When
humans visit Mars, they'll have to watch out for towering
electrified dust devils.
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July 14, 2005: Ah,
Martian summer! Finally, the days are long, just like on dear
old Earth. And daytime highs rocket all the way up to a balmy
20°C (68°F) from the summer nighttime low of -90°C (-130°F),
meaning you and your fellow astronauts can warm up your machinery
earlier to get a good start on mining operations.
But
those warm daytime temperatures also bring alive the Martian
devils.
Dust
devils, that is.
Right:
Artist Nilton Renno's concept of a Martian dust devil. [More]
You
were caught in one just yesterday--and a devilishly terrifying
experience it was. This was no little Arizona desert whirlwind,
only a few tens of meters high and a few meters across and
past you in seconds.
No,
what hit you yesterday was a monster column towering kilometers
high and hundreds of meters wide, 10 times larger than any
tornado on Earth. Red-brown sand and dust whipping around
faster than 30 meters per second (70 miles per hour) dropped
visibility to zero, scouring your faceplate, driving dust
into every fold and wrinkle of your spacesuit. For 15 minutes
you huddled and endured the buffeting. The scariest part was
the incessant crackling and flashing of miniature lightning
bolts nipping at you and your rover, and the loud static on
your radio that prevented you from calling for help.
Could
this really happen? According to NASA's Vision
for Space Exploration, astronauts are going to visit Mars
in the decades ahead. And when they get there, dust devils
will be waiting.
"The
sand in the lower part of a Martian dust devil would be the
biggest hazard," says Mark T. Lemmon, associate research
scientist in the Department of Atmospheric Sciences at Texas
A & M University. "The atmospheric pressure on Mars
is only 1 percent that at sea level [on Earth], so you wouldn't
feel much wind against you. But you'd still be pinged by high-speed
material."
Moreover,
the moving dust and sand may become electrically charged,
to the point of "arcing to your spacesuit or vehicle,
and creating electromagnetic interference," adds William
M. Farrell of NASA's Goddard Space Flight Center.
Dust
devils on Mars form the same way they do in deserts on Earth.
"You need strong surface heating, so the ground can get
hotter than the air above it," explains Lemmon. Heated
less-dense air close to the ground rises, punching through
the layer of cooler denser air above; rising plumes of hot
air and falling plumes of cool air begin circulating vertically
in convection cells. Now, if a horizontal gust of wind blows
through, "it turns the convection cells on their sides,
so they begin spinning horizontally, forming vertical columns--and
starting a dust devil."
![see caption](images/dustdevils/diagram2.gif)
Above:
A simplified sketch of dust devil formation. [More]
Hot
air rising through the center of the column powers the whirling
air ever faster--fast enough to begin picking up sand. Sand
scouring the ground then dislodges flour-fine dust, and the
central column of hot rising air buoys that dust high aloft.
Once prevailing horizontal winds begin pushing the dust devil
across the ground, look out!
"If
you were standing next to the Spirit rover right now [in Gusev
Crater] in the middle of the day, you might see half a dozen
dust devils," says Lemmon. Each Martian spring or summer
day, dust devils begin appearing about 10 AM as the ground
heats, and start abating about 3 PM as the ground cools (Mars's
solar day of 24 hours 39 minutes is only 39 minutes longer
than Earth’s). Although the exact frequency and duration of
Martian dust devils is unknown, photographs from Mars Global
Surveyor in orbit reveal innumerable wandering tracks at all
latitudes on the planet. These tracks crisscross the surface
where dust devils have scoured away loose surface material
to reveal different-colored soil beneath.
Moreover,
actual dust devils have been photographed from orbit--some
of them as large as 1 to 2 kilometers across at their base
and (from their shadows) clearly towering 8 to 10 km high.
What
intrigues Farrell from having chased dust devils in the Arizona
desert, however, is the strange fact that terrestrial dust
devils are electrically charged--and Martian dust devils might
be, too.
![see caption](images/dustdevils/dustdevilmovie_med.gif)
Above:
A Martian dust devil spins across Gusev Crater just before
noon on March 15, 2005. Photo credit: Mars rover Spirit. [More]
Dust
devils get their charge from grains of sand and dust rubbing
together in the whirlwind. When certain pairs of unlike materials
rub together, one material gives up some of its electrons
(negative charges) to the other material. Such separation
of electric charges is called triboelectric charging, the
prefix "tribo" (pronounced TRY-bo) meaning "rubbing."
Triboelectric charging makes your hair stand on end when you
rub a balloon against your head. Dust and sand, like plastic
and hair, form a tribolelectric pair. (Dust and sand aren't
necessarily made of the same stuff, notes Lemmon, because
"dust can be blown in from anywhere.") Smaller dust
particles tend to charge negative, taking away electrons from
the larger sand grains.
Because
the rising central column of hot air that powers the dust
devil carries the negatively-charged dust upward and leaves
the heavier positively-charged sand swirling near the base,
the charges get separated, creating an electric field. "On
Earth, with instruments we've measured electric fields on
the order of 20 thousand volts per meter (20 kV/m),"
Farrell says. That's peanuts compared to the electric fields
in terrestrial thunderstorms, where lightning doesn't flash
until electric fields get 100 times greater--enough to ionize
(break apart) air molecules.
But
a mere 20 kV/m "is very close to the breakdown of the
thin Martian atmosphere," Farrell points out. More significantly,
Martian dust devils are so much bigger than their terrestrial
counterparts that their stored electrical energy may be much
higher. "How would those fields discharge?" he asks.
"Would you have Martian lightning inside the dust devils?"
Even if lightning wouldn't ordinarily occur naturally, the
presence of an astronaut or rover or habitat might induce
filamentary discharges, or local arcing. "The thing you'd
really have to watch out for is corners, where electric fields
can get very strong," he adds. "You might want to
make your vehicle or habitat rounded."
Right:
Dust devil tracks, photographed from orbit by Mars Global
Surveyor on Oct. 14, 2004. [More]
Another
consideration for astronauts on Mars would be "radio
static as charged grains hit bare-wire antennas," Farrell
warns. And after the dust devil passed over and was gone,
a lasting souvenir of its passage would be an increased adhesion
of dust to spacesuits, vehicles, and habitats via electrostatic
cling--the same phenomenon that causes socks to stick together
when pulled out of a clothes dryer--making cleanup difficult
before reentering a habitat.
Because
Martian dust devils can tower 8 to 10 kilometers high, planetary
meteorologists now think the devils may be responsible for
throwing so much dust high into the Martian atmosphere. Importantly
for astronauts, that dust may be carrying negative charges
high into the atmosphere as well. Charge building up at the
storm top could pose a hazard to a rocket taking off from
Mars, as happened to Apollo 12 in November 1969 when it lifted
off from Florida during a thunderstorm: the rocket exhaust
ionized or broke down the air molecules, leaving a trail of
charged molecules all the way down to the ground, triggering
a lightning bolt that struck the spacecraft.
"Early
sea navigators, like Columbus, understood that their ships
had to be designed for extreme weather conditions," Farrell
points out. "To design a mission to Mars, we need to
know the extremes of Martian weather--and those extremes appear
to be in the form of dust storms and devils."
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Author: Trudy E.
Bell | Editor:
Dr. Tony Phillips | Credit: Science@NASA
|