Parameters of Dust Particles in the Martian Atmosphere

A critical analysis of the methods and results of estimating the optical thickness of the dust component in the Martian atmosphere τ0, the particle size r0, and the imaginary part of the refractive index nihas shown the following. (1) Observational data on the brightness distribution over the Martian disk as well as the phase dependences of diffusely reflected light and the azimuthal dependences of diffusely transmitted light are most appropriate to use only for verifying the reliability of the aerosol parameters determined by other methods. (2) If the morning and evening fogs in the atmosphere are disregarded, the Bouguer–Lambert–Beer method used to analyze the solar-brightness attenuation measured on the planetary surface yields overestimated extraatmospheric solar intensity I0and atmospheric optical depth τ0. At the Viking 1landing site, I0and τ0could be overestimated by a factor of 1.7 and by 0.35, respectively. (3) The aerosol size determined by analyzing measurements of the azimuthal dependences for the Martian sky brightness at low elevations of the Sun most likely corresponds to the fog particles. (4) If overestimated values of I0were used to standardize the observations of the solar radiation transmitted by the Martian atmosphere, then niwere also overestimated; using overestimated τ0also affected the reliability of the latter. (5) The problem of reliability of the available τ0and r0estimates for periods of high atmospheric transparency is yet to be solved. For the highest activity of the dust storm in 1971, it was found that 4.5 ≤ r0≤ 7.5 μm for the lognormal particle size distribution with σ2= 0.2 and the optical thickness of a dust cloud τ0≥ 15. (6) The spectral values of the apparent albedo of Mars measured in October 1971 at a phase angle of 42° in the spectral range 0.250 ≤ λ ≤ 0.717 allowed the imaginary part of the refractive index to be estimated in terms of a model of a dust cloud composed of spherical particles with the lognormal size distribution with r0= 4.5 μm and σ2= 0.2.