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 Febru ary of 1995. These spectra yield ozone column abundances, cloud opacit ies (0.2-0.4 at low latitudes), and polar seasonal ice albedos from so uthern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (L(s)) of 63.5 degr ees, corresponding to the late northern spring season on Mars, and ver y near to Mars aphelion. The most important result of these observatio ns is the measurement of low-latitude ozone abundances (3.1(-0.5)(+2.1 )), which are significantly (greater than or equal to 100%) elevated r elative to the northern fall (L(s) = 208 degrees, pre-perihelion) IR o zone measurements of Espenak et al. [1991] in 1988. The implied perihe lion-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 w ith photochemical modeling analysis of Clancy and Nair [this issue], w hich predicts large annual variations in Mars photochemistry due to or bital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diag nostic of the altitudes at which Mars ozone densities vary with L(s), which is a key aspect of the Clancy and Nair model prediction. Further more, it is the uncertain ozone density profile which leads to the lar ge asymmetric uncertainties in the derived FOS and IR ozone columns. A n 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 t he north polar seasonal CO2 cap is 0.18+/-0.07, which is roughly 10 ti mes the ultraviolet albedo of the silicate surface of Mars, but only o ne quarter the visible albedo of the seasonal CO2 ice cap.