36th DPS Meeting, 8-12 November 2004
Session 37 Mars Atmosphere
Poster II, Thursday, November 11, 2004, 4:15-7:00pm, Exhibition Hall 1A

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[37.08] Atmospheric optical depths from HRSC stereo images of Gusev crater and elsewhere on Mars

N. M. Hoekzema, W. J. Markiewicz, A. Inada, S. H. Hviid, H. U. Keller (MPS, Katlenburg-Lindau, Germany), K. Gwinner, H. Hoffmann (DLR, Berlin, Germany), J. A. Meima (BGR, Hanover, Germany), G. Neukum (FU Berlin, Germany), HRSC Team, MER Science Team

Stereo images taken with the High Resolution Stereo Camera (HRSC) onboard the Mars Express orbiter offer a unique and powerful way to study Mars and its atmosphere. Since the atmosphere contains large and variable amounts of aerosols that scatter light and influence the images, image analysis requires careful consideration of atmospheric effects. An essential parameter is the optical depth, which often is retrievable from stereo observations with the ‘so called’ stereo method. This method uses contrast differences between stereo images to estimate optical depths. Software for this purpose has been developed at the MPS in Lindau Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR. For validation, we compared optical depths retrieved from HRSC images with measurements from the Spirit rover in Gusev. On Jan 16 2004 Spirit looked up into the Sun and measured the local optical depth at 0.87—0.89. That same day, during orbit 24 of MEX, HRSC observed Gusev. HRSC’s stereo images yielded 0.91 ± 0.04, in good agreement with Spirit’s ground truth.

Spirit landed in a contrast-rich region. For accurate retrievals, sufficient contrast is essential. Low contrast regions, say <5% in RMS, generally yield large errors. In addition, careful consideration of topography proves crucial. The stereo-method works best on flat terrain since then the surface looks almost the same in all images, even though these were observed from different viewing angles. Retrievals from regions with a lot of sharp topography, such as cliffs and crater edges, generally are unreliable since such features can look very different from different perspectives. We use HRSC stereo images, and the Digital Terrain Models derived from these, to study effects from topography and contrast.

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Bulletin of the American Astronomical Society, 36 #4
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