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[22P.07] An Investigation of Mars NIR Spectral Features using Absolutely Calibrated Images

D.R. Klassen (Rowan University), J. F. Bell, III (Cornell University)

We used the NSFCAM 256x256 InSb array camera at the NASA Infrared Telescope Facility to gather near-infrared (NIR) spectral image sets of Mars through the 1995 opposition. In previous studies with these data [1-6] we noted several interesting spectral features, some of which are diagnostic volatile absorption bands that allow the discrimination between CO₂ or H₂O ices. Band depth maps of these regions show polar and morning and evening limb ices composed of water and some indication of polar CO₂ ices. Other features, near 3.33 and 3.4\micron, appear to be confined to particular geographic regions; specifically Syrtis Major. However, the images used in these previous studies were calibrated to either the disk average or only to a rough scaled reflectance by simple division by solar-type star data gathered at the same time as the images. This only allowed determinations of spectral features either relative to some global average of the feature, or to some unit not directly comparable to other published data. For at least three of our observation nights the conditions and data are sufficient to absolutely calibrate the images to radiance factors. For this work we reinvestigate the spectra and band depth mapping results using these absolutely calibrated images. In general we find that bright regions have peak radiance factors of 0.5 to 0.6 at 2.25\micron\ and 0.3 to 0.4 at 3.5\micron; dark regions have radiance factors of 0.2 to 0.25 at 2.25\micron\ and 0.1 to 0.15 at 3.5\micron. Overall, precision errors are about 0.025 in radiance factor and absolute errors are at the 10-15% level. These results are consistent with previous studies that found radiance factors of 0.35 in Tharsis, 0.47 in Elysium, and 0.26 in dark regions at 2.25\micron\ [7,8] and 0.3 in bright regions and 0.1 in dark regions at 3.5\micron\ [8]. These absolute flux values will allow direct comparison of these results to radiative transfer models of the behavior of the surface and atmosphere and also provide an independent measurement for comparison to, and calibration of, imaging and spectral data acquired by spacecraft orbiting Mars. {\bf References} [1] Bell III J. F., {\it et al.} (1995) BAAS, 27, 1091. [2] Klassen D. R. {\it et al.}(1995) BAAS, 27, 1061. [3] Bell III J. F. {\it et al.} (1996a) J. Geophys. Res., 101, 9227. [4] Bell III J. F. {\it et al.} (1996b) BAAS, 28, 1063. [5] Klassen D. R. (1997) Ph.D. Dissertation, Univ. Wyoming. [6] Klassen D. R. {\it et al.} (1998) LPSC XXIX, abstract no. 1658. [7] Bell III J. F. {\it et al.} (1994) Icarus, 111, 106. [8] Roush T. L. {\it et al.} (1992) Icarus, 99, 42.