Contribution of Aerosols to Near-IR Remote Sensing of the Mars Surface Composition, Derived From the OMEGA/Mars Express Dataset.

The OMEGA instrument on board Mars Express observes the surface in visible and near-infrared wavelengths where light is strongly scattered by aerosols. Therefore, aerosol contributions have to be considered when interpreting the OMEGA spectra in terms of surface mineralogy. We have developed a model of near-IR Martian aerosols based on a Monte-Carlo approach. This model has been used to retrieve surface Lambert albedo and optical depth when different geometries of observation are available. The Monte-Carlo approach is well suited for taking into account multiple scattering, which plays a major role at high incidence/emergence. In the polar regions of Mars, we have separated the contribution of dust in the surface ice and airborne dust using consecutives observations (< 1° of LS) in the nadir pointing mode with significant difference in solar incidences. The contribution of surface dust is found to be low in most regions of the South Polar cap (except the "cryptic region"). The signal at 2.6μm, in the saturated CO2 band, has therefore been used to map the optical depth of atmospheric dust over the south cap with a short time scale (~ 1° of LS). Emission phase functions have been analysed to separate aerosols and surface contribution with a Lambert hypothesis for the surface. For moderate phase angles, the variations of the reflectance factor with emergence are mainly due to aerosols, with only a minor role of the bidirectional properties of the surface. These methods have been tested in the Meridiani region. They provide satisfactory determinations of the optical depth at 1μm when compared to direct solar extinction measurements performed by Pancam/Opportunity at 0.88μm. This model could prove useful for processing image cubes from the CRISM imaging spectrometer on board MRO, which will implement emission phase function observation as a nominal observation strategy.