Mars aerosol optical thickness retrieved from measurements of the polarization inversion angle and the shape of dust particles

The estimates of the optical thickness of the "clear" atmosphere of Mars ob tained from ground-based polarimetric observations are characterized by ver y low values. The sensitivity of polarization to the size and shape of scat terers raises the question of the effect of the irregular shape of solid ae rosol particles on the optical thickness retrieved from the spectral depend ence of the polarization inversion angle for Mars (the phase angle at which the polarization of the scattered radiation changes sign). The Stokes scat tering matrix for irregular dust particles is calculated using the T-matrix method. The model of randomly oriented oblate polydisperse spheroids used in this study indicates a weak dependence of the polarization inversion ang le on the optical thickness of aerosols tau(d): an increase in tau(d) from 0.05 to 0.15 results in an inversion angle increase of less than 1 degrees, whereas for spherical particles this change is about 5 degrees. It is reas onable to expect that particles with more pronounced irregular shapes than spheroids may lead to more substantial depolarization and hence an even wea ker sensitivity of the inversion angle to variations in the aerosol optical thickness. Thus, very low estimates of the optical thickness of the "clear " atmosphere of Mars retrieved from ground-based polarimetric observations using a spherical-shape approach for aerosol particles cannot be considered reliable. Ignoring the dust particle shape introduces similar uncertaintie s in the models of the Martian atmosphere as those caused by traditionally discussed errors in such parameters as refractive index or size distributio n. (C) 1999 Elsevier Science Ltd. All rights reserved.