THE VERTICAL STRUCTURE OF MARTIAN AEROSOLS EXPLORED USING A 3D SPHERICAL MONTE-CARLO MODEL AND OBSERVATIONS AT THE TERMINATOR BY OMEGA. Mathieu

Introduction: The OMEGA instrument onboard Mars Express has obtained several nadir observations of the surface of Mars at the boundary between day and night (the “terminator”) since early 2004. Even under clear atmospheric conditions, the path length of photons in the aerosols layer is high for such observations. In the dayside, almost all solar photons heading toward the surface are stopped by aerosols. The surface is then lighted up by photons scattered high in the atmosphere, where the aerosol layer is thin enough. These aerosols also send photons toward the night side of the planet, which makes it possible to see the surface after the sunset. These observations are sensitive to the vertical structure of aerosols. Previous studies of the vertical distribution of aerosols have revealed that the particle size of atmospheric dust decreases with altitude [1] and that discrete layers of water ice are common [2]. A 3D spherical Monte-Carlo model: We have developed a 3D Monte-Carlo model of radiative transfer in an atmosphere populated by suspended particles. The Monte-Carlo approach includes multiple scattering without simplifying assumptions [3]. The model is used to populate look tables of reflectance seen from zenith both in the dayside and in the night side.