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R.T. Clancy, M. Wolff, B. Whitney (Space Science Institute), B. Cantor (Malin Space Science Systems)
The Pathfinder descent entry detected extremely cold temperatures at 70-100 km altitudes (Schofield et al., 1997), suggesting the possibility of frequent CO2 ice cloud occurrence in the 70-80 km altitude region (Clancy and Sandor, 1998). However, prior to recent MGS limb measurements, it remained unclear whether CO2 or water ice aerosols are ever present at >60 km altitudes. Dust aerosols have been identified at such high altitudes in Mariner 9 (Anderson and Leovy, 1978), Viking (Jaquin et al., 1986), and MGS limb measurements (Clancy, 2003), associated with the 1971, 1977, and 2001 planet-encircling dust storms, respectively. The highest detached ice cloud identified from Viking limb data occurred at a projected tangent altitude of 55km, at 16S, 72W and Ls = 176\arcdeg (Jaquin et al., 1986; recently modeled by Montmessin et al., 2002). The seasonal period and location of this detached limb cloud appears consistent with 55-75 km detached limb clouds that we have observed as prominent in MGS TES solarband limb scans and MGS MOC wide-angle (WA) limb images (1-3 pm local times). Their occurrence frequency approaches unity at the beginning and end of the aphelion northern summer season on Mars (centered at Ls =30\arcdeg and 150\arcdeg), where they are confined to equatorial (15S-15N) latitudes and two relatively narrow longitude ranges (330-20W and 50-120W). Cloud altitudes of 65-75km are derived from the distribution of projected limb heights. Peak optical (visible) depths are quite significant (0.01) for the low pressure region of formation (~1 microbar). It remains unclear as to their composition (water or CO2), although coadded MGS thermal IR spectra indicate potential for water ice identification and particle size information. We will present the spatial and temporal distribution of these mesospheric clouds indicated in the MSG TES and MOC visible limb data, and derived vertical profiles of optical depth and particle size based upon a monte carlo spherical RT analysis of the coincident TES solarband and thermal IR limb measurements.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.