Detecting Dust Devils on Mars
Jet Propulsion Laboratory
June 25, 2001
NASA's Mars Global Surveyor spacecraft recently caught sight of a dust devil dancing across the Martian surface. While it isn't the first of the tornado-like weather systems to be imaged, it is yet another reminder that Mars is an ever-changing planet.
Dr. Ken Edgett, a staff scientist at Malin Space Science Systems in San Diego, Calif., regularly tracks the dust devils and studies surface features. As the operator for the Surveyor's orbiter camera, he is one of the first to see fascinating images of the red planet. Dr. Edgett recently discussed the importance of dust devils and how they are transforming the look of Mars.
Q: First of all, what is a dust devil?
A: A dust devil is something that happens both on Earth and on Mars and looks somewhat like a mini-tornado. As with tornadoes, dust devils are spinning columns of air. Such a column is called a vortex--you might see the same effect when you let water run down a bathtub drain. Unlike tornadoes, dust devils aren't usually associated with storms.You typically see them on dry, sunny summer days when there is anywhere from a little to no breeze. You might say they look something like that Tazmanian Devil cartoon character - he spins 'round and 'round like a tornado when he moves.
A dust devil is actually a visual apparition of a wind vortex. If there isn't any dust on the ground, a vortex might still form but no one would see it. An example of a vortex without dust might be the scene in the film American Beauty where the plastic shopping bag is caught on videotape, spinning, spiraling, and dancing in the air. Dust devil vortices form when the air is fairly calm and the ground is heated by sunlight-this heats the air immediately above ground. Hot air rises up the outside of the spinning column, while cooler air descends through its middle. If a vortex passes over a dusty surface, it will pick up the dust and become a visible feature---a dust devil.
Q: Are Martian dust devils different than devils on Earth?
A: The Martian surface is so much more dusty than Earth because here we have rain to wash away most of the dust that settles out of the sky, but on Mars it doesn't rain. What's neat about the Martian dust devils is that they create "art". All that extra dust on the ground means that the dust devils leave tracks behind them where they have either picked up dust or disturbed the dust lying about on the surface. Most of the time these tracks are darker than the surroundings, but sometimes they are lighter---it just depends upon whether the surface under the thin coating of dust is brighter or darker than the dust itself. In some places on Mars, you can get hundreds of crisscrossing dust devil tracks, they make a pattern that some say resembles Jackson Pollack paintings, others say resembles something their 2-year old might do with crayons.
Q: How do you detect dust devils in the Global Surveyor data?
A: NASA's Mars Global Surveyor spacecraft has the Mars orbiter camera, that's actually three-cameras-in-one.
The two wide-angle cameras are used every day to take a global portrait of Mars; we use these to document changes in weather and frost patterns. The high-resolution camera, on the other hand, is used to see things up close. Its main purpose is to examine the geology and geomorphology---the shape of the landforms. Every once in a while, however, one of these cameras captures a dust devil in action. The high-resolution camera has a very narrow field of view---we can only see areas about 3 kilometers (1.9 miles) across, so no one really expected we'd ever see a dust devil with this camera. But we have.
More amazing to us, sometimes the dust devils are so big that you can see them with the wide-angle cameras meaning that they are wide enough to cover a couple football fields and stand several kilometers high.
Q: Did you know dust devils existed before Global Surveyor?
A: Vortices, though no one knows if they had dust in them or not, were detected by the meteorology experiments on the two Viking landers in the late 1970s. Similar detections occurred during the 1997 Mars Pathfinder mission. Some of these went right over the lander without causing damage. In the mid-1980s, researchers at Cornell University realized that some of the Viking pictures taken from orbit showed dust devils---from orbit what you see is usually a round, fuzzy-looking cloud that is casting a very long, columnar shadow. Researchers at the University of Nevada in Reno have suggested that a few of Mars Pathfinder's images also detected dust devils---these appear as actual columns of dust moving across the distant landscape.
