Mars ozone measurements near the 1995 aphelion: Hubble space telescope ultraviolet spectroscopy with the faint object spectrograph

Ultraviolet (225-330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2-0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (L(s)) of 63.5 degrees, corresponding to the late northern spring season on Mars, and very near to Mars aphelion. The most important result of these observations is the measurement of low-latitude ozone abundances (3.1(-0.5)(+2.1)), which are significantly (greater than or equal to 100%) elevated relative to the northern fall (L(s) = 208 degrees, pre-perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion-to-aphelion increase in the global Mars ozone column (from 1.51(-1.0)(+0.4) to 3.1(-0.5)(+2.1) mu m atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with L(s), which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3+/-2.5 mu m atm is retrieved for a high northern latitude region (71-75 degrees N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18+/-0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.