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[11P.09] Martian Atmospheric Water Vapor: The 1996-1998 Apparition

E. S. Barker (McDonald Obs., Univ. of Texas at Austin)

Between October 1996 and November 1997, seasonal coverage of Martian water vapor in L_s concentrated on the northern spring summer season with samples at L_s = 25^\circ, 46^\circ, 67^\circ, 74^\circ, 83^\circ before Mars' opposition; and on the northern late-summer, early-fall season at L_s = 132^\circ, 143^\circ, 166^\circ, 181^\circ, 212^\circ after opposition. The slit was placed either in the N-S direction along the central meridian to study the latitudinal distribution or E-W to study the diurnal behavior. High-dispersion, CCD echelle spectra yielded 87 continuous H₂O vapor profiles across the Martian disk.

To improve the inter-comparability of ground-based datasets (Arizona, Texas), we are using: (1) HITRAN96 line strengths for the (211) band of H₂O which are 20-40% stronger (depending on the line and effective temperature) than those used previously; and (2) the Mars GCM (Haberle, personal communication) to predict the effective atmospheric temperature for each L_s and sampled Martian latitude and longitude. Evidence for the following summary conclusions will be presented.\\ \bullet\hspace {1mm}Spatial variations of 4x or greater are present at all sampled seasons.\\ \bullet\hspace {1mm}During the northern spring season, the latitude region containing the maximum amount of water vapor moves northward as the season progress, but the maximum is at least 30^\circ north of sub-solar latitude.\\ \bullet\hspace {1mm}During the pre-opposition period (L_s = 25^\circ to 83^\circ), nominal amounts of Martian water vapor were observed, increasing to maximum amounts of (20 to 25\mum of ppt. H₂O) at higher northern latitudes (55^\circ to 75^\circ).\\ \bullet\hspace {1mm}Barely detectable 1-3\mum amounts were measured during these seasons at all southern latitudes (0^\circ to -30^\circ).\\ \bullet\hspace {1mm}Measurements of the diurnal change at different Martian latitudes showed quite low abundances (2-5\mum) above both the morning and evening terminator regions with a maximum about 1 hour after local noon.\\ \bullet\hspace {1mm}Ground-based measurements showed 10.5±1.5\mum of ppt. H₂O above the Pathfinder site on July 5, 1997.\\ Research supported by NASA grant NAG5-6140.