Without the bustle of humans and animals, Mars is nearly silent, save the gusty wind. So, the SuperCam team also decided to analyze the sounds coming from their own scientific equipment.
Part of SuperCam’s job is to zap nearby rocks with a laser and record the acoustic and optical signals that reverberate back, in order to determine hardness and chemical composition. The mic was also able to pick up farther away sounds, including the whir of the blades on NASA’s Ingenuity helicopter — the first aircraft to make a powered, controlled flight on another planet.
Interpreting the noise
Based on the noises coming from the laser and the helicopter, the researchers were able to determine that the speed of sound is much slower on Mars than it is on Earth. Moreover, different frequencies travel at different speeds. On Earth, sounds typically disperse at about 767 mph. On Mars, however, high-pitched sounds move at 559 mph and low-pitched ones move even slower at 537 mph. A similar phenomenon occurs on Earth as well, but only at frequencies outside our hearing range, so we don’t usually notice the discrepancy.
Chide says that this difference in speed between low and high frequencies would be most apparent at long distances. If you were to attend a concert on Mars, for example, and stand a few hundred feet away from the stage, you would receive the high frequencies a few milliseconds before the low ones, which would lead to sound distortion. The band would also have to play quite loud in order for the music to reach you because sounds on Mars don’t carry nearly as far as they do on Earth.
These otherworldly acoustic patterns are due to the peculiar atmosphere on Mars. Unlike Earth’s atmosphere, which is primarily oxygen and nitrogen, the martian atmosphere is 96 percent carbon dioxide (CO2), and extremely cold and thin. This combination of factors causes the CO2 molecules to vibrate in such a way that they absorb higher-frequency sounds, preventing those noises from traveling long distances.
Given that temperature has such a drastic effect on sound propagation, Chide and his colleagues suspect that the speed of sound on Mars will vary by season and even time of day. As the dust storm season approaches, the researchers anticipate their microphones will detect additional wind and temperature changes. SuperCam’s mic has unprecedented sensitivity, and can detect pressure fluctuations at scales 1,000 times smaller than ever before recorded on Mars. As a result, the mic can sense tiny eddies of wind called “micro-turbulence,” which whisk up dust — sculpting the planet’s surface, mixing chemicals and aerosols in the atmosphere, and controlling the temperature by absorbing solar radiation.
Beyond the theoretical
Prior to these sound recordings (which totaled just under six hours), models of the martian soundscape were purely theoretical and offered conflicting predictions. Andi Petculescu, a physicist from the University of Louisiana at Lafayette who studies acoustic properties of planetary atmospheres and was not involved in the research, considers the new study to be “a breakthrough” that will help develop more consistent acoustic models.
Given that the cold, thin, CO2-rich atmosphere renders Mars acoustically “unfriendly,” Petculescu was impressed that the researchers were able to obtain such clean signals from their microphones. He advocates equipping future spacecraft with acoustic sensors in order to learn more about the sound properties of other atmospheres. For example, Saturn’s largest moon Titan has a denser atmosphere and a wealth of background noises to record, including methane rainstorms. In 2027, NASA is scheduled to launch its Dragonfly mission, which will carry two microphones on its meteorological experiment, complementing previous recordings from the Huygens Probe.
“The study of acoustics is coming of age in planetary science, and we are learning new things about the way sound propagates in different atmospheres,” explains Roger Wiens, a co-author on the recent study and principal investigator of SuperCam.
The physics of sound will be important to understand in advance of humans setting foot on the Red Planet, he adds, in order to deduce information like wind direction and speed — and even gauge the health of scientific instruments by listening to the sounds they make as they operate. When humans finally get there, we’ll likely have to use devices like radios to communicate because sound won’t travel very far. As Wiens puts it: “You can't yell to somebody at the other side of the block.” Not to mention, you’d be shouting from inside your space helmet.