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.
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