More than a dozen spacecraft have successfully landed on Mars since the Soviet Union’s Mars 3 lander touched down in 1971. Over the past 50 years, these landers and rovers have taken images, measured radiation, sampled Martian soil, and collected numerous other observations. But none had listened to the Red Planet until NASA’s Perseverance rover deployed the first operational microphones on Mars after landing in Jezero Crater in February 2021.
Perseverance, its microphones, and a bit of serendipity have now led to another first, as scientists report recording the first audio of a dust devil on the Red Planet. The turbulent air column swept directly over the rover on 27 September 2021. Combined with concurrent images taken with the rover’s SuperCam camera and other sensors, scientists were able to gain new insights into the Martian atmosphere and the planet’s dust cycles and sketch out the dimensions of the dust devil itself, which rose 118 meters into the sky and was almost 25 meters wide.
“We really hit the jackpot with this encounter.”
“We really hit the jackpot with this encounter,” wrote Naomi Murdoch, a researcher with the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO) in Toulouse, France, in an email to Eos. “The camera was looking in the perfect direction, all of the meteorology sensors were on, and the microphone was also on.” The microphone is switched on for only a couple minutes eight times a month, so the recording was serendipitous.
Murdoch is the lead author of a study detailing the dust devil, which was published today in the journal Nature Communications.
Dust devils—swirling vortices twisting around upward flowing columns of air—are common on Mars. They are part of the planet’s dust cycle, according to Murdoch, which is connected to the global Martian climate. Dust devils lift dust into the air and could contribute to the larger regional and even global dust storms that periodically darken the Red Planet, cooling daytime temperatures. “These dust grains are electrostatically charged, and they don’t want to get lofted into the atmosphere,” said Christopher Edwards, a planetary scientist at Northern Arizona University who was not involved with the study. “So what people have proposed is that dust devils are one mechanism to kind of get these things up.”
The mini storms can have large impacts on surface missions. “If there is too much dust lifted, it can damage instruments,” Murdoch said. However, having some dust lifting is important for cleaning off the solar panels on leaders and rovers. NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander, operating on the planet since November 2018, will soon end because too much dust has collected on its solar panels.
A better understanding of wind speeds and the number of particles swirling in Martian dust devils could help scientists and engineers plan for future Mars research.
“With this dust devil recording, we have demonstrated that the microphone can directly observe the process of dust lifting and characterize the conditions under which such lifting occurs,” Murdoch said. The researchers were even able to count the number of dust grains that struck the rover as the dust devil passed. “To date, no other instrument has been able to quantify wind-blown grain fluxes on Mars.”
“This is a really cool study,” said Joseph Battalio, a planetary research scientist at Yale who was not involved in the study. “I hope, at the very least, this will be the first of many from Perseverance and that they will create this nice, big database, and we can better understand this process of dust lifting.”
From Spectroscopy to Meteorology
Microphones were included on NASA’s Mars Polar Lander (1999) and Phoenix (2008) missions, but the Polar Lander failed upon landing, and the Phoenix microphone was never switched on because of fears it might interfere with other instruments. Perseverance’s multiple microphones “are, therefore, the first to function and record sounds on Mars,” Murdoch said.
The microphone that caught the dust devil wasn’t originally intended as a device for studying Martian meteorology, at least not primarily. The microphone is part of the rover’s SuperCam, which, mounted on the rover’s mast, uses a laser to blast rocks and measure their chemical composition.
“It’s called laser-induced breakdown spectroscopy,” or LIBS, said planetary scientist Roger Wiens, the principal investigator for the SuperCam instrument and a professor of Earth, atmospheric, and planetary sciences at Purdue University.
“Not only does it give us those compositions, but the laser flash gives a little shock wave that actually blows the dust off of that particular spot in the rock, so that we can do a dust-free measurement.” By using a microphone to listen to the “zap” of the laser, the SuperCam can ensure the laser is optimally focused. Changes in the sound can also help determine how hard a rock might be, Wiens said.
Wind Blowing in the Mic
“It’s just the way it would sound to us.”
But the team at ISAE-SUPAERO that designed and built the SuperCam microphone knew it could help scientists study the Martian atmosphere in new levels of detail, according to Murdoch. “The microphone is sensitive to the wind, and so we can use the strength of the signal from the microphone to determine the wind speed,” she said. “Also, because the microphone measurements are at higher frequency than any other meteorological instruments, we are able to ‘see’ the very rapid wind fluctuations that other instruments cannot measure.”
The SuperCam microphone detected peak wind speeds of 8 meters per second as the dust devil passed directly over the rover. A recording of the event revealed two periods of low-frequency winds as the leading and trailing walls passed, separated by a quiet period when the rover was in the eye of the vortex.
Because the microphone is sensitive to the range of frequencies audible to the human ear—around 20 hertz to 12.5 kilohertz—the recording sounds remarkably like a dust devil on Earth, according to Wiens, who said he experienced quite a few terrestrial devils in his youth in the panhandle of Oklahoma. “It’s just the way it would sound to us,” he said. “It’s just great. It is so sort of humanlike, in a sense.”
Tuning In to the Speeds of Sound
Capturing the sounds of a dust devil was not the first audio revelation made possible by the SuperCam microphone. The instrument recorded the sounds of the Ingenuity helicopter, the small rotorcraft that landed on Mars along with Perseverance, from about 70 meters away.
“The propagation of sound on Mars was not necessarily easily determined from theory,” Wiens said. “The theory basically predicted that we would really not have a chance of hearing it. And we did, we heard it. So we were all just shocked and astonished and happy.”
The helicopter recordings ultimately helped researchers determine that there are two speeds of sound on Mars; low-pitch sounds travel at 240 meters per second, whereas higher-pitched sounds travel at 250 meters per second.
Listening Across the Solar System
The scientific successes of the SuperCam microphone all but guarantee that audio instruments will find their way onto future planetary science missions, Wiens said. Microscopic microphones weighing less than a gram each could be included on future missions without taking up much space, yielding important meteorological data. “So [they] could go on some very, very tiny missions,” Wiens said.
In addition to future Mars missions, microphones are already being discussed as important instruments for studying the dense, hot atmosphere of Venus and the thick, hazy atmosphere of Saturn’s moon Titan, he said. Current plans for NASA’s robotic rotorcraft mission to Titan, Dragonfly, include microphones, though the spacecraft won’t reach the distant moon until 2034.
In the meantime, Murdoch said, the research she and her colleagues published today should help establish audio sensing as an important part of instrument suites for Mars missions going forward. “In our team at ISAE-SUPAERO, we have been convinced for a very long time that microphones would open up a new and exciting field of atmospheric research,” she said. “This dust devil encounter clearly demonstrates just how valuable acoustic data are in planetary exploration.”
—Jon Kelvey (@Jonkelvey), Science Writer