When Mars Global Surveyor arrived in late 1997, we started seeing in our high-resolution pictures, thin narrow tracks running across the surface in some places, especially in the dust-covered areas. We suspected these were caused by dust devils, but we had no proof. Later on we were able to photograph some of these tracks a second time, and lo and behold, they had changed! In some cases there were more tracks, in others the original tracks had completely disappeared and were replaced by new ones, like some giant Martian Etch-a-sketch. "There can't be that many dust devils on Mars, can there?" we wondered. But indeed it's true that in some areas out in the deserts of Arizona and Nevada, people have recorded hundreds of dust devils over periods of only a few weeks. So you can have lots of dust devils, it happens on Earth.
In December of 1999, we got our first really good image that actually caught a dust devil in the act of creating one of these dark streaks. We were thrilled! After that, we've now seen dozens of cases where dust devils are creating streaks--usually dark streaks, but in early April 2001 we got one in Amazonis Planitia that was making a very faint bright streak.
Q: Speaking about that dust devil image in Amazonis Planitia, what was it like when you first saw that image?
A: I was tickled. It is always neat to see these dust devils in Global Surveyor images. We don't get them every day, so when we spot them they always create a buzz among the camera operations staff -- "come see what I found!" This particular dust devil from April was exciting because it isn't a round, fuzzy cloud. It's a twisted thing that casts a dark, bent shadow. Because the camera is looking straight down, the shadow is what gives the best impression of the shape of the thing. Bent dust devils like this aren't unusual, but neat nonetheless. The bending is caused by differences in the wind at different levels in the lower Martian atmosphere at the time the dust devil was moving across the landscape.
Q: If that same dust devil appeared on Earth would it do any damage?
A: This particular dust devil probably wouldn't cause any real damage, though in the April image it was clearly picking up dust and creating a faint, bright streak. If you went and stood in the way and the dust devil came over you, you'd certainly feel it, though. I once drove my car into a dust devil down along I-8 near Yuma, Arizona, and it definitely jiggled the car around. Do not try this at home! I should say, however, that there are documented cases on Earth where dust devils, as opposed to tornadoes, have caused some damage, including buildings, but usually this is not the case. Some stronger dust devils can have winds comparable to small tornadoes.
Q: Why do you study dust devils?
A: Dust devils are one of the mechanisms by which dust is moved around and redistributed on Mars. They are part of a process that is active today, meaning that Mars is not a "dead" planet but has things that are happening right now. Dust devils may contribute some of that dust that gives the sky its pinkish color. Dust devils also appear to play a role in cleaning off dark surfaces. For hundreds of years, people saw in telescopes that Mars' surface markings would change over the course of a year. In spring, areas would get darker and then get lighter in autumn. Once upon a time, it was thought that the "wave of darkening" was caused by springtime growth of vegetation. We now know that blowing dust is what causes these changes, and with Mars Global Surveyor's high-resolution images, it now appears that some areas darken because dust devils come along in the spring and summer months to clean dust off that accumulated in autumn and winter or, at least, that's what I think we're seeing with this camera.
Q: Do you have a favorite dust devil image?
A: Yes, it was taken October 14, 1999, in the western Daedalia Planum region. I just happen to like this one because it is very dramatic, though it is not creating a streak on the surface--they don't all make streaks. When it first came in, I was really moved by the experience of seeing an event that had taken place on Mars just a few hours earlier.
Q: What is it about dust devils that surprise you?
A: The fact that we can catch them in action! We see such a limited amount of the surface with the high-resolution camera, to date we've photographed less than 2 percent of the surface, yet we have seen dozens of dust devils and thousands of streaks that we think are produced by them. This must mean that dust devils are very common all over Mars. It surprises me that we even see their streaks at the top of the giant volcano, Olympus Mons, where the atmosphere is so thin---about 10 times thinner than at the Mars Pathfinder site--that you are almost in a vacuum. When you get lucky and catch a dust devil in one of these images, you get an eerie chill down your spine. These are dynamic things and you just happened to catch one at the time the spacecraft flew overhead. Dust devils give me a chill when I see them out in nature on Earth--they often seem to have a mind of their own. They might come toward you, then go away from you, as if teasing you. To see these on Mars gives me that same sense of being tantalized and teased. The dust devil you capture today is something that will not be there tomorrow.