**DPS Pasadena Meeting 2000, 23-27 October 2000**

*Session 59. Mars Surface and Satellites Posters*

Displayed, 1:00pm, Monday - 1:00pm, Friday, Highlighted Tuesday and Thursday, 3:30-6:30pm, C101-C105, C211
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## [59.09] On the Problem of Light Scattering by Planetary Regolith

*W.J. Markiewicz (Max-Planck-Institute for Aeronomy, Germany), E.V. Petrova (Space Science Institute, Russia and Max-Planck-Institute for Aeronomy, Germany), H.U. Keller (Max-Planck-Institute for Aeronomy, Germany)*

A number of approximate models, and mainly the Hapke
bidirectional reflection function, have been widely used to
analyze photometric observations of planetary surfaces.
Although these models are able to fit the data well with a
small number of free parameters, they do not contain
explicitly such crucial physical parameters as grain size or
refractive index. A direct comparison of the Hapke model
results with accurate numerical solutions of the radiative
transfer equation for Henyey-Greenstein phase functions has
revealed that the errors are small only for almost isotropic
scattering when the asymmetry parameter is less than 0.2.
However it is known that realistic phase functions of the
soil particles are strongly forward scattering.

Using geometric optics approximation, we have found that a
single scattering phase function of soil grains can be
successfully modeled by a shape mixture of randomly oriented
polydisperse spheroids. In order to take into account the
effect of packing density in a regolith, we used the
so-called static structure factor which depends on the
particle size and the filling factor. The main effect of
increasing packing density is to suppress the forward
scattering diffraction component of the phase function and
to increase the surface albedo. We used an accurate
radiative transfer technique to compute bidirectional
reflectance of a regolith layer composed of particles with
specified physical properties (size, refractive index and
packing density).

Although the problem of non-unique solution, which is
inherent for derivation of the media properties from the
measurements of the scattered light intensity, still
remains, we would like to stress that the used exact
procedure allows us to fit the observation data with a set
of real characteristics of the regolith rather than with
abstract parameters of the widely used approximations.

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