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.