The Perseverance rover captured the world’s imagination when it recorded sounds from the surface of Mars shortly after its arrival on the red planet in 2021. It recorded sounds of the Martian wind, as well as the noises it made itself, and it even managed to capture the sounds of the Ingenuity helicopter in action. Now, scientists have analyzed these recordings to learn about how sound propagates on Mars, and found that the speed of sound isn’t constant there — it depends on the sound’s pitch.
One of the challenges of recording sounds on Mars is that because the atmosphere is so thin there, scientists were unsure if it was going to be possible to record sounds at all. The atmosphere is made up mostly of carbon dioxide, which tends to absorb sound waves as well. So the fact that the microphones on Perseverance were able to record Ingenuity from a distance of 80 meters was a surprise and a delight.
But this means that the recordings which are available tend to be quiet. “Mars is very quiet because of low atmospheric pressure,” said coauthor of the study Baptiste Chide of Los Alamos National Laboratory in New Mexico in a statement. “But the pressure changes with the seasons on Mars.” That means we can expect changes to the sounds recorded in future. “We are entering a high-pressure season,” Chide said. “Maybe the acoustic environment on Mars will be less quiet than it was when we landed.”
The strangest finding from the study is that the speed of sound on Mars is variable. Here on Earth, the speed of sound is 767 mph. But on Mars, the speed sound travels at depends on its pitch: Low-pitched sounds travel at about 537 mph, and higher-pitched sounds move considerably faster at 559 mph. This seems to be due to the extreme nature of the thin, cold atmosphere.
The recordings were made using Perseverance’s two microphones: One on its SuperCam instrument, used to hear the sounds made when a laser strikes its rock target to perform spectroscopy, and a second which records the sounds of puffs of air from the Gaseous Dust Removal Tool which clears rock surfaces of debris. The SuperCam microphone is the main one being used for the science work.
“The microphone is now used several times a day and performs extremely well; its overall performance is better than what we had modeled and even tested in a Mars-like environment on Earth,” said David Mimoun, professor at Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO) and lead of the team that developed the microphone experiment. “We could even record the humming of the Mars helicopter at long distance.”
The viability of researching sounds on Mars opens new avenues of research. “It’s a new sense of investigation we’ve never used before on Mars,” said Sylvestre Maurice, an astrophysicist at the University of Toulouse in France and lead author of the study. “I expect many discoveries to come, using the atmosphere as a source of sound and the medium of propagation.”
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