TES Mineralogy in the Wings of the CO2 Band. Constraining the Mineralogy of Type 2 Terrain.

In search for further telltale mineralogical absorptions to constrain the mineralogy of the Type 2 terrain, we have corrected for the atmospheric absorption in the wings of the 15-μ m CO₂ band. Bandfield et al. (Science, Vol. 287, Issue 5458, pp. 1626-1630, 2000) identified two primary types of terrain in TES spectra, one associated primarily with highland material (Type 1 terrain), and the second associated with lowland material (Type 2 terrain). The spectrum of Type 1 terrain was shown to match that of basalt, whereas the best match for the spectrum of Type 2 terrain was shown to be basaltic andesite. Wyatt and McSween (Nature, Volume 417, Issue 6886, pp. 263-266, 2002) later showed that the spectrum of weathered basalt could provide an equally good match for the Type 2 terrain. Most recently, Ruff (Mars 6, #3258, 2003) searched the 530 cm^-1 region for evidence of clay-type minerals such as Montmorillonite, Nontronite, and Fe-smectite, and found no absorption whose global distribution could correlate to that of the Type 2 terrain. Although the 530 cm^-1 region does contain mineralogical absorptions of interest, it is not the limit, and additional absorptions by plausible alteration minerals can occur further into the shoulders of the CO₂ band. In this work, we corrected the 500-600 cm^-1 region for the effect of the low wavenumber CO₂ wing. Because such atmospheric work requires accurate knowledge of the wavenumbers in the spectra, we recalibrated the TES wavenumbers using the atmospheric absorption of water vapor at high and low wavenumbers, and CO₂ hot- and isotope- bands at the intermediate wavenumbers. This method showed discrepancies (which increased at large wavenumbers) between our results and those reported by the TES team, leading us to adopt our calibrated wavenumbers for this work. To correct for the wings in the CO₂ band, we used the correlated-k method along with the retrievals of aerosol loading and vertical temperature distributions that were originally described by Conrath et al. (JGR, V.105, E4, pp.9509-9520, 2000) and provided by the PDS. We applied this correction and a band-finding algorithm to the data from the first 3000 orbits. Preliminary results suggest the presence of an absorption band centered at 563 cm^-1. This absorption exists in some sheet silicates such as smectite clays (ie: kaolinite, dickite) and micas (muscovite), some iron oxides (ie: hematite, magnetite), and some phosphates (apatite). Further work will involve the inclusion of more observations, mapping of this feature index and a search for features in the higher wavenumber wing of the CO₂ band.