The Scale-height Of Optical-depth In Valles Marineris As Derived From Shadows In HRSC Images
Abstract
The optical depth of the Martian atmosphere can be estimated from the brightness of shadows with the so called "shadow method". We investigated the accuracy of this method by analyzing a set of stereo and color images observed on July 21, 2005 with the High Resolution Stereo Camera of the Mars-Express orbiter. The images show part of Valles Marineris during late afternoon and contain numerous shadows. Whereas the analyzed regions span height differences of about ten kilometers, we could study the relation between altitude and shadow method measurements. If the optical depth and gas-pressure have similar scale heights, then the accuracy with which our measurements can reproduce the pressure-scale-height tells about the accuracy of the shadow method.
Various GCMs suggest a local pressure scale height of around 13 km at that moment, while the red and the five panchromatic stereo images all yielded similar scale heights with an average of 12.2 ± 0.7 km. Thus, in the color of the panchromatics (yellow to red) and in red the shadow method yielded good results. The scale height derived from the NIR image is too low: 10.6 ± 0.4 km, we speculate that this is an effect from airborne dust particles that are on average larger in the lower than in the higher atmosphere. The scale heights that were derived from blue and green images were unrealistically high: 17.0 ± 0.7 km and 14.5 ± 0.5 km respectively. This may be caused by thin white high altitude hazes, or indicate that the accuracy of the shadow method varies with optical depth in these colors.- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #41
- Pub Date:
- September 2009
- Bibcode:
- 2009DPS....41.4904H