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J. J. Caldwell, T. Ouvarova (York U.), S. K. Atreya, A.-S. Wong, N. O. Renno (U. Michigan), P. James (U. Toledo)
Wong et al. (JGR,2003) have discussed the characteristics of plausible Martian outgassing products on Mars, based on terrestrial analogs. Among the gases considered were SO2 and CH4. These gases have strong bands in the UV (SO2) or near IR (CH4) within the ranges of the HST spectrographs. We investigate here the advantage of the UV accessibility of the Hubble Space Telescope and its high spatial resolution capability to detect these gases in local concentrations higher than their global average upper limits, using archival STIS CCD spectra of Mars. For the available data, acquired in 1997 and 2001, the distance to Mars ranged from 0.58 to 1.79 AU. The STIS pixel width is 0.05 arc sec on the sky, so that a sub-Earth pixel on Mars is a square with sides ranging from 21 to 65 km. STIS slit widths for available Mars data are either 4 or 10 pixels wide, increasing the effective pixel dimension by those factors in the dispersion direction. Further, the telescope PSF has a radius of 0.1 arc sec for 70% energy inclusion, increasing the resolved area by about 4 pixels in each dimension. The effectively resolved region on Mars therefore varies from about 14,000 km2 (4 x 10-4 times Mars cross-section) to about 840,000 km2. We therefore expect, for data taken when Mars was relatively close to the Earth, to improve upon current global upper limits (from the Mariner 9 IRIS) for SO2 (200 ppb) and CH4 (20 ppb). The HST/STIS has significant UV grating scatter effects which invalidate data below about 2200Å. This precludes detecting some strong SO2 lines. We employ STIS spectra of the Moon, using the same instrument settings as the Mars data, as a Solar surrogate to produce albedo curves that are independent of aliasing between different instruments. We also compare adjacent spectra on Mars for spatially differential absorption. Finally, we make use of the STIS exposure time calculator to estimate possible improvement using STIS MAMA detectors.
The author(s) of this abstract have provided an email address for comments about the abstract: caldwell@nereid.sal.phys.yorku.ca
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.