Wired Science | 28 July 2010
First in-situ analysis of dust devil tracks on Earth and their comparison with tracks on Mars - D Riess et alThe ephemeral dark trails left in desert sand by dust devils are produced when the whirlwinds blow tiny particles of lighter-colored silt and dust off larger sand grains, a new study shows. Even removing a layer of dust and silt only a few micrometers thick can produce a dark trail visible with satellites, recent field studies suggest.
Dust devil trails have been spotted on space-based images of both Mars and Earth. Oddly, scientists are much more familiar with the phenomenon on Mars, says Dennis Reiss, a geographer at Westfälische Wilhelms Universität Münster in Germany.
Not only are dust devils larger and the resulting trails wider on the Red Planet, the atmospheric processes that erase such blemishes are weaker there. While Martian trails may persist for weeks, those on Earth typically disappear in a day or two. “We’re lucky to see them at all on Earth,” Reiss adds.
In the July 28 Geophysical Research Letters, Reiss and his colleagues document their analyses of a fresh dust devil trail. For their first-of-its-kind field study, the researchers went to a desert region of northwestern China, where such trails are commonly seen on satellite images and are easily detected at ground level. After finding a dark trail that hadn’t been seen the previous afternoon, they used handheld microscopes to look at sand in the trail and at a comparable site just a few meters outside the dark streak.
While much of the sand outside of the trail was coated with micrometer-sized particles of silt, clay and dust, most of the sand inside the trail had been whisked clean of such particles. Because those tiny surface particles typically are light-colored, the cleansed sand, when seen from a distance, appears much darker than the nearby material, says Reiss. The team estimates that if the material removed by the dust devil were instead spread evenly over the whirlwind’s path, that layer would measure about 2 micrometers thick.
Scientists have made similar observations of a dust devil trail on Mars using instruments on a robotic rover, but those sensors didn’t have nearly the resolution of the microscopes used in the field study in China, says Reiss.
- Geophysical Research Letters 37 L14203 (July 2010) DOI: 10.1029/2010GL044016