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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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