Why audio is the next frontier in Mars exploration

You’ve seen the photos of Mars from the surface: The red-tinted dust, the sharp mountains, the desolate rock formations. For as long as we’ve had rovers on the red planet, since Viking 1 rolled onto martian soil in the 1970s, we’ve been fascinated by imagery of this alien world.

But now there’s a new way for us to experience Mars from here on Earth, and that’s by listening to it. Since its arrival on Mars in February 2021, NASA’s Perseverance rover has recorded sounds of itself in action and, last month, it was able to record audio of the Ingenuity helicopter in flight for the first time.

More than just a fun way to imagine yourself on another planet, these audio recordings can teach us not only about the rover’s hardware and how it’s doing, but also about Mars itself.

To hear about how and why they recorded audio from another planet, we spoke to two experts overseeing the process: Roger Wiens, principle investigator for SuperCam, a remote-sensing instrument for analyzing the composition of rocks which sits on the top of the Perseverance’s mast, and David Mimoun, science lead for the SuperCam Mars microphone.

The power of sound

There has long been interest in recording sounds from Mars, Mimoun told us, in order to engage the public in planetary science. That’s because scientists were “very much aware of the power of sound on the human mind,” and they knew that helping people to hear the soundscape from another planet would be a special experience.

But it wasn’t easy to convince the mission operators, NASA’s Jet Propulsion Laboratory, to include a microphone on a Mars rover. That’s because every gram of weight is precious when you’re launching something into space, and the rover designers wanted to only include equipment that was very likely to work and to produce useful scientific data.

And the biggest problem was that most people believed that recording sound on Mars just wouldn’t work, because of the martian atmosphere.

The challenge of recording sounds on Mars

Not only is the martian atmosphere very thin (around just 1% of the density of the atmosphere on Earth, which makes it poor at transmitting sound waves), but its composition causes problems as well. Most of the atmosphere there is carbon dioxide, which “absorbs sound much more than the nitrogen that 80% of our air consists of here on Earth,” said Wiens.

“For both of those reasons, the sound just doesn’t do as well on Mars as it does on Earth,” he explained. That makes it a big challenge to hear distant sounds, and that meant it was an uphill battle to convince Mars mission managers to include a microphone on the off chance it would work.

When the SuperCam team was designing the instrument, there were only a handful of scientific papers predicting how sound propagation on Mars might work, and they weren’t optimistic about how far sounds waves would travel. Based on these theories, it seemed unlikely that SuperCam’s microphone would be able to pick up the sound of Ingenuity in flight.

“The predictions were pretty dismal,” Wiens said.

And of course you need a special microphone that can handle extreme swings in temperature while using minimal power. It needs to be outside the rover to hear the sounds of Mars, rather than inside the rover where it’s snug and warm. It has a tiny boom that protrudes out from the rover body so it can pick up more sounds, but even so, no one was confident it would be able to pick up sounds from as far away as 80 meters — which was the distance between the rover and the Ingenuity helicopter when the recording was made.

How a microphone tells you about rock structure

To get a microphone approved for their instrument, the SuperCam team had to have a scientific justification for its inclusion. As important as public outreach might be, it’s not usually enough of a motivation to justify taking up highly precious room and weight. But it turns out that audio recordings can be directly valuable to study the geology of Mars.

That’s because analyzing rocks on another planet is a complicated business. One way the SuperCam instrument in the Perseverance rover analyzes rock samples is through a technique called laser-induced breakdown spectroscopy, in which a high-powered laser is fired at a rock to turn into into plasma. The light given off by that plasma can then tell you what that rock is made of.

But when you blow up a rock, you inevitably destroy its structure, which means you miss some important information. That’s because, Mimoun said, “you can have rocks with the same composition but with various structures which are different.”

For example, there are types of clays called phyllosilicates that can have similar composition as basalts, but a different structure — because one type has been exposed to water, and the other has not. Mimoun pointed out that the “SuperCam is able to determine whether this is a basaltic rock, but by only using the laser light, it is not able to say if it has seen water.”

That’s where the microphone comes in. The SuperCam microphone was designed to listen to the sound of the laser hitting the rocks, which can tell you information about the sample like the hardness of the rock and whether its surface has experienced weathering. From that, you can learn about the structure of the rock as well as its composition.

The little microphone that could

During the Ingenuity helicopter’s fourth test flight, on April 30, 2021, while the helicopter team was preparing to take Ingenuity for an almost two-minute-long ride, the SuperCam team turned on their microphone. The helicopter and rover were a fair distance apart at the time (262 feet, or 80 meters), but they decided to try for a recording anyway.

