Minimal aerosol loading and global increases in atmospheric ozone during the 1996-1997 Martian northern spring season

We employed the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) to observe middle ultraviolet (220-330 nm) spectral scans of Mars for two periods during the 1996-1997 northern spring season. Analysis of these data yields quantitative measurements of atmospheric column opacities for aerosols and ozone over a wide range of latitudes (40 degrees S-70 degrees N) in two Mars seasons (solar longitudes, L-s of 10 degrees and 61 degrees). The most significant findings of the analysis are: (1) the global Mars atmosphere exhibited minimal aerosol loading during this season (variable opacities of 0.02-0.08 for clouds, or 0.05-0.2 for dust); (2) the low-to-mid latitude ozone abundance in the Mars atmosphere increased by roughly a factor of two (from 1.8 to 3.6 mu m-atm) between L-s = 10 degrees and L-s = 61 degrees. These results support previous predictions (R. T Clancy and H. Nair 1996, J. Geophys. Res. 101, 12785-12790) and observations R. T. Clancy et al. 1996b, J. Geophys. Res. 101, 12777-12783) of orbital (L-s) variations in Mars global ozone abundances, as driven by orbital variations in the global altitudes of water vapor saturation (R. T. Clancy et al. 1996a, Icarus 122, 36-62). They also support conclusions of very low dust levels in the Mars atmosphere around aphelion (Clancy et al. 1996a), with upper limits for background dust opacities which are several times lower than described for this season by the Viking-based dusty model of the Mars climate (e.g., tau < 0.2, here; versus 0.6; D. S. Colburn et al. 1989, Icarus 79, 159-189). Also of interest is the observation of substantially reduced cloud opacities for the "aphelion cloud belt" in early northern spring (L-s similar to 60 degrees) in 1997, versus 1995 or 1991. (C) 1999 Academic Press.