Mini-lightning strikes created by whirling dust devils on Mars have been detected accidentally by the microphone on board the Perseverance rover.
The chance discovery is direct evidence of a form of lightning on Mars, researchers say in a report published in Nature. They describe how the rover's microphone picked up signs of electrical arcs just a few centimeters long, which were accompanied by audible shockwaves.
"There's been a very big mystery about lightning on Mars for a long time. It's probably one of the biggest mysteries about Mars," says Daniel Mitchard, a lightning researcher at Cardiff University in the United Kingdom, who wasn't part of the research team but wrote an accompanying commentary for the journal.
"The key thing here," he explains, "is that we actually have a rover on the surface of Mars that appears to have detected something that fits our idea of what we think lightning on Mars would look like."
Besides Earth, flashes of lightning have been seen in the atmospheres of Jupiter and Saturn, and lightning has also been detected on Neptune and Uranus. But finding lightning has proven more elusive on our closest planetary neighbors — even though experimenters in the 1970s did lab work that suggested lightning should exist on Mars.
For example, when researchers put volcanic sand into a flask and pumped it down to Martian atmospheric pressures, swirling the sand in the flask created a glow that could be seen in the dark, says Ralph Lorenz, a planetary scientist with the Johns Hopkins University Applied Physics Laboratory.
The glow came from electrical charges caused by the friction between the bits of sand. If you had a bigger buildup of electric charge, he says, that could produce a more sudden discharge, like what happens with spark plugs in a car, or on a larger scale, lightning. After all, even on Earth, lightning can occur in turbulent clouds of volcanic ash.
"So there's no reason that blowing dust or sand on Mars shouldn't become electrically charged," says Lorenz.
Recently, he and some colleagues were reviewing audio picked up by the Perseverance rover, a car-size robot that's been trundling around on the red planet since 2021. It's got a microphone, and a few years ago scientists reported hearing the sounds of a whirling dust devil passing over the rover.
Besides the wind and the hiss of the dust, Lorenz says, there was a brief sound of a snap or crack in the middle of the encounter. "We just assumed it was a big sand grain or a small gravel grain just, you know, hitting the structure," he says.
But not too long later, one of their team members attended a science conference and heard a talk about atmospheric electricity. "I thought that if there were discharges, we could hear them. And then, I remembered this recording," says Baptiste Chide, who is with the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France.
So he did some experiments here on Earth, using an electrostatic generator, to see how electric discharges would affect the microphone. What he saw was the same signals that had been captured on Mars; there was a distinctive pattern of a brief electrical interference followed by the acoustic signal of a shockwave.
Fifty-five such events were picked up by the microphone over two Martian years, the researchers say, and the sparks were usually associated with dust devils and the fronts of dust storms.
The electrical arcs would feel and sound like strong static electricity sparks, says Chide. If an astronaut was on Mars, it might be possible to see them, although "small discharges are hard to see in strong sunshine, and it's the sunniest times of day that have most dust devils and maybe most of the strong discharge events. That said, some events were at night," he says.
The researchers think it's important to study this atmospheric electrical activity to understand the hazards it could pose to future robotic or human missions. While most space hardware is designed to be robust, they note that the Soviet Mars 3 mission landed during a dust storm and only operated for about 20 seconds on the surface before suddenly and mysteriously ending its transmission.
"Something changed in 20 seconds," says Lorenz. "Could it have been an electrical discharge event? I don't think we can rule that out."
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