In Discover Magazine, an analysis of Mars audio reveals that the sounds on the red planet follow different rules

Almost 50 years ago, the Viking 1 lander captured the first image of Mars. Yet until recently, it remained silent. Scientists can now listen to the Red Planet's atmosphere and unique sound propagation patterns from over a million miles away with two microphones mounted on the Perseverance rover. Acoustics is still a relatively new field of study.

However, NASA has only ever equipped a few missions with dedicated microphones despite using sound waves to map the ocean floor, infer wind patterns, track lightning, and do other things. There were problems with the first Mars mission in 1999, and there were problems with the second, launched in 2007.

Perseverance touched down in early 2021, and researchers were able to gather a Martian soundscape then. NASA has posted the eerie audio recordings on its site. They published their findings in Nature on April 1. One of Perseverance's lead authors, Baptiste Chide, a postdoc at Los Alamos National Laboratory, says NASA had to be coaxed into adding the microphones.

A team at Los Alamos built an instrument called SuperCam with the help of planetary scientists Sylvestre Maurice and David Mimoun, and Chide finished his Ph.D. in Toulouse in 2020. The SuperCam is a microphone mounted on Perseverance's mast; the other microphone is in its side camera.

Chide was tasked with devising the scientific rationale that would convince NASA of the value of the microphones as a Ph.D. student. A risky bet, but a successful one: Chide's entire thesis project hinged on getting those mics into space. In addition to the sounds coming from the SuperCam team's scientific equipment, Mars is nearly silent without humans and animals.

To determine hardness and chemical composition, SuperCam shoots a laser at nearby rocks and records reverberating acoustic and optical signals. NASA's Ingenuity helicopter, which became the first powered and controlled aircraft on another planet, was also able to pick up sounds farther away.

On Mars, sound travels much slower than it does on Earth, according to laser and helicopter sounds. Different frequencies travel at different speeds, too. Sound travels at about 767 mph on Earth, but 559 mph on Mars and 537 mph on Earth. On Earth, similar phenomena occur, but at frequencies above our hearing range, so we don't notice them.

Long distances are the most likely place for this difference between high and low frequencies. The high frequencies would arrive a few milliseconds before the low ones if you were to attend a concert on Mars, for example. To hear the music on Mars, the band will have to play quite loudly. Sound on Mars doesn't carry nearly as far as it does on Earth.

In contrast to the atmosphere on Earth, which is primarily oxygen and nitrogen, Mars' atmosphere is 96 percent carbon dioxide (CO2) and extremely cold and thin, contributing to these otherworldly acoustic patterns. CO2 molecules vibrate so that higher-frequency sounds are absorbed, preventing them from traveling long distances.

Since temperature drastically affects sound propagation, Chide and his colleagues suspect the speed of sound on Mars will vary depending on the season. The researchers anticipate their microphones will detect additional wind and temperature changes during dust storms.

Despite being 1,000 times smaller than ever recorded on Mars, SuperCam's microphone has unprecedented sensitivity. Micro-turbulence is created by tiny eddies of wind - which sculpt the surface of the planet, mix chemicals and aerosols in the atmosphere, and control temperature by absorbing sunlight.

These sound recordings provided conflicting predictions about the Martian soundscape before they were purely theoretical. According to Andi Petculescu, a physics professor from Louisiana State University who studies the acoustics of planets, the new study is a 'breakthrough' that will lead to more consistent acoustic models.

As Mars has a thin, cold atmosphere filled with CO2, Petculescu is amazed at the researchers' ability to obtain such clean signals. To learn more about other atmospheres, he advocates equipping future spacecraft with acoustic sensors.

Saturn's largest moon Titan, for example, has a dense atmosphere and methane rainstorms to record. Dragonfly is NASA's meteorological experiment that will carry two microphones, complementing Huygens Probe's recordings from earlier missions. During SuperCam's development, Roger Wiens, a co-author of the recent study and a principal investigator, said that the study of acoustics is reaching a new level of sophistication.

A better understanding of sound will help humans get information such as wind direction and speed before they land on the Red Planet, he says. Listening to the sounds scientific instruments make can also help them determine their health.  To communicate there, humans will likely need devices like radios because sound can't travel very far. A space helmet makes it impossible to yell at someone across the street.