There’s a lovely scene in Interstellar—one of the best space movies in history; don’t argue with me—when the NASA pilot tasked with saving the day hands a pair of headphones to his fellow space traveler, a physicist, who’s having a difficult time on their perilous journey through space. When the physicist puts the buds in, he and the audience hear the distinct sounds of Earth: crickets, rain, the low rumble of thunder. The moment is meant to soothe the worried character, but it is also a reminder to all of us that our planet is very noisy and the rest of the solar system—the rest of the universe, really—is not. Sound, or the absence of it, is one more way our home fulfills Carl Sagan’s description of Earth as “a lonely speck in the great enveloping cosmic dark.”
So it might seem strange that anyone would want to send a microphone to deep space, but scientists and engineers have done just that. They put two microphones on a Mars rover and dispatched it to the red planet, dropping it off at the edge of a crater, near an area that was a river delta billions of years ago. The Perseverance rover arrived last year, stretched its wheels, and got to work examining interesting-looking rocks, with the mics listening in. Then the spacecraft beamed the recordings home, where scientists put on some headphones and pressed “Play.”
At first, they heard nothing. It was so quiet, in fact, that scientists thought the mics might be broken. Mars, it turns out, is extremely quiet. (NASA, describing it in the most melancholy way possible, says, “Mostly, a deep silence prevails.”) But the red planet is not totally silent, and as scientists listened longer, they heard the whirring murmurs of the Perseverance rover as it moved its robotic arm; the crunch of its wheels over russet ground; the sharp, crackling noise of its laser zapping a rock; the puff of a little instrument that blows the shavings away afterward. Even the hum of helicopter blades moving through the air—Ingenuity, a small experimental spacecraft in the distance, proving that robots can fly on Mars. And then, in between those distinctly mechanical noises, something else: the low rumble of a gentle wind.
After decades of dispatching robots with cameras to show us what Mars looks like, humankind has now extended one more form of perception to the red planet’s surface. We have decades’ worth of images, but to hear the natural sounds that go with them is “a really visceral experience,” Nina Lanza, a planetary geologist at Los Alamos National Laboratory who works on the Perseverance rover, told me. “We can at once feel that it’s extremely familiar in some ways, but also weirdly alien in ways that it’s a little hard to put your finger on,” she said. We’re tuning in to a world with its very own soundscape, shaped by its own distinct qualities.
On a scientific level, listening to another planet allows researchers to study how components such as atmospheric gases, surface pressure, and temperature combine to produce sound, an effect that scientists understand well here at home. The Mars tapes have allowed scientists, for the first time, to conduct a serious analysis of how sound works on Mars. They’ve confirmed their predictions that sound moves slower on Mars than it does on Earth, thanks to the red planet’s thin, super-cold atmosphere, which is mostly composed of carbon dioxide. Their analysis also showed that sound, particularly at high pitches, doesn’t carry very far at all. In such a low-pressure atmosphere, “you just have to work harder to vibrate these molecules farther, so things will be quiet,” Lanza said. The sounds on Mars change by the season too: The river delta where Perseverance resides is heading toward Martian autumn, and the resulting changes in atmospheric pressure should make the wind—and all the other noises—sound louder to the rover.
Based on the new analysis, I desperately wanted to know what song would, as the kids say these days, absolutely slap if it were played on the surface of Mars. (Not that any humans would actually be able to listen to it with their own ears; future astronauts will need to wear helmets any time they venture out into the Martian landscape.) Nothing too high-pitched, Lanza said. “A bass-heavy song—that will sound much better on Mars.”
David Mimoun, a scientist at France’s Higher Institute of Aeronautics and Space who led the development of the Perseverance rover’s main microphone, told me he has seen some resistance to miking up Mars missions because mission managers weren’t convinced the equipment would be scientifically useful. Microphones seemed superfluous, and past attempts to use them hadn't worked out. In 1999, a NASA mission bound for the planet’s south pole carried the first microphone to Mars, but the spacecraft failed to land and never called home. Another miked-up mission successfully touched down in 2008, but the listening instrument was never turned on, because engineers were nervous about potential electrical problems.
But Mimoun believed that mics could provide important data, and, as this mission has shown, he was right. Scientists have learned more about the Martian environment by listening to it with their own ears, filling in gaps in their understanding of our planetary neighbor. The sound of a laser zapping a rock changes depending on the composition of that rock, and scientists have analyzed the acoustic data to better understand the material that the rover has encountered, Lanza said. The mission’s engineers can also use the tapes to check on Perseverance’s health; if the rover starts making weird, unexpected noises, engineers might want to troubleshoot, like a mechanic searching for the source of trouble in a grumbling car. “We are not playing with toys,” Mimoun told me.
An admirable stance, given that NASA missions like Perseverance rely on taxpayer funding. But even if they didn’t provide scientifically important measurements, the microphones would still give us something of value. If the overwhelming majority of humans only get to live out our existence on this one planet, let’s at least hear what the other worlds in our cosmic neighborhood sound like, to the extent that technology and atmospheric conditions make it possible. I want to hear the rumble of a volcano on Venus; the methane lakes lapping on the shores of Titan, a moon of Saturn; gusts of autumn winds sweeping across the basins of Mars. Unlike, say, a high-resolution camera, a mic is a fairly low-lift effort, and it can uncover a burst of wonder.
Perseverance will spend its time on Mars exploring the dried-up river delta, investigating the rocks and sediments that scientists are most excited about—the kind that might hold the fossilized imprints of microbial life. A few billion years ago, Perseverance would have encountered a completely different scene. The spacecraft’s cameras would have seen a beautiful river, and its microphones would have caught the rippling noise of the water, and maybe even a subtle burbling at the surface—the tiny respirations of little creatures below. That Mars exists only in our imagination now. But modern-day Mars has its own soundtrack, and it’s worth a listen.