Mars: the role of water ice clouds on surface remote sensing

The possible effect of water ice clouds on spectroscopic measurements of the martian surface is evaluated. We have analysed imaging spectroscopic data of Mars in the 0.4-1.05 mum wavelength domain, obtained during the opposition in March 1997. The images show the classical albedo units in the red, while in the blue the surface contrast almost vanishes. Condensates on Hellas, north pole, Elysium and Acidalia, probably due to H2O ice crystals, are visible in the blue images. Spectral analysis shows the presence of an effect, named "albedo reversal", occurring on Syrtis Major, Vastitas Borealis and Elysium. While this effect has been historically attributed to the spectral properties of the surface, from our study it appears that an atmosphere contribution could exist. We attribute the enhanced UV-VIS brightness to phase properties of airborne dust-H2O ice particles.