The Dust Devils of Mars Could Pack a Seismic Punch
Could the seismic fingerprint of dust devils detected on Earth be used to decipher the tiny tornadoes racing across the Martian surface?
During experiments on a dry California lake bed, seismologists were able to detect dust devils racing across the barren surface - the first time a seismic signal of these tiny atmospheric events has ever been recorded.
Now the researchers, whose work has been published in the journal Bulletin of the Seismological Society of America, are eyeing Mars to track down the seismic signals of the tiny tornadoes that zigzag across the Red Planet's surface.
Dust devils are fairly common on Earth; they pop up on open plains and deserts and seem to have a life of their own. Spawned from a thin, heated layer of earth, tiny vortexes of air can spiral upward into the cooler layers creating often impressive mini-tornadoes. These whirlwinds can pick up dust and other small debris, carrying it high into the atmosphere.
Though impressive on Earth, the undisputed king of dust devils is Mars. The Red Planet is constantly shaped by aeolian (wind-blown) processes, and in many regions dust devils are a common occurrence. Indeed, recent studies have shown that Martian dust devils, which can reach as high as several miles, play a key atmospheric role in transporting fine particles of dust from the surface, injecting it high into the atmosphere. This "dust cycle" could have implications for the planet's global climate.
Also, these whirlwinds change the albedo (reflectiveness) of Mars' surface - dark lanes of recently exposed regolith, or Mars "soil", are often left in the dust devils' wake - potentially altering the solar heating effect on the Martian surface, therefore impacting atmospheric temperatures.
Phantom Cleaners Mars missions, past and present, are very familiar with these ghostly whirling dervishes. NASA's Mars Exploration Rover Spirit became very used to seeing dust devils race around its area of scientific study in Gusev Crater before the mission was declared lost in 2010.
Spirit's sister rover Opportunity, which still roves on Meridiani Planum over a decade since landing, is indebted to Mars' dust devils - its dust-clogged solar panels have undergone numerous "cleaning events" where the updraft from these whirlwinds have blown over the rover, hoovering the dust away. These cleaning events are one of the contributing factors to the veteran rover's astonishing longevity.
From space, NASA's Mars Reconnaissance Orbiter is very familiar with the "Etch A Sketch" patterns created by swarms of dust devils, dark channels running haphazardly across bright plains and dune fields, crisscrossing one another, often converging to create vast, dark plains of disturbed dirt.
"While on Earth dust devils are generally just an occasional nuisance and meteorological curiosity," said Ralph Lorenz, of Johns Hopkins University in Baltimore, Md., in a Seismological Society of America press release, "on Mars, they are major agents of dust-raising, which is a major factor in the climate, and in the operation of solar-powered vehicles on Mars."
So we know that Mars is a windy world, capable of producing some of the most impressive dust devil specimens in the solar system, wouldn't it be great if we could detect them without the need of a robotic camera to be pointing in the right direction at the right time? They are, after all, random occurrences, we only really get to study their effects after they have come, gone and disturbed the dirt like a phantom leaf blower.
Seismic Wind-Print It just so happens that NASA is launching its next Mars lander next year, called the InSight mission. InSight will land on Mars to help us form a better idea about what lies beneath all that regolith. The lander will use a drill to bore through the topsoil, providing us with a first-ever look at heat-flow from the planet's interior up to 5 meters below the surface. It will also have a seismograph to study motions from within the planet. It's thought that, although the Red Planet is generally tectonically dead, "marsquakes" do occur and this will be our first opportunity to study them in depth.
And it turns out that InSight may also be a ready-made dust devil detector.
During their studies of dust devils on Earth, Lorenz and his colleagues planted a seismometer in the desert near the Goldstone Deep Space Communications Complex outside of Barstow, Calif. The area is remote, away from traffic and was fensed off to prevent wild donkeys and other wildlife from disturbing the dry lake bed. In the area surrounding the seismometer, the researchers also set up 8 air pressure gauges so they could correlate any rapid drops in pressure with a seismic signal.
When a dust devil forms, it creates a mini-low pressure region over the ground - the warm surface causes the air to rise and the air will start to rotate. Like an ice skater pulling her arms in while spinning on the spot, the spinning air will form a vortex, pulling air aloft (and any dust with it). This vortex will therefore be registered as a low pressure "blip" as it whirls across the surface.
Using this pressure sensor-seismometer combo, the researchers were able to detect 2 distinct dust devil pressure drops 10 minutes apart. They also registered seismic signatures as the surface layers were slightly distorted by the low pressure region the dust devils created. As the dust devil passed, the "sucked up" the top layers of desert. It turns out that the seismometer is crazy sensitive to any slight tilts in the ground created by this suction effect, registering a slope change of only 12 millionths of a degree.
