Twenty years' worth of imagery captured by two orbiters circling Mars has revealed raging winds on the red planet.
Wind on the barren planet would be invisible if it weren't for Mars' iconic red dust, which was caught in the wind's vortices, creating a phenomenon known as dust devils.
These tornado-like whirlwinds also occur on Earth, but the new catalogue of Martian dust devils, published this week in the journal Science Advances, shows that Martian dust devils seem to move much faster and are more abundant on a global scale, said lead study author Valentin Bickel, a Center for Space and Habitability fellow at the University of Bern in Switzerland.
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Researchers compiled the new publicly available catalogue by using images taken by the European Space Agency's (ESA) Mars Express since 2004 and ExoMars Trace Gas Orbiter since 2016.
They trained a neural network, or machine learning modelled on the human brain, to spot the vortices in the orbital data, then checked each one to create a map of 1039 dust devils across Mars, including examples atop ancient volcanoes and in open plains.
The team was also able to determine the direction of motion for 373 of the rotating dust-filled wind columns.
They determined that the Martian dust devils and their accompanying winds can move up to about 160km/h, much faster than any dust devils ever clocked by rovers exploring the red planet's surface.
"This observation implies that those winds are likely able to lift a substantial amount of dust from the surface into the atmosphere," Bickel wrote in an email.
"We found a new puzzle piece that helps us to better understand the Martian dust cycle: where, when and how much dust is lifted off the surface and is injected into the atmosphere."
Tracking the movement of dust on Mars is crucial for planning future robotic and human exploration of the red planet, experts say.
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A window into whirlwinds
Studying dust on Mars provides scientists with a way to model the planet's climate without actually being on the ground.
On Earth, rain helps clear dust from the air. But on Mars, dust can linger in the atmosphere for much longer once it's lofted by wind, travelling across the entire planet.
And once dust is in the atmosphere, it affects Mars' climate and weather. Dust prevents sunlight from reaching the surface, causing daytime temperatures to be cooler, and it insulates the planet to keep temperatures warmer at night.
Now, based on the new research, scientists suspect dust devils may play a larger role in lofting dust into the Martian air than previously thought.
Dust devils have long fascinated Bickel because they provide a window into the dynamics of Mars' atmosphere closest to the planet's surface – and they can be seen from orbit.
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"I think it is incredible that we are able to observe and track moving whirlwinds of dust on a different planet," Bickel said. "By observing dust devils from orbit we can learn many things, such as wind speed and direction, which would otherwise be invisible."
The team's analysis showed that while dust devils occurred across the planet, many were found in Amazonis Planitia, one of the smoothest plains on Mars that is covered with dust and sand.
Dust devils form when hot air near the surface rises and spins, picking up dust along the way, Bickel said.
"It appears that Amazonis Planitia offers the ideal conditions for dust devil formation, as it's a vast, very flat region that receives a lot of illumination during the summer," he said.
The team also determined that dust devils have seasonality, with intense whirlwind activity taking place during the spring and summer months in both the northern and southern hemispheres.
Typically, the vortices just last a few minutes and occur during daytime, between 11am and 2pm local time – similar to dust devils that occur in arid, dusty places on Earth in the summer.
An advantageous aerial view
Neither orbiter carries instruments that were designed to clock wind speeds on Mars. But Bickel and his team uncovered treasure within what might normally be thought of as a nuisance.
Both orbiters create images by combining views from different channels that look at Mars in specific colours or directions. For example, Mars Express can create a single image from nine image channels, and there can be delays of between seven and 19 seconds from one channel to another.
So if there is movement on Mars, such as a dust devil, it produces noticeable "colour offsets" in the final mosaic image – and a way to track the speed and motion of the dust devils.
Faster dust devils tend to travel in very straight lines, while slower dust devils wobble from left to right, Bickel said.
Previous measurements of dust devils on Mars showed that they typically had a sustained speed below 50km,/h with a rare maximum of 100km/h. But the latest data shows a much higher maximum for the dust devils as well as the winds surrounding them.
"These strong, straight-line winds are very likely to bring a considerable amount of dust into the Martian atmosphere – much more than previously assumed," Bickel said. "Our data show where and when the winds on Mars seem to be strong enough to lift dust from the surface."
But Mars' atmosphere is more than 100 times thinner than Earth's, meaning that even strong winds would feel more like a breeze to us. Without a substantial atmosphere, wind lacks force — but on Mars, it's just enough to pick up dust.
"A dust devil would certainly not be able to kick you off your feet," Bickel said.
Picking the perfect landing site
Dust remains a primary concern for missions heading to Mars. Suspended Martian dust can lead to planet-encircling dust storms, a phenomenon which ended the Opportunity mission in 2019. And a buildup of dust on the stationary lander InSight's solar panels put an end to that mission in 2022.
Sometimes, dust devils can be helpful. By chance, the vortices helped clear dust from solar panels on the Spirit rover in 2009.
Bickel said that new images from orbital data will continue to be added to the catalogue so it can serve as a resource for future mission planning.
"Our measurements could help scientists build up an understanding of wind conditions at a landing site before touchdown, which could help them estimate how much dust might settle on a rover's solar panels – and therefore how often they should self-clean," Bickel said.
Data from the new study is already being used to help determine the optimal landing site for ESA's ExoMars Rosalind Franklin rover, expected to touch down on Mars in 2030.
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