Mariner 9 Ultraviolet Spectrometer Experiment: Afternoon terminator observations of Mars

Analysis of the Mariner 9 ultraviolet spectrometer twilight data of Mars indicates the presence of two characteristic types of atmospheric scattering properties as measured at 3050 A. Measurements in the mid-latitudes from 15°S to 50°N require a scattering layer at an altitude of 60–90 km in order to explain the intensity variation with solar depression angle. A scattering layer of optical thickness 0.005 combined with a homogeneous lower atmosphere having a nominal 10-km scale height will explain the mid-latitude measurements. North of 50° latitude the terminator intensity is enhanced and decreases rapidly with increasing solar depression angle. The terminator scattering at northern latitudes can be explained by a homogeneous scattering medium with an optical thickness ≥0.1, a single scattering albedo of 0.3–0.5, and an effective scale height of 6–8 km. Atmospheric water ice or carbon dioxide ice probably accounts for both the mid-latitude scattering layer and the enhanced scattering in the northern latitude lower atmosphere.