Interestingly, according to the press release, a dust devil measuring just 10 meters in diameter "can cause a drop in pressure equivalent to removing the weight of a small car from the ground surface."
"So a large dust devil can cause a very significant change in the loading of the ground, and it is no surprise the ground deforms by a tiny amount," said Lorenz. "In essence, the dust devil sucks on the ground, pulling it upwards like a tablecloth pinched between thumb and forefinger. So the ground tilts away from the dust devil."
Now the researchers have correlated a drop in pressure related to the passage of dust devils with their seismic signature on Earth, they hope that analysis of seismic signals detected by InSight might reveal the passage of Martian dust devils. If we know what to look for, we might finally be able to get up-close and personal with these windy events on Mars and begin to understand just how much of an impact they have.
A large dust devil towers over the Martian surface on the plain of Amazonis Planitia, as seen in this High-Resolution Imaging Science Experiment (HiRISE) camera observation taken in 2012. The shadow indicates this dust plume reached a height of 12 miles although it was only 140 yards in diameter.
Ten years ago this week, NASA's Mars Reconnaissance Orbiter began a very special mission: to provide regular high-definition views of the Red Planet. Since arriving, it's spotted a lot of neat stuff -- spacecraft descending under parachutes, rovers on the surface and even dust devils. These are just a handful of some of the best pictures.
And by the way, if you have a neat location idea for MRO to photograph,
-- a public program for people to tell the spacecraft's
camera where to point.
NASA and the University of Arizona (which runs HiRISE) are used to precision operations. This was showcased spectacularly on Aug. 5, 2012, when the Curiosity rover landed on Mars.
safely to the surface of the Red Planet, starting a mission at Gale Crater that continues today. Previously in 2008, HiRISE also caught a view of the Phoenix lander under its parachute.
Several Mars spacecraft were on the lookout when comet C/2013 A1 Siding Spring gave the Red Planet a close shave.
was acquired from as close as 86,000 miles (138,000 kilometers) from the nucleus. That's very close in astronomical terms, the equivalent of flying a third of the way to the moon from Earth.
Twelve years after the Beagle lander was supposed to arrive at the Red Planet, the MRO spotted the long-lost spacecraft (which stopped communicating during landing). The image shows that the lander made it to the surface
. It also arrived within its landing target, a circle with a radius of roughly three miles (five kilometers).
While MRO's primary target is Mars, occasionally it has caught glimpses of the planet's moons (Phobos and Deimos). In this March 23, 2008 picture,
-- as well as a bunch of troughs and crater chains that are likely unrelated to the impactor that created Stickney. The gravity of Mars is expected to tear Phobos apart in about 100 million years.
The NASA Phoenix lander was expected to last three months on the surface after landing in May 2008, but actually made it to about five. Then the spacecraft fell silent as sunlight diminished for winter. NASA kept trying to hail it until this 2010 MRO image showing damage to the lander. It is believed that
after likely hundreds of pounds of carbon-dioxide ice coated the spacecraft.
This shows a fairly common feature on Mars
, which form when winds tend to blow in one direction. It gives researchers a sense of where the dominent winds were when the features were formed. MRO is able to track seasonal changes on dunes such as these, which are near the north pole and get frosty in the winter.
Look at the brown smudge on the right side of this picture, then move your eyes left until you see a small gray dot. That's the Opportunity rover on the surface, near Victoria crater! This picture was taken by MRO in the weeks after arriving at Mars on Oct. 3, 2006. If you look carefully to the left of Opportunity and "around the corner" of the dark feature to the left and below the rover,
. The spacecraft also regularly
Even though MRO is in orbit around the planet, it can spot fairly small features on Mars --
! Based on the size of the shadow, it is estimated this dust devil was more than half a mile (800 meters) high, and roughly 30 yards or meters in diameter. It was spotted in the late afternoon in the north, during a time when Mars was far from the sun.
This image from Coprates Chasma is a clear example of
, which scientists believe could be evidence of flowing liquid water on Mars. While the atmosphere of the Red Planet is believed to be too thin for water to survive on the surface for long, ice melting could create temporary water flows that leave this dark evidence behind.
One benefit of MRO being at Mars at a long time is it can track change, such as this 100-foot (30-meter) crater that popped up. Roughly 200 craters happen a year when space rocks careen into the surface, but the
. It was formed sometime between July 2010 and May 2012, between MRO imaging campaigns of the region.