Hear the wind on Mars

wWhen I first heard it, it sounded like a short, shrill quote. A temporary lull in an outside event captured by an old video camera. A sound slide on a windy afternoon anywhere on Earth.

The voice was actually blowing wind through the microphone. But the wind is on another planet. Wind on Mars. The recording, released by NASA in December 2022, marks the first time humans have heard sound made by Mars.

Scientists have known for years that the surface of the Red Planet is alive with winds, creating spiral dust devils and giant storms that sweep across the planet. But until now they had to watch these events unfold in frustrated silence. “It’s as if you’ve experienced something with one of your senses for a long time, but you’ve never experienced it with another person,” says Robert Wiens, professor of planetary sciences at Purdue University. “Now we have a second sense experience.”

Wiens is one of the authors of the recent paper describing NASA’s new earworm¹ As well as the principal scientist in the NASA Perseverance rover’s suite of observing instruments, collectively known as SuperCam, designed to gather new information about Mars.

The interesting thing about winds on Mars is that they are actually very gentle.

To conserve resources, the rover’s microphone rarely listened—it ran for less than 85 minutes during the entire first year of the mission. But on Sept. 27, 2021, at 11:02 Martian local time, he happened to be navigating one of his short listening laps when a dust devil, about 387 feet tall, swept through not just nearby but the rover itself. and the microphone onAlmost At the level of a human ear,” Wens points out, not only the devil’s sides and calm eyes, but also the sound of his dust.

“It’s one thing to hear a gust of wind, but to know it’s part of this pattern of dust devils that we’ve seen for many years on Mars was a dream come true,” Wiens says.

sWhen our rovers are equipped with lasers, ground-penetrating radar, and X-ray fluorimeters, it may seem like a fairly low ound channel to collect data from our solar system. But to Wayne’s ears, the audio is rich with information that would otherwise be hard to come by.

“In the recording, we can actually hear the grains of sand hitting the rover,” Wiens says. “The wind itself tends to make a lower note, and then the grains of sand strike a higher pitch.” The scientists used these separate sounds to estimate the density of dust grains in the entire dust devil.

There is one voice in the recording that he and his colleagues are still struggling to decipher. “There’s one kind of loud throb in the middle,” he says. It could either be a grain of sand hitting the microphone directly – or an electrostatic crackle of charge that occurs in dry dust clouds.

The full weather station was also running on Perseverance at the time of the dust devil’s passage, so the scientists were able to match the audio recording to temperature, pressure, wind speed and direction. Notes Alexandre Stott, a co-author on the paper and a postdoctoral researcher at the University of Toulouse, writing in Nature Portfolio’s Astronomy Community.² Exactly 199 years before this Martian storm was recorded, on the same day in September, French linguist Jean-François Champollion reported that he had deciphered the Rosetta Stone. “In a similar fashion, the Perseverance rover uses multiple techniques and measurements to decipher the Martian atmosphere,” Stott wrote.

wWhen we hear the wind on Earth, what we hear—aside from how it affects the things around us (leaves rustling against each other, for example)—is essentially the jumble of particles of the atmosphere flowing themselves when they are derailed by the structure of our ears. (We can shape this sound by turning our heads to or away from the wind—or perpendicular to it, which creates different amounts and types of turbulence around our ears.) The same physical dynamics, it turns out, work with a microphone on Mars. “It makes this turbulent sound as it flows through the corners of the box,” Wiens explains of the detected Martian wind.

The interesting thing about winds on Mars is that despite the menacing appearance of the dust storms they generate, they are actually very gentle. With atmospheric pressure only 1 percent of Earth’s, even the planet’s most powerful storm, at 60 mph, carries no pressure and will likely feel like little more than a light jolt to us. (Providing reassurance that setting for The Martian—in which a violent windstorm causes so much damage that it basically sucks in the world — you’ll never encounter any real operations there.)

Sound is extremely rare in the universe.

This lower density of gas molecules is likely the reason why dust devils are so frequent on Mars. Just like on Earth, Martian molecules like to flow from high to low pressure. These pressure differences arise when there are large temperature differences. Because the air molecules on Mars are so few and far between, they are unable to retain heat from the sun as effectively as the denser molecules here in our atmosphere. So there is a dramatic temperature gradient over the ground — often extending 35 degrees Fahrenheit by several feet. “So it can be mildly warm near your feet, and it can be freezing on your eyelashes,” Wiens says. These hyperlocal temperature differences create just the right conditions for the uplift of swirling dust devils.

As languid and dusty as it is, this wind is actually a useful collaborator. Dust on Mars contains ultrafine particles, just the kind that can coat the solar panels that power much of the surface-based science. So a storm here and there can help undo these essential ingredients. Which added an unexpectedly long lifespan to some of humanity’s missions. In January 2004, NASA’s Spirit and Opportunity rover landed on Mars with a life expectancy of about 90 days. At the time, “we didn’t know the dust devils were going to come and clean those baseboards,” says Wiens. But the two rovers swarmed the bustling Martian surface until 2010 and 2018, respectively. However, not all regions of the planet are full of breezes. The InSight lander, which reached Elysium Planitia’s flat plain in late 2018, was decommissioned in late December 2022 because accumulating dust eventually blocked sunlight.

HArth is a noisy planet. But without life, the sounds here would be limited to the sounds of wind, water, and the occasional geological event. On Mars, the only sound you’re likely to hear is the wind. (Most other acoustic studies on Mars have been of man-made noise: plasma laser bolts, spinning helicopter blades.) Recognizing sound on other planets can help scientists understand more about how these pressure waves can be used in future research.

But sound is extremely rare in the universe. It needs fairly tightly packed particles – like those in the atmosphere – to spread out. “So the moon isn’t good for that,” Wiens says. Nor is Mercury. Even supernovae are essentially silent. When I ask what he would choose if he could hear anything in the galaxy, he pauses for a moment to look at places that might have sound.

Finally, he landed on Titan, one of Saturn’s most attractive moons. Perhaps that was no coincidence. NASA is currently building a helicopter, called the Dragonfly, that will begin its journey to Titan in just a few years. The Dragonfly will be equipped with microphones for careful listening, and maybe not just for the wind. Scientists believe that Titan has very cold liquid methane on its surface. “So if we are really lucky, we will hear the sound of the liquid, and also the sound of the wind. But who knows,” Wiens thinks.

For now, Perseverance will continue to listen, a few fleeting moments at a time, on Mars. Wiens says any new sounds it picks up will bring new insights into the atmosphere and physics beyond our planet. Because driving all of this, he told me, is not just gathering information for future base camps on Mars, but also “curiosity about how the physics of the universe works.”

Katherine Harmon Courage is the Deputy Editor-in-Chief Nautilus. Follow her on Twitter @KHCourage.

Main image: NASA/JPL

References

1. Murdoch, N. (ed.). et al. The voice of the Mars Dust Demon. Nature Communications 137505 (2022).

2. Stott, A.; Listen to the dust devils on Mars. astronomycommunity.nature.com (2022).