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C.R. Stoker, K. Rages (NASA Ames Research Center)
We developed a method to to remove the illumination effects of highly reddened (compared to the solar spectrum) diffuse light caused by dust scattering in the Martian atmosphere from spectra obtained by the multispectral IMP camera. We use a radiative transfer computer code to model the direct (solar) and diffuse (sky) illumination on surfaces as separate contributions to total reflected light intensity. Subtracting the component of the scattered light due to the diffuse illumination produces spectra as they would appear if there were no dust in the atmosphere, provided we know the scattering geometry and surface scattering properties. To verify our model, we compared its results with calibrated spectra of radiometric targets on the spacecraft, which have known reflectance properties. To study how the reflected color from surfaces varies as a function of time due to direct and diffuse components of the illumination, and to demonstrate the method for producing spectra as they would appear on Mars with no atmospheric dust, we created a geometric model of a truncated dodecahedron having Lambertian reflection properties. This model, which approximates the shape of the rock Yogi, is used to show how the reflected color from various faces of the rock may vary with time of day. We produced digital terrain models from 670nm-filter stereo pairs using the method described in Stoker et al.( JGR 104,8889,1999), used these to deduce surface normal vectors, and compute scattering geometry for each considered spectra. We have applied the color-correction procedure to images of Photometry Flats and Yogi, using the Hapke parameters deduced by Johnson et al. ( JGR 104, 8809,1999) for dark soil and gray rocks. We present results from this study which successfully demonstrate our approach for color correction but show that improved Hapke parameters must be deduced while taking into account the diffuse illumination source.
This work is funded by the NASA MDAP program.