Optical properties of the Martian aerosols as derived from Imager for Mars Pathfinder midday sky brightness data

The Imager for Mars Pathfinder (IMP) obtained data on the midday sky brightness in filters centered at 443.6, 481.0, 670.8, 896.1 and 965.3 nm. Useful data sets were returned on sols 27, 40, 56, 65, 68, 74, and 82. Data from sol 56 were fitted with multiple scattering radiative transfer calculations, to extract the size distribution, optical properties and shape of the aerosols suspended in the atmosphere. The derived effective radius of the particles is about 1.71 + 0.29/ - 0.26 mu m with an effective variance of nu(eff) = 0.25 + 0.05/ - 0.1. The estimated values of the refractive index and shape parameters are close to those derived from Viking and Phobos data. This in turn implies that dust plays a significant and relatively constant role in the energy budget of the Martian atmosphere over the last two decades. Estimates of the optical depth agree well with those obtained independently from direct IMP imaging of the Sun. The derived single scattering phase function is more compatible with plate (clay) like particles rather than equal dimensional particles. The presented analysis assumes a simple single-component dust atmosphere. The data-model residuals exhibit, albeit weak, wavelength dependence. This dependence can be interpreted as an indication that during the time the analyzed images were taken, the dust particle distribution was bimodal or that the Martian atmosphere contained a second component, possibly submicron ice particles, in the aerosol's population.