A lonely rover toiling among the sands of Mars has now answered an age-old question: If lightning crackles on the red planet and no one hears it, does it still make a sound?
In recordings obtained by NASA's Perseverance rover, scientists have identified, for the first time, electrical discharges captured during Mars's wild dust events and whirling dust devils – not once, but 55 times over two Martian years of observation.
Crucially, the dusty weather in which these events appeared reveals the specific conditions required to generate electricity in the thin, bone-dry atmosphere of Mars – long suspected but never directly demonstrated until now.
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Lightning is thought to occur when turbulent conditions in an atmosphere jostle particles around, rubbing them together to generate charge. Eventually, so much charge builds up that it has to go somewhere, producing a discharge.
Lightning is ubiquitous here on Earth, generating some of the most wildly beautiful weather on the planet.
It's most strongly associated with clouds of water vapor, but wetness isn't required. Lightning discharges rage in the giant spumes of ash belched forth by volcanoes, for instance.
Even sandstorms – whose dry silicate particles are insulators, not conductors – can generate enough charge to trigger electrical discharges.
Scientists had proposed that similar mechanisms could be at play on Mars, even though its predominantly carbon dioxide atmosphere is much thinner and drier than Earth's.
After all, lightning has been recorded on Jupiter and Saturn, and tentatively detected on Neptune and Uranus, all of which also differ from Earth quite significantly (lightning on Venus remains an open question).
Models suggest that if electrical discharges occur on Mars, they're most likely near the surface, where atmospheric pressure is highest.
Fortuitously, we happen to have active rovers on the Martian surface – and one of them, Perseverance, has an instrument capable of detecting signs of lightning.
Led by planetary scientist Baptiste Chide of the University of Toulouse in France, a team of scientists analyzed data collected by Perseverance's SuperCam microphone, a device capable of recording sound data and electromagnetic interference.
They pored over 28 hours of microphone recordings, searching for signs of electrical discharge among the planet's swirling dust.
They found 55 events, 7 of which captured a distinctive electrical discharge signature in full. First, the instrument records a sudden electronic "blip" caused by electromagnetic interference as the electrical discharge tangles with the microphone's wiring. This blip is followed by a relaxation, or ringdown, lasting about 8 milliseconds.
The seven events Perseverance captured in full concluded with the acoustic signature of a tiny sonic boom created by the electric discharge heating and expanding the air around it – a little teeny tiny clap of thunder.
To ensure that the recordings do indeed come from miniature lightning discharges, the researchers used a SuperCam replica here on Earth. They recorded electrical discharges, replicating the profile of the Mars recordings.
Interestingly, a high concentration of dust in the atmosphere wasn't enough on its own to produce electricity.
The vast majority of the events – 54 of the 55 – occurred during the top 30 percent of the strongest winds Perseverance recorded on Mars during the study period, with most associated with the fronts of dust storms.
Meanwhile, 16 electrical discharges were recorded during Perseverance's two encounters with dust devils.
Based on six of the seven recorded thunderclaps, most discharges were tiny, just 0.1 to 150 nanojoules. The seventh acoustic event was the largest, clocking in at 40 millijoules – consistent with a discharge from the rover into the ground, likely associated with a charge buildup from particles rubbing against Perseverance itself.
An average bolt of cloud-to-ground lightning on Earth, by contrast, discharges about a billion joules. So lightning, as it manifests on Mars, is vastly different from lightning on Earth – but it does exist, and this has some interesting implications.
An obvious one is that it can help inform the design of future Mars exploration technology to protect it from the electrical discharges we now know occur. As they've done for early Earth, planetary scientists can now model more accurately the chemical reactions in Mars' atmosphere that would be mediated by electrical discharge.
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On a more speculative level, some current theories about life's emergence on Earth invoke lightning as a delivery system for the necessary ingredients to nudge a collection of molecules into biology. If lightning exists on Mars, astrobiologists can factor that into their estimates of the probability of life there, too.
"This study opens a notable field of investigation for the atmosphere of Mars… and motivates the development of new atmospheric models to account for electrical phenomena and their consequences in the Martian atmosphere," Chide and colleagues conclude.
The findings have been published in Nature.