“People did not believe that sound could propagate enough on Mars to be recorded. And that proved to be completely false with the recording of the helicopter,” Mimoun said. “So that’s fun! This is science.”

The team based their theoretical work off two studies, both of which predicted that at 80 meters, they’d be unlikely to hear anything at all. But that’s not what happened. The microphone picked up the clear sounds of the helicopter in flight, audible over the sound of the martian wind:

“It turns out that both of those theories were wrong, in our favor,” Wiens said. “So we were very happy about that. It really means there is a lot of good acoustic theory that is being tested and proven and tried by these measurements. This is the only time that we will have on Mars a discrete sound source that is that distance away from the rover.”

The idea was to use Ingenuity as a kind of beacon. It was putting out sounds from a known distance, so by recording the audio, researchers could see how the sound propagated from that location to the rover. “It can really tell us a lot more about how acoustics work on other planets, Mars especially,” Wiens said.

And sound gives you a lot of useful data you don’t get otherwise, particularly about the frequency spectrum. With the recording of Ingenuity, the researchers could detect the speed of the blades and see that frequency going down as the helicopter came down toward the ground. “You can’t actually capture the spinning of the blades even with the high-speed cameras we have on the rover,” Wiens said. “So we could tell them more about their blades than they could see with a camera using the audio.”

Plus, you can learn a lot about the martian atmosphere by listening — which is of particular interest to Mimoun, who works in atmospheric science. “It gives a lot of information about turbulence, about the atmosphere, the speed of sound. You can measure the wind speed at a very high frequency, which the usual sensor cannot,” he said. “So it adds a lot.”

Different modes of exploration

The fact that audio propagation on Mars is more favorable that we thought opens all sorts of possibilities for future exploration. When they were shooting at a rock, Wiens’ group noticed an echo in the data, and realized that they could use sound to do sonar on Mars. Like bats, future rovers could navigate in the dark using sound.

This would allow the exploration of new environments, such as the lava tubes on Mars, which are like enormous caves that are formed when lava flows under the planet’s surface. These mysterious passages beneath the surface have been identified as a potential place where life could have survived, or which could be used as a base by future human explorers to keep themselves safe from radiation.

As humans, we tend to be very focused on visual information, but we’ll be able to explore more areas of Mars and beyond if we consider the information that can be gained via other senses, too. “We, as humans, focus on certain senses, but if you look at other animals, they have optimized these other senses in ways we haven’t,” Wiens said. “And we can think about that for different modes of exploration, both on Earth and elsewhere.”

And there’s the importance of sound in hardware maintenance. If you’ve ever heard your hard drive making weird noises and decided you ought to do a backup of your data before the drive fails, you already know how valuable audio information can be when assessing mechanical equipment.

“It turns out we are actually getting recordings of different parts of the Perseverance rover,” Wiens said. “That includes the spinning of the turbo pump on the MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) instrument, and mechanisms in the Mastcam-Z stereo cameras, like the zoom and focus for the lenses and the filter wheels that give different filters. We can listen for all of those.”

Listening to all of this hardware can provide a baseline of what the rover sounds like when it’s healthy. If anything goes wrong in the future, experts will be able to compare the new sounds to this baseline and hopefully figure out what the problem is.

For example, the audio recordings of the MOXIE instrument, which is an experiment to create oxygen from the planet’s atmosphere, can act as an early-detection system for potential problems. “You can now hear oxygen being made on Mars!” joked Michael Hecht, principal invesyigator of MOXIE. “Mostly what we’re listening to is the compressor, which is sucking in air, and that’s what we were after. Because if you’re going to have a problem with a pump or a compressor, the first way you know it is because it sounds funny, right? You don’t need to be a rocket scientist to know that. So we want to hear it.”

The future of Mars audio

One of the great things about collecting audio information is how relatively cheap and easy it is to add a microphone to a mission. It can be a valuable source of data and there’s little cost or damage if it doesn’t work.

The researchers we spoke to agreed that there was potential for great scientific benefit from adding a tiny microphone to a mission, and Mimoun predicted that most future planetary missions, and almost certainly all future Mars missions, will have a microphone on board: “Now that people are convinced that it works, and it works quite well, it’s worth taking these few additional grams. I would bet a bottle of good champagne that any Mars mission will take a microphone now. I think it opens a new window.”

And then there’s the importance of engaging with the public — hearing the sounds of a place makes it feel so much more real and so much closer, and it’s a key way to make people feel involved in Mars exploration. Sound can be like nothing else for that.

“This is something which is engaging and brings people closer to what we do,” said Mimoun. “When we heard the very first sounds, it felt like Mars was closer.”

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