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

*Session 51. Mars Atmosphere Posters*

Displayed, 1:00pm, Monday - 1:00pm, Friday, Highlighted Tuesday and Thursday, 3:30-6:30pm, C101-C105, C211
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## [51.09] On the shape of martian dust and water ice aerosols

*K. M. Pitman (Louisiana State University), M. J. Wolff, R. T. Clancy (Space Science Institute), G. C. Clayton (LSU)*

Researchers have often calculated radiative properties of
Martian aerosols using either Mie theory for homogeneous
spheres or semi-empirical theories. Given that these
atmospheric particles are randomly oriented, this approach
seems fairly reasonable. However, \textit{the idea that
randomly oriented nonspherical particles have scattering
properties equivalent to even a select subset of spheres is
demonstratably false} (Bohren and Huffman 1983; Bohren and
Koh 1985, Appl. Optics, 24, 1023). Fortunately, recent
computational developments now enable us to directly compute
scattering properties for nonspherical particles.

We have combined a numerical approach for axisymmetric
particle shapes, i.e., cylinders, disks, spheroids
(Waterman's T-Matrix approach as improved by Mishchenko and
collaborators; cf., Mishchenko et al. 1997, JGR, 102, D14,
16,831), with a multiple-scattering radiative transfer
algorithm to constrain the shape of water ice and dust
aerosols. We utilize a two-stage iterative process. First,
we empirically derive a scattering phase function for each
aerosol component (starting with some ``guess'') from
radiative transfer models of MGS Thermal Emission
Spectrometer Emission Phase Function (EPF) sequences (for
details on this step, see Clancy et al., DPS 2000). Next, we
perform a series of scattering calculations, adjusting our
parameters to arrive at a ``best-fit'' theoretical phase
function.

In this presentation, we provide details on the second step
in our analysis, including the derived phase functions (for
several characteristic EPF sequences) as well as the
particle properties of the best-fit theoretical models. We
provide a sensitivity analysis for the EPF model-data
comparisons in terms of perturbations in the particle
properties (i.e., range of axial ratios, sizes, refractive
indices, etc).

This work is supported through NASA grant NAGS-9820 (MJW)
and JPL contract no. 961471 (RTC).

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