Polarimetry of Mars in High-Transparency Periods: How Reliable Are the Estimates of Aerosol Optical Properties?

The influence of various factors on the interpretation of polarimetric observations of Mars performed under high-transparency atmospheric conditions was investigated. It was demonstrated that the technique proposed by Morozhenko (1974) for the data interpretation does not allow the optical characteristics of cloud particles to be estimated unambiguously. In particular, it turns out that both the model of a thin layer composed of very small silicate particles and the model of very small ice particles in a thin layer fit the data of measurements at a phase angle of 25°. Numerical simulations showed that reliable information on the reflectance characteristics of the Martian surface in a wide spectral range, including blue and ultraviolet wavelengths, is necessary to obtain reliable estimates of aerosol optical properties. Our calculations confirmed that the scattering on ice-cloud particles could introduce a significant ambiguity in the interpretation of the available polarimetric data. Besides, small particles, both ice and silicate, existing in the upper atmospheric layer can substantially mask the layer of larger particles below, which becomes invisible for remote sensing.