Martian aerosol properties from the Phobos/ISM experiment.

Observations with the infrared spectrometer (ISM) on board the Phobos 2 spacecraft show evidence for a near-infrared, spectrally-varying contribution of aerosols in the Martian spectra, which are probably suspended dust in the atmosphere. In the conditions of ISM observing sequences close to backscattering geometry, the main effect of the scattering is to add to the surface spectrum a backscattering dust component, with a strong decreasing slope with its contribution declining between 1 and 2.5 μm. A complete scattering model for the Martian aerosols has been developed. Finally, a consistent picture of Martian dust at time of Phobos 2 observations, in March 1989, is obtained. It is found that the main difficulty of the study is to separate the surface contribution from the suspended dust spectra. The results are found to be consistent with a low amount of dust in the atmosphere (τ ≈ 0.2) and a size distribution of dust particles with an effective radius r_eff ≈ 1.25 μm.