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Dust Devils on Mars

Wednesday, March 2, 2005
Matt
Balme (PSI)

Dust devils are small-scale convective vortices found mainly in arid areas. They form when strong insolation causing a reversal of the near-surface vertical temperature gradient (that is, the air near the ground becomes warmer than the air above it) is combined with a source of vertical vorticity to initiate a 'swirl'. Such swirls can then develop into a self-sustaining vortex that, if intense enough to lift surface material, can become dust loaded and hence is termed a 'dust devil'. Dust devils are common phenomena on Earth and recent images from orbiter and lander spacecraft shows this is also true for Mars.

While the amount of material dust devils lift into suspension on Earth is generally considered to be only a minor part of the atmospheric dust budget, on Mars they are often thought to be more important contributors. This is because of the apparent paradox between how dusty Mars' atmosphere is and the lack of observed boundary layer winds strong enough to lift such fine dust. Dust devils seem therefore a likely candidate for the mechanism that entrains this material.

This talk presents results from terrestrial field work and laboratory experiments to estimate the ability of dust devils to lift materials. The results suggest that dust devils are more efficient at lifting fine dust than boundary layer winds, this difference being attributed to the low pressure core found within dust devils that could act like a 'vacuum cleaner' to provide extra lift. Also, results from a survey of dust devil tracks in two large study areas on Mars conclusively show that dust devils are more active in the spring and summer, as is the case on Earth, and suggest that there is considerable spatial variation of dust devil activity. Parametric estimates of the global amount of material dust devils can lift on Mars suggests that there are still too many unknowns to determine this figure accurately. Ongoing work to solve this problem includes laboratory and numerical modelling of formation rates of dust devils and searches for active dust devils in orbiter and lander images.

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