Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p51a0339c&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P51A-0339
Other
0300 Atmospheric Composition And Structure, 0305 Aerosols And Particles (0345, 4801), 3300 Meteorology And Atmospheric Dynamics, 5415 Erosion And Weathering, 5465 Rings And Dust
Scientific paper
A wind tunnel was used to study the effects of a heated surface, thereby creating an unstable near-surface atmosphere, on the threshold of aeolian-blown (windblown) dust-size particles (1-2 mm) under Mars-simulated pressure. Unstable conditions on Mars typically arise during the mid to late afternoon hours due to the accumulation of daytime solar-radiation. When the surface is warmer than the atmosphere just above it, vertical turbulence is increased. Thus, loose dust particles can be more easily lofted and mixed at a threshold wind speed lower than that known under neutral atmospheric conditions. For this wind-tunnel study, unstable (heated) surface conditions were simulated based on the negative temperature gradients and surface bulk Richardson numbers estimated from the Mars Pathfinder Lander (MPL) mission data during the mid-afternoon to early evening Mars period. According to other missions, evidence of highly active dust suspension during this part of the Mars daytime hours was recorded, including the presence of "dust devils". Experiments were performed in the Martian Surface Wind Tunnel (MARSWIT) located at NASA Ames Research Center, Moffett Field, California. Based on data acquired from the MPL site, the mean surface pressure was found to be 6.75 mb. Thus, simulations in MARSWIT were conducted at 10-mb atmospheric pressure using air, which agrees with a dynamically similar environment of 6.5 mb on Mars. In order to attain the necessary vertical temperature gradients that would develop an unstable layer, a test bed was heated by sub-surface heaters. Three surface roughness conditions were simulated, over which not only dust threshold was measured but also velocity and temperature profiles were acquired under various heating levels. Boundary layer measurements and analysis conducted under neutral conditions were used to estimate roughness height, zo, and the friction speed, u*, for all stability conditions. Dust threshold tests were conducted using a surrogate Mars soil, Carbondale Red Clay (CRC), which has a mean particle diameter of about 1 to 2 mm in dust form. According to boundary-layer analysis, two test beds, having zo = 0.015 mm and 0.09 mm, generated hydraulically smooth-wall turbulent flow. Under neutral stability conditions, the corresponding dust threshold frictions speeds for these two surface conditions were u* = 1.63 m/s and 1.61 m/s, respectively. Heated-surface experiments also showed that the two smoother test beds developed a decreasing trend in threshold wind speed, from 30 m/s at neutral conditions to 8 and 20 m/s, respectively, at increased surface heating levels. A third bed, zo = 0.018 mm, observed the classical rough-wall "law-of-the-wall" trend. This rougher test surface, however, portrayed the opposite effect, where threshold increased for greater instability conditions. The major difference between the first two smooth beds and this rougher bed was the application of conductive roughness elements (steel nuts), which initially caused a lower threshold value of u* = 0.77 m/s at neutral conditions, but then increased with higher surface heating. This latter result should be viewed as preliminary.
Coquilla R. V.
White Roscoe B.
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