Dust deflation by dust devils on Mars derived from optical depth measurements using the shadow method in HiRISE images

We measured the optical depth of three separate dust devils and their surroundings with the so called "shadow method" in HiRISE images. The calculated optical depths of the dust devils range from 0.29±0.18 to 1.20±0.38. Conservative calculations of the minimum and maximum dust loads are in the range of 4-122 mg m−3. Assuming reliable upper and lower boundary values of vertical speeds within the dust devils between 0.1 and 10 ms−1 based on terrestrial and Martian studies we derived dust fluxes in the range of 6.3-1221 mg m−2 s−1 (PSP_004285_1375), from 0.38-162 mg m−2 s−1 (ESP_013545_1110), and from 3.2-581 mg m−2 s−1 (ESP_016306_2410) for the three dust devils. Our dust load and dust flux calculations for the three dust devils are in good agreement to previous studies. Two of the analyzed dust devils left continuous dark tracks on the surface. For these dust devils we could calculate how much dust was removed by using the minimum and maximum dust fluxes in combination with measured horizontal speeds of these dust devils. Our results indicate that a dust removal of an equivalent layer of less than 2 µm (or less than one monolayer) is sufficient for the formation of dust devil tracks on Mars. This value might be used in future studies to estimate the contribution of dust devils to the global dust entrainment into the atmosphere on Mars.