Video: This is what it sounded like when a sand devil hit the Mars rover Perseverance. Source: NPG Press
Alien Sounds: The Perseverance rover recorded the sounds of a dust-laden mini tornado on Mars. The whispering encounter with the dust devil was recorded simultaneously by a camera and sensors. By combining the data, a research team was then able to more precisely determine the characteristics of the approximately 25-meter-wide air vortex. The information can contribute to the understanding of the atmospheric dust transport and is therefore also of importance for the exploration of Mars, the scientists say.
They run through the landscape like spooky hoses: On Earth, especially in dry regions, there are sometimes small air turbulences that make themselves felt through their cargo – they pull dust particles up and carry them with them. That is why the little relatives of tornadoes are also called dust devils. It has been known for some time that these atmospheric phenomena also swirl across the surface of our neighboring planet. They form when warm Martian air rises near the surface and begins to rotate. In the Jezero crater alone, the Mars rover Perseverance has spotted around 100 dust devils since landing in February 2021. Now, however, for the first time he has also recorded a specimen acoustically, reports the international research team.
The vortex passed directly over the rover on September 27, 2021 and was “overheard” by a microphone belonging to Perseverance’s “sensor head” equipped with various instruments. “We can also learn a lot from sound information. You can also get a sense of what it’s like on Mars,” says senior author Roger Wiens of Purdue University in West Lafayette. As he and his colleagues report, the microphone is not always switched on, but instead records the sound ambience at the location for about three minutes every few days. “Normal” wind noise was already recorded. The fact that the microphone was switched on when a dust devil whirled over Perseverance was a stroke of luck, emphasizes Wiens. At the same time, the encounter was also captured by the rover’s navigation camera and several other sensors, allowing for a more complex analysis of the event.
Perseverance Overhears a “Windy Alien”
A sequence of two wind noises and a light crackling can be heard in the audio recording. The first effervescence apparently arose when the vortex structure arrived at the microphone. After that, there was a short silence – it is due to the doldrums in the eye of the small tornado, the researchers explain. This was followed a few seconds later by another roaring sound caused by the oppositely directed airflow from the “rear” part of the sand devil on its way. In addition, the impacts of dust particles carried by the eddy could be heard, the scientists report.
The scientists were finally able to characterize the dust devil by combining the information with the camera recordings and the other sensor data as well as by modelling. It was about 25 meters wide, more than 100 meters high and was traveling at a speed of around 20 kilometers per hour. “The wind speed in it reached around 40 kilometers per hour, which roughly corresponds to what you can find in a dust devil on Earth,” says Wiens. “The difference, however, is that the air pressure on Mars is so much lower than ours. As a result, the winds are fast, but they generate only about 1 percent of the pressure that the same wind speed would have on Earth. On Mars, however, it is sufficient to throw particles into the air and create a dust devil,” says the scientist.
Insight into dusty dynamics
The results can now contribute to knowledge about the atmospheric dynamics of our neighboring planet, the scientists say: “A better understanding of dust uplift and atmospheric transport is key for an accurate simulation of the particle cycle and for the prediction of dust storms. Also, understanding is important for future exploration, as grain impacts are associated with the degradation of hardware on the Martian surface,” the team writes.
Favorable effects are apparently also possible, however, because wind movements can also whirl away dust that has been deposited. For example, breezes may have blown dust off the solar panels of other rovers, especially Opportunity and Spirit. “Rover teams experienced a slow decline in performance over several days to weeks, but then there was a jump. That was probably when the wind cleared the solar panels,” says Wiens. Clearly, however, one cannot hope for cleansing air vortices everywhere. “Just like on Earth, there are different weather conditions on Mars in different areas,” says Wiens. For example, no dust devil has been sighted in the region of Elysium Planitia, where the InSIght mission landed.
In order to gain further insights into the characteristics of these atmospheric phenomena on Mars, the scientists are now hoping for more strokes of luck during the course of the Perseverance mission: Additional microphone recordings of encounters with dust devils could enable comparative studies, the researchers say.
Source: Purdue University, professional article: Nature Communications, doi: 10.1038/s41467-022-35100-z