The Perseverance rover is currently navigating the uneven surface of Mars, and it's hardly a peaceful journey. Bangs, pings, and rattles from the rover's six wheels can be plainly heard in NASA's tracks. The truck is noisy because of the metal wheels and the fact that it is travelling over pebbles.
These audio recordings can inform us about the rover's hardware and how it's doing, and about Mars itself. They're more than simply a fun way to envision yourself on another planet.
Roger Wiens, principal investigator for SuperCam, a remote-sensing device that rests atop the Perseverance's mast and analyzes the composition of rocks, and David Mimoun, science lead for the SuperCam Mars microphone, explain why they need to record the sounds of Mars.
Challenge of Recording Sounds on Mars
According to Mimoun, there has long been a desire to record sounds from Mars to engage the public in planetary research. That's because scientists were "well aware of the impact of sound on the human mind," and they realized that assisting humans in hearing the sounds of another planet would be a unique experience.
However, persuading the mission's managers, NASA's Jet Propulsion Laboratory, to include a microphone on a Mars rover was not simple. That's because when you're flying something into space, every gram counts. The rover's designers wanted to include only technology that was very likely to work and produce significant scientific data.
The primary issue was that most people assumed that recording sound on Mars would be impossible due to the planet's atmosphere.
The Martian atmosphere is incredibly thin, which makes it poor at transmitting sound waves. Hence, its structure also poses issues. The majority of the atmosphere is carbon dioxide, which absorbs sound much more than the nitrogen that makes up 80% of our air here on Earth, Wiens said.
"For both of those reasons, the sound just doesn't do as well on Mars as it does on Earth," Wiens told Digital Trends.
It's challenging to detect distant sounds because of this, so persuading Mars mission managers to add a microphone on the off chance it may function was a tough sell.
Only a few scientific studies described how sound propagation on Mars would work when the SuperCam team was developing the equipment, and they weren't optimistic about how far sound waves might travel. According to these beliefs, SuperCam's microphone would not pick up the sound of Ingenuity in flight.
"The predictions were pretty dismal," Wiens added in the same Digital Trends report.
Of course, you'll need a special microphone that can withstand large temperature changes while using very little power. To hear the noises of Mars, it must be outside the rover, rather than inside, where it is cozy and warm. It includes a small boom that protrudes from the rover body to pick up more sounds, but no one expected it to be able to pick up sounds from as far away as 80 meters - the distance between the rover and the Ingenuity helicopter at the time of the recording.
Can a Microphone Tell You About Rock Structure?
The SuperCam team needed a scientific reason for including a microphone in their device to get it authorized. Even though public outreach is vital, it is rarely enough of an incentive to justify taking up valuable space and weight. However, it has been discovered that audio recordings can be used to understand the geology of Mars.
This is because examining rocks on another planet is a difficult task. A laser-induced breakdown spectroscopy technique is one method the SuperCam equipment on the Perseverance rover studies rock samples. It's a high-powered laser that is shot at a rock to turn it into plasma. The light emitted by that plasma can then be used to determine the composition of the rock.
When scientists blow up a rock, they're inevitably destroying its structure, which means they're missing out on some crucial information. That's because, as Mimoun explained, they can have rocks with the same composition but different structures.
For example, phyllosilicate clays might have a similar composition to basalts but a distinct structure because one has been exposed to water while the other has not. The SuperCam can determine whether this is a basaltic rock," Mimoun said. But it can't say if it has seen water because it only uses laser light.
This is where the microphone enters the picture. The SuperCam microphone was created to listen to the sound of the laser hitting the rocks, which can reveal information about the sample such as the rock's hardness and whether or not its surface has weathered. You can learn about the structure and composition of the rock by doing so.
The Microphone That Could
The SuperCam team turned on its microphone during the fourth test flight of the Ingenuity helicopter as the helicopter team prepared to take Ingenuity for a nearly two-minute journey. Although the helicopter and rover were separated by a considerable distance (262 feet or 80 meters) at the time, they opted to attempt a recording regardless.
The team's theoretical work was based on two studies, both of which projected that they would be unlikely to hear anything at 80 meters. That, however, was not the case. Over the sound of the martian wind, the microphone picked up the crisp sounds of the chopper in flight.
The Ingenuity team's plan was to use Ingenuity as a sort of lighthouse. It was emitting sounds from a known distance, and researchers were able to see how the sound travelled from that position to the rover by recording the audio. "It has the potential to tell us a lot more about how acoustics work on other planets, particularly Mars," Wiens added.
And sound provides you with a wealth of information that you wouldn't otherwise have access to, particularly within the frequency spectrum. The researchers were able to determine the speed of the blades and saw that frequency drop as the chopper approached the ground thanks to Ingenuity's recording. Even with the high-speed cameras we have on the rover, Wiens added that you couldn't capture the spinning of the blades. Therefore, NASA could use the audio to tell them more about their blades than they could see with a camera."
Furthermore, listening can teach you a lot about the martian environment, which is very interesting to Mimoun, who works in atmospheric research.
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