Microphones on Mars are for the first time revealing the sounds of another planet.
Microphones on NASA's Perseverance rover have recorded soundscapes revealing an audio environment on Mars quite different from that on Earth. In an unprocessed recording taken two days after landing, you’ll hear a faint thump followed by gusty winds blowing on the microphone.
The thump may come from something moving inside the rover, but most of the noise the microphone picks up comes from the wind, says Alex Stott (University of Toulouse), a member of the large international team that published analysis of the Martian recordings published online on April 1st in Nature. The group is using the soundscapes to study Mars, its atmosphere, and their interaction with the rover.
Theorists had modeled the acoustics of the Martian atmosphere before, but measurements to test the models were lacking. It was not for the lack of trying: NASA's Mars Polar Lander carried microphones but crashed in 1999, and the Phoenix Lander carried microphones when it landed successfully in 2008, but technical problems prevented them from working.
Third try’s the charm: The two microphones on Perseverance started working soon after it landed on February 18, 2021. One on the mast of its SuperCam instrument was turned on the day after landing, after the mast was deployed; it sits 2.1 meters above the ground. The second rests a meter above the ground on the Entry, Descent and Landing Camera and was turned on the second day after landing. In those positions, they pick up natural sounds from Martian winds as well as noises from the rover.
Sounds of Mars
The soundscape is sculpted by the propagation of sound waves, which depends on the pressure and composition of the gas they travel through. The air on Mars has a pressure less than 1% that of Earth’s atmosphere and is comprised of some 95% carbon dioxide, so it’s no surprise that sound behaves differently on the two planets. The differences combine to damp down sound waves about 100 times more on Mars than on Earth.
Martian winds are turbulent, generating sound and forming eddies or whirls, which shrink in scale and gradually dissipate their energy, says Stott. The thin atmosphere fosters turbulent convection and rapid pressure variations during the day, but radiative cooling is strong at night, producing stable conditions that inhibit turbulence.
Perseverance can measure the speed of sound as it fires laser pulses to vaporize rock several meters away. The microphones record the ensuing shock waves produced by the laser’s heat as clicks. Clocking the time between laser pulse and click enables scientists to measure the speed of sound, confirming that it’s 240 meters per second (540 mph), slower than the 340 m/s (760 mph) on Earth.
Measurements also show that Martian sound has other odd quirks. One is that the speed of sound jumps about 10 m/s as the sound frequency increases above 240 hertz because of vibrational resonances in the carbon dioxide that makes up most of the Martian air. That difference may seem small, but it's near the center of the audible spectrum and large enough to seem off-key to the human ear. “A similar [jump] happens on Earth, but it happens at higher frequencies and in the stratosphere,” Stott says, so nobody notices.
Sound transmission in Martian air is weaker than on Earth and drops sharply with frequency, so Mars is a quiet place. NASA’s Mars2020 website thus demonstrates the differences by contrasting common terrestrial sounds to models of what they would sound like on Mars. The sound of city traffic, for example, would drop somewhat on Mars, but birdsong would drop to almost nothing.
(Just for fun, you can even record yourself and see what you'd sound like on Mars using those same sound models!)
Sounds of Perseverance
The microphones on Mars also collect data to monitor instrument performance and test rover equipment. To test prospects for generating oxygen on Mars, NASA is running an experiment called MOXIE. “It has a lot of pumps on it,” says Stott, and sounds from the machinery can help engineers on Earth monitor its behavior, like listening to vibrations from a washing machine. The microphones also proved sensitive enough to detect noise from the small experimental helicopter Ingenuity when it became the first aircraft to fly on another planet.
“We use sound as a new way of exploring,” Stott says. Mars represents the first opportunity we’ve had to do so after many tries. It may be a while before we can explore other bodies in our solar system in the same way.
Saturn's largest moon Titan has a nitrogen-methane atmosphere with a ground pressure about 50% higher than Earth's. Ground pressure of the superheated carbon-dioxide atmosphere of Venus is more than 90 times that of Earth. But upcoming missions to these worlds, including Dragonfly to Titan and two orbiters and an atmospheric probe to Venus, will not include microphones just yet.