Spectrophotometry of Mars in the KRFM experiment of the Phobos mission : some properties of the particles of atmospheric aerosols and the surface

Abstract

A new analysis of the data of the KRFM experiment ( Phobos mission) is presented. The five complete photometric profiles (eight wavelengths for each) obtained in this experiment have been processed, and the calibration and the method of modelling calculations have been specified. The optical depth of the constant haze has been estimated to be within the range 0.1 < τ < 0.2. This is essentially smaller than the value predicted for the same season on the basis of the Viking landers' measurements. The imaginary part of the refraction index of the particles in the constant haze changes smoothly from 0.015 at 315 nm to 0.005 at 550 nm. This behaviour corresponds to some basaltic material, containing several percent of iron oxides (limonite, geothite, hematite). Their presence in the Martian soil was recognized a long time ago, but our study gives some new quantitative estimations of Fe 2O 3 content. The clouds above the large volcanoes have an optical depth ∼0.1 and contain about 5 × 10 −6 g cm −2 of ice particles. All the evaluations of the constant haze particles' properties have been made assuming a gamma size distribution of particles with modal radius r m = 0.4 μm and α = γ = 1. For the description of the surface's photometric properties in the whole range 315–550 nm, the roughness factor q = 0.5 is acceptable as a first approximation, although one cannot exclude that q rises with increasing wavelength. The averaged radius of the optically active particles of the surface has been evaluated as 6–7 μm. The method for the calculation of photometric profiles of Mars used in the paper is more reliable than any published version we know, because it takes into account the main optical properties of the Martian atmosphere (the presence of absorption, strong forward scattering) and surface (including a non-Lambertian reflection law).