Renard, J.B., Levasseur-Regourd, J.C., Worms, J.C., J. Quant. Spectros. Transfer. 79, 679, 2003 [10] Levasseur-Regourd, A.C., Adv. Space Res. 31, 2599, 2003