Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p33c..04g&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P33C-04
Physics
3346 Planetary Meteorology (5445, 5739), 5405 Atmospheres (0343, 1060), 5464 Remote Sensing, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
The last four years have provided the opportunity to study Mars through the concurrent operation of orbiters and rovers to observe processes related to active winds. Data have been obtained on the characteristics of active sand and dust at the MER sites for comparisons with features and active processes seen from orbit. Combined with modeling, results give new insight into surface modification by windblown material. For example, the operation area of Spirit seen from orbit is criss-crossed with dark linear features thought to be tracks left by dust devils (DD). The rover traversed one track and obtained Microscopic Imager data showing that sand grains within the track are relatively clear of dust, while those outside the track are partly mantled with dust. Subsequent observations show that as active DD cross the plains (entraining dust into the atmosphere) they leave behind low-albedo tracks, which are later gradually obscured by settling of dust. The rate of deposition can be calculated from the solar panel output and appears to be continuous, proportional to optical depth τ, minus a constant. Dust removal from the panels is in discrete episodes. Observations of terrain and the atmosphere were made from the ground and from orbit: 1) before the dust devil "season" (τ = 0.578), 2) during active dust devil formation (τ = 0.918), 3) after active dust devil formation, but during active dust clouds (τ = 2.061), and 4) after clearing of dust from the atmosphere (τ = 0.487). Results were compared with temperatures derived from the Thermal Emission Spectrometer for the relevant seasons. Results suggest that spring heating of the surface enhances DD formation, but as atmospheric dust-loading progresses, active DD "shut-off," possibly because atmospheric conditions become thermally stable. In addition to dust activity, movement of coarser grains (i.e., "sand") has also been observed. For example, MOC, HiRISE, THEMIS, and HRSC images from orbit show abundant bedforms, revealed by Spirit to be composed of grains larger than a few hundred microns in diameter. Imaging of Spirit's deck after periods of aeolian activity showed the presence of similar grains, as well as the bounce marks of their passage in the deposited dust suggesting emplacement by saltation; showing that at least some sands currently are active. This interpretation was verified by a sequence of images from Spirit which showed active movement of small ripples across the surface. Despite these results, questions remain regarding current versus relict aeolian features, and the specific pathways of aeolian transport in complex terrains, such as the Columbia Hills in Gusev crater. These questions are being addressed through current research and the acquisition of new data from both the ground and orbit.
Arvidson Ray
Cabrol Nathalie
Christensen Paul
de Souza Paulo
Geissler Philip
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