dust devils

All posts tagged dust devils

There were lots of great things about the movie “The Martian” (and a few inaccurate things), but one of the best things, for aeolian scientists like myself anyway, was the depiction of ubiquitous, enormous dust devils.

Mark Watney survery the desolate martian landscape.

Mars loves to make dust devils. It’s relatively easy for sunlight to heat the atmosphere and get it churning, and thick dust deposits blanket enormous regions on Mars.

A map of dust deposits on Mars. From https://sci.esa.int/web/mars-express/-/51861-mars-dust-map.

Dust devils on Mars help keep the atmosphere dusty, which warms the climate and helps drive weather. However, as surprising as it might be, we don’t totally understand how dust devils actually lift dust.

Sure it’s true that dust devils are windy, but when you actually plug the windspeeds measured in dust devils into the dust-lifting equations, the amount of dust they *should* lift can be much less than what they *do* lift. So some other mechanism besides just wind must help lift dust in devils.

A dust devil on Mars. From https://en.wikipedia.org/wiki/Dust_devil.

One possibility is that dust devils act like vacuum cleaners and actually suck dust up off the martian surface. See, a lot of the dust sitting on the surface of Mars has been sitting there for a long time, not moving. As a result, the dusty surface can become vacuum-packed, trapping some gas in between the dust grains.

The vacuum cleaner effect illustrated. The sub-surface pressure p2 is greater than the pressure at the center of the dust devil p1. From Bila et al. (2020).

At the center of a dust devil is a small dip in the atmospheric pressure (created by the convecting air inside the dust devil). So when a dust devil skitters over the hermetically sealed dust surface, the trapped gas pressure can launch the dust into the air, where the devil can pick it up.

But this vacuum cleaner effect is still just a hypothesis, so to test this idea, the Experimental Astrophysics group at University of Duisburg-Essen, experts in astrophysical dust experiments, set up a test chamber to mimic the martian surface under a low-pressure (1% of Earth’s) martian atmosphere.

The experimental set-up. The dust dispenser poured dust particles onto a fine mesh membrane, with ambient pressure p1 on one side, and higher pressure p2 on the other. From Bila et al. (2020).

They created a thin layer of small dust grains on a membrane, with a pressure differential across the membrane, to see if a small pressure differential could really lift the dust grains up. The answer is yes!

Dust grains of different sizes lofted by a pressure differential.

Now whether this experiment accurately replicates conditions on Mars is not totally clear, but some of the measurements made by the soon-to-be-launched and recently named Mars 2020 rover Perseverance may help to test the idea.

In addition to collecting geological samples for later return to Earth, Perseverance will collect high-resolution imagery of dust and mineral grains on the surface of Mars. It will also continuously measure meteorological conditions, which we know from past missions can reveal the presence of dust devils. So in addition to telling us about the possibility of past life on Mars, Perseverance may also help us test whether there are dust devil vacuum cleaners on Mars.

Naming the Mars 2020 rover. From https://www.youtube.com/watch?v=Muj4s7BCiMI.

A martian dust devil observed by the HiRISE instrument, in orbit around Mars.

Dust devils, whirling columns of fine particulates, have captured imaginations going back at least to the ancient Greeks, but their inner workings continue to confound and surprise scientists. Dust devils are common in arid regions on Earth, and on Mars, where “arid” doesn’t begin to describe the climate, dust devils are ubiquitous.

Especially puzzling, dust devils are better at lifting dust into the air than they ought to be. For example, in lab-simulated vortices, even when the winds are barely above a breeze, small dust grains seem to miraculously levitate and dance. So it seems that some force other than just wind must be important for lofting dust in devils. During our research group meeting today, we discussed a recent study by Gabriele Franzese and colleagues looking at one possibility: electric fields within dust devils.

As dust grains clatter around within the turbulent body of a devil, they can collide over and over again, which can transfer charge between the grains similar to the process that generates static electricity. And, for reasons that aren’t well understood, small grains like to collect negative charge.

Since small grains can be more easily lifted than large ones, small, negatively charged grains end up at higher altitude than large grains in dust devils, resulting in charge separation and a electric field. In the same way static electricity can lift small pieces of paper, these electric fields can draw in more dust grains and help explain the surprising ability of devils to lift shrouds of dust.

From Figure 5 of Franzese et al. (2018), the electric field associated with a dust devil.

For their field study of active dust devils, Franzese and colleagues set up meteorological equipment in the deserts of Morroco and left it there, steadily measuring wind speeds, dust loading, and electric fields. As dust devils skittered past their instruments, they registered as dips and spikes in the data logs. After recovering the instruments and analyzing the data, Franzese and colleagues found more than 500 dust devils had visited their instruments over a three month time-span.

 

The dust devils displayed a wide variety of shapes, sizes, and, most importantly, electric fields. The picture at left shows the electric field measured for one particularly strong dust devil. In this case, the devil exhibited an enormous electric field, 12,000 V/m. For comparison, such strong electric fields usually seen within storm clouds. Franzese and colleagues show that the strength of field measured for a devil correlates one-to-one with the amount of dust within the devil, so it seems likely electric fields do play some role in the lofting the dust.

Since humidity in the air can wick away static charge, dust devils on arid Mars probably exhibit even stronger electric fields than on Earth, which may help explain why martian devils are so much more common there: even faint whirlwinds manage to lift dust. These same electric fields could also present a danger to human exploration of Mars, though, potentially damaging sensitive electronics. Or at the very least, making a case of the Mondays even worse.