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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.
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| A bright, circular dust devil appears in Melas Chasmas on 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.
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| A streaked Martian surface, possibly caused in part by dust devils |
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.
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| A dust devil spotted in Amazonis Planitia in April 2001 |
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.
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| Mars dust devil and its shadow |
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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