**Properties of Extraterrestrial Regoliths from Light Scattering
Observations**

A.Ch. Levasseur-Regourd, E. Hadamcik, J. Lasue

Aeronomie CNRS-IPSL, Univ. P. & M. Curie (Paris VI), France

Accurate estimations of the physical properties of regoliths found on
different regions of the Moon and Mars are required to prepare future
human exploration. Unique information about some physical properties
(e.g. albedo, texture and size distribution) may be provided by in-situ
and remote light scattering observations in the visible domain. Solar
light scattered by such irregular media 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 and Martian (as well as 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. Spatially resolved observations have
allowed Shkuratov et al. (1992) to point out some anomalies and to
correlate them to specific terrains.

We have been using models of fractal aggregates to compute the light
scattering properties of aggregates of spheroidal (monodisperse or
core-mantle) particles. Results will be discussed as a function of the
size and the porosity. Monte-Carlo techniques have been developed by
Muinonen (2004) to take into account multiple scattering for spherical
monodisperse particles. They are being compared with laboratory
measurements on large porous aggregates of silica spheres.

Experimental simulations indeed provide a complementary approach, since
they avoid computations of multiple scattering by irregular particles,
as well as various assumptions. We will present typical results we
obtained on various samples, including NASA JSC Lunar and Martian soil
analogues, with the PROGRA^2^ instrument (Worms et al., 1999; Hadamcik
et al., 2003). The weak near-opposition negative branch characteristics
depend upon the material and the packing density of the particles, in
agreement with Shkuratov et al. results (2002)**.** The maximum in
polarization increases with the average size of compact particles, up to
a level that depends upon the refracted light absorption, i.e. upon the
characteristics of the sample. The maximum in polarization decreases
with the albedo for comparable particles, the decrease being stronger
for compact than for fluffy particles. The trend is the same for porous
layers than for packed ones, although the polarization may then be
lower.

Finally, we will present the possibilities offered by the ESA ICAPS
multi-user research laboratory to analyze, through an innovative Light
Scattering Unit (Levasseur-Regourd, 2003), the optical properties of
samples of dust aggregates and regolitic type fragments.

**Acknowledgements**. This work is supported by CNES and ESA.

**References.** Hadamcik et al., J. Quant. Spectros. Transfer. 79, 679,
2003; Levasseur-Regourd, Adv. Space Res. 31, 2599, 2003; Muinonen, Waves
in random media (in press), 2004; Shkuratov et al., Icarus 99, 468,
1992; Shkuratov et al., Icarus 159, 396-416, 2002*;* Worms et al.,
Icarus 142, 281, 1999.