The ultraviolet albedo of Mars

Ultraviolet observations have proven successful at studying the composition and structure of the upper atmospheres of the giant planets and Venus. In the case of Mars, the analysis of UV data has concentrated on the spatial distribution and temporal behavior of ozone. The potential of far ultraviolet (FUV) data (shortward of 200 nm) for measuring the atmospheric structure, in particular the vertical distribution of aerosols, is demonstrated. For illustrative purposes, a simple two layer model (atmosphere and surface) is computed using the atmospheric composition measured at the Viking landing sites. This model is then compared to the albedo observed with the International Ultraviolet Explorer (IUE). This comparison clearly shows that a clear atmosphere with the nominal Viking mixing ratios of CO2, O2, and H2O is incompatible with the IUE data shortward of 200 nm. To obtain a better match between data and model, an aerosol has to be introduced that effectively scatters at these wavelengths. The required reduction in effective CO2 pathlength is a factor of 5. The comparison also points out the inadequacy of the IUE to provide data in the critical wavelength region between 195 nm and 210 nm. Such data will be forthcoming from the Hubble Space Telescope and will provide the opportunity to infer the vertical distribution of aerosols on Mars on a global scale.