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 Tasmanian 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 sunlightthis 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.
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.
Dr. Ken Edgett of Malin Space Science Systems
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.
Next page: Before Global Surveyor
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