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R.T. Clancy, B.J. Sandor (Space Science Institute), G.H. Moriarty-Schieven (NRC of Canada, JAC -Hilo)
The late August 2003 opposition of Mars, which occurs very near Mars perihelion, presents its largest angular diameter (25 arcsec) over the previous and subsequent 20 years. Sub-millimeter observations from the James Clerk Maxwell Telescope (JCMT) on August 27 (also scheduled for September 3) will provide 345 Ghz CO line integrations at five beam positions on the Mars disk. Differencing spectral line absorptions observed at east, west, south, and north offset positions from a disk center spectrum yields highly accurate measurements of projected doppler velocities relative to the disk center. As demonstrated in similar Venus mesospheric (90-110 km altitude) wind measurements obtained from JCMT in March 2001 and November 2002 (Clancy et al., 2002), this method provides excellent sensitivity (5 m/sec at 40-80 km altitudes) for short integration periods (10-15 minutes). Systematic uncertainties associated with the absorption lineshape and spectrometer baseline and channel characteristics are minimized, and the steep sub-millimeter line core shapes provide improved doppler shift sensitivity relative to millimeter measurements. Direct wind measurements for the Mars atmosphere are extremely important for validation of Mars general circulation models (GCM, e.g. Forget et al., 1999), yet remain beyond current spacecraft mission capabilities. Lellouch et al. (1993) obtained equinoctial (Ls=200) wind determinations in significant disagreement with Mars GCM predictions, employing 230 Ghz CO doppler line shifts from IRAM. JCMT sub-millimeter CO doppler shifts observed during the August 2003 Mars opposition should be much more accurate, with critical zonal and meridional resolution during the key southern summer season. Atmospheric pressure-temperature profiles (0-75km) will also be retrieved from each disk position 12CO spectrum, complementing the Ls dependence of disk average measurements obtained from previous whole disk JCMT Mars 12CO observations. In addition to doppler winds measurements, we will obtain 5 hour integrations on the strong 362 Ghz transition of H2O2. H2O2 is the only potentially observable member of the critical HOx family of photochemical radicals in the Mars atmosphere. At its expected 10 ppb abundance levels, it is very difficult to measure but surprisingly low upper limits (3-5 ppb) have recently been inferred from high resolution spectroscopic observations at 1230 cm-1 (Encrenaz et al., 2003). JCMT sub-millimeter H2O2 line integrations during the 2003 opposition have the advantages of high line strengths, a filled antenna beam, and the Mars season of maximized H2O2 concentrations (Clancy and Nair, 1996).
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.