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 dat a yields quantitative measurements of atmospheric column opacities for aero sols and ozone over a wide range of latitudes (40 degrees S-70 degrees N) i n two Mars seasons (solar longitudes, L-s of 10 degrees and 61 degrees). Th e most significant findings of the analysis are: (1) the global Mars atmosp here exhibited minimal aerosol loading during this season (variable opaciti es of 0.02-0.08 for clouds, or 0.05-0.2 for dust); (2) the low-to-mid latit ude 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 degre es. These results support previous predictions (R. T Clancy and H. Nair 199 6, 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 Ma rs global ozone abundances, as driven by orbital variations in the global a ltitudes 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 a tmosphere around aphelion (Clancy et al. 1996a), with upper limits for back ground 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 o f interest is the observation of substantially reduced cloud opacities for the "aphelion cloud belt" in early northern spring (L-s similar to 60 degre es) in 1997, versus 1995 or 1991. (C) 1999 Academic Press.