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J.L. Piatek, K.R. Stockstill, J.E. Moersch (U. of Tennessee)
Analysis of 100m resolution THEMIS IR data is key to understanding the surface composition of Mars. One of the difficulties in analysis of this data is the separation of the surface and atmsopheric components. The total radiance reaching the instrument (Lmeas) can be represented as: Lmeas = \tau\lbrack\epsilon\cdot L(BBsurf) + (1-\epsilon)\cdot L\downarrow\rbrack + L\uparrow where \epsilon is surface emissivity, L(BBsurf) is radiance emitted from the surface, L\downarrow is atmospheric emission which is reflected off of the surface (represented by (1-\epsilon)), \tau is the atmospheric opacity, and L\uparrow is radiance emitted by the atmosphere towards the spacecraft. All of these terms are wavelength-dependent, and the radiance (L) terms are all temperature-dependent.
A two-step process can be used to separate the atmospheric and surficial components of the radiance. First, the additive components (\tau (1-\epsilon)L\downarrow and L\uparrow, as well as any instrument offset) are removed, using a method similar to that of Bandfield et al. (in press). This is accomplished by designating a region of interest (ROI) in the image where the surface emissivity is expected not to vary, but where the surface temperature does (e.g., a crater). The "radiance offset" term is then determined by solving the resulting set of linear equations. This term is then assumed to contain the atmospheric emission terms, which depend on wavelength, temperature, and atmospheric opacity. The radiance offset values are treated as a spectrum, and a normalized emissivity method is used to identify a blackbody temperature. The blackbody contribution is then removed, and the result used to calculate \tau.
Results from the application of this correction will be compared with previous results (i.e. TES spectra and images) to determine if this method is capable of producing reliable separations of surface and atmosphere for use in spectral studies of the Martian surface. \end
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.