Physics
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999sosyr..33..260p&link_type=abstract
Solar System Research, vol. 33, p. 260 (1999)
Physics
Scientific paper
Estimates of the optical thickness of the quiet atmosphere of Mars obtained from ground-based polarimetric observations in a visible spectral range are characterized by very low values. The sensitivity of polarization to the size and shape of scatterers raises the question of the effect of the nonspherical shape of solid particles of the Martian aerosol on the optical thickness estimate retrieved from the spectral dependence of the polarization inversion angle. The scattering-matrix elements for irregular particles (randomly oriented spheroids) are calculated using the T-matrix method. This model of dust particles leads to a weak dependence of the inversion angle of the polarization of radiation scattered by the surface and the atmosphere on the optical thickness of dust aerosols tau_d if this thickness is small: an increase in tau_d from 0.05 to 0.15 results in an inversion angle increase of less than 1 deg, while for spherical particles this change is about 5 deg. For higher values of optical thickness (up to 0.6), the inversion angle increases for the spheroid models by several degrees, too, which is confirmed by observations during dust storms on Mars. It is natural to expect that particles with more pronounced irregular shapes than spheroids may lead to a more substantial depolarization and hence to an even weaker sensitivity of the inversion angle to variations in tau_d. Thus, very low estimates of the optical thickness of the quiet atmosphere of Mars derived from ground-based polarimetric observations using a spherical shape approach for aerosol particles cannot be considered reliable. Ignoring the dust particle shape introduces similar uncertainties in the models of the Martian atmosphere such as those caused by traditionally discussed errors in such parameters as the refractive index, size distribution, etc.
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