On September 27, 2021, a dust devil whirled past the Perseverance rover on Mars. Not only did the rover catch the dust devil with its cameras and weather sensors, but it also picked up the faint, eerie sounds of the dust devil with its microphone, the first instrument of its kind to record sounds on Mars.
“We were convinced that the microphone would give us a lot of new observations of atmospheric features on Mars that we had not been able to see before,” says Naomi Murdoch of the University of Toulouse, lead author of a Nature communication paper about the research, he said The edge. “And we are not disappointed!”
Rovers have been exploring Mars for decades with cameras, spectrographs and weather sensors, giving us a better picture than ever before of what the Red Planet’s environment looks like. Now sounds are added to the list. This combination of data will allow researchers to understand more about these dusty phenomena and the implications they could have for future robotic and manned missions.
Dust devils arise from atmospheric conditions common on Mars. “You have to have a really big temperature gradient between the ground and the air,” Murdoch explains. “So the ground gets really warm, and this heats up the air, which causes the air to start rising. And as the air starts to rise, things start spinning, and that’s when you get this whirlwind-like movement.
This process also takes place here on Earth, but what is striking about dust devils on Mars is how large they can grow. The dust devil recently discovered by Perseverance was 25 meters wide and 118 meters long (82 feet by 387 feet), which puts it squarely in the average size zone for dust storms on Mars. But they can also grow much larger, as dust on Mars can be whipped up in massive global dust storms.
“One of the big problems we have right now is that we can’t predict dust storms very well,” Murdoch said. And that has implications for everything – from trying to spacecraft land safely on Mars to try keep dust from vital solar panels to keep robot missions going. Current models in particular struggle to predict the major global dust storms, and that seems to be because it is difficult to model the forces that lift dust from the planet’s surface.
Researchers know that forces such as wind shear and dust devils can pull dust from the surface, but there are plenty of open questions about exactly how this happens. One particularly strange finding is that while dust devils are common in the Jezero crater where Perseverance is located, they are particularly rare in Elysium Planitia, the area the InSight lander is in, and it’s not clear why.
That’s where the microphone comes in. “What our microphone is sensitive to is the wind speed in particular. So when we see gusts of wind, we see a big increase in the sound amplitude on the microphone, just like when you’re on the phone and it’s windy,” explains Murdoch. “We use that background noise to study the wind.”
With so many science instruments on Perseverance taking time, the windows available for microphone recordings are only a few minutes long each. To maximize their chances of detecting a dust devil during these brief periods, the microphone team timed those windows for periods in the afternoon, when dust devils are most active. Each window is only a few minutes long and they only had eight windows a month – so it was a combination of careful planning and a healthy dose of luck that this recent dust devil was captured.
In addition to picking up data on wind conditions, the microphone also detected the sound of small impacts: some dust particles hitting the area around the microphone and making a perceptible ping. By counting each of these impacts, the researchers could see how dense these particles were in the dust devil — a measurement that no other instrument has been able to make, and which could help model how dust devils lift particles from the surface.
This research is just an early example in the burgeoning field of using acoustic data in planetary exploration. For a planet or moon with an atmosphere, acoustic instruments can collect data at a high sampling rate, allowing observations of rapidly changing events such as wind gusts compared to instruments such as wind sensors, which operate on timescales of a few seconds.
“Our sampling rate is much higher than any of the mainstream weather sensors you’ll find on planetary rovers and planetary landers,” Murdoch said. “With a microphone, it’s a bit like using a microscope. We look at what happens on these very short timescales.”
There is also the human fascination that comes with hearing the sounds of another planet. NASAs playlist of Perseverance sounds allows everyone to experience the sounds of the Red Planet, from the whistling of the wind to the humming of the rover itself, all made possible by SuperCam’s microphone.
“The bandwidth of our microphone is exactly the same bandwidth as that of the human ear, so the sounds we hear are not modified in any way. They’re the sounds you’d hear if you were on another planet,” Murdoch said. “And that’s really cool.”