On August 30, something moved in the background of a series of images taken by one of the Perseverance Rover’s Navcams. On a planet with no life (at least not large enough to see) and little in the way of meteorological activity, that’s unusual. The movement came from a dust devil seen over a rise known as Thorofare Ridge. It’s not the first time the Martian rover has spotted one of these events, but the shape is unusually sharp.
Dust devils are caused by the same processes on Mars as on Earth. Sunlight warms the ground, causing the air closest to it to rise, with cold air coming down to fill the space. Under the right circumstances, a vortex is produced and the air spins around as it rises. When the ground is covered in something light enough – dust or sand – it gets drawn up into the whirlwind, making it visible before eventually settling somewhere else as the devil loses power. They can make a major contribution to redistributing sand around Mars.
Despite the reduced heat of the Sun at Mars’ distance and the thinness of the atmosphere, dust devils are reasonably common on Mars. Last year Perseverance even filmed three dust devils dancing across the nearby plain at once. Later in the year, the rover picked up the sound of one. They’re not limited to Perseverance’s home at Jezero Crater – Curiosity has spotted them too, and there is even a possibility they occur on Titan.
Nevertheless, it’s a big (ok, decent-sized) planet and Perseverance can only film a tiny part of it. Despite recently setting the Martian speed record, the rover also can’t move at a pace that would allow it to get close to a dust devil once it spotted one, even if onboard systems could recognize them in time.
Instead, they tend to dance in the background of images like this until someone at NASA notices one.
Although the physics of dust devils has a lot in common with tornadoes and waterspouts, even on Earth they are usually fairly harmless. On Mars, the atmosphere is so thin a dust devil would struggle to damage a paper bag, let alone a hefty rover.
On the other hand, they are useful for understanding Martian atmospheric dynamics. NASA scientists have calculated this particular dust devil was 4 kilometers (2.5 miles) away and moving at a speed of 19 kilometers per hour (12 miles per hour). It’s also larger than it looks at an estimated 60 meters (200 feet) across.
The section we can see is 118 meters (387 feet) high, with the rest cut off by the top of the field of view. However, Mark Lemmon of the Space Science Institute said in a statement; “We don’t see the top of the dust devil, but the shadow it throws gives us a good indication of its height. Most are vertical columns. If this dust devil were configured that way, its shadow would indicate it is about 1.2 miles (2 kilometers) in height.”
That’s many times higher than most get on Earth, but Martian dust devils can grow very high. They mostly occur in the relevant hemisphere’s spring or summer (it’s currently summer at Perseverance’s location).