NASA’s Perseverance rover first recorded Martian sounds on February 19, 2021, the day after its arrival. These sounds fall within the human audible spectrum, between 20 Hz and 20 kHz. They revealed that Mars is quiet, in fact so quiet that on several occasions the Perseverance scientists thought the microphone was no longer working. By studying the propagation of sounds generated by Perseverance itself, including the rover’s mechanical whine and click in a light Martian wind, the whir of rotors on the Mars helicopter Ingenuity, and the crackling strike of a rock-zapping laser, the researchers were able to accurately characterize the acoustic properties of the Martian atmosphere. They found that the speed of sound is lower on Mars than on Earth: 240 m/s, as compared to 340 m/s on our planet; however, the most surprising thing was that there are two distinct values for the speed of sound on Mars that are 10 m/s apart below and above 240 Hz, a unique characteristic of low-pressure carbon dioxide-dominated atmosphere.
“It’s a new sense of investigation we’ve never used before on Mars,” said Dr. Sylvestre Maurice, an astrophysicist at the University of Toulouse.
“I expect many discoveries to come, using the atmosphere as a source of sound and the medium of propagation.”
Most of the sounds in the study were recorded using the microphone on Perseverance’s SuperCam, mounted on the head of the rover’s mast.
The study also refers to sounds recorded by another microphone mounted on the chassis of the rover. This second microphone recently recorded the puffs and pings of the rover’s Gaseous Dust Removal Tool (gDRT), which blows shavings off rocks that the rover has scraped in order to examine.
The result of the recordings: a new understanding of strange characteristics of the Martian atmosphere, where the speed of sound is slower than on Earth — and varies with pitch (or frequency).
On Earth, sounds typically travel at 343 m/sec. But on Mars, low-pitched sounds travel at about 240 m/sec, while higher-pitched sounds move at 250 m/sec.
The variable sound speeds on Mars are an effect of the thin, cold, carbon dioxide atmosphere.
Prior to the mission, scientists expected the Martian atmosphere would influence sound speed, but the phenomenon had never been observed until these recordings were made.
Another effect of this thin atmosphere: sounds carry only a short distance, and higher-pitched tones carry hardly at all.
On Earth, sound might drop off after about 65 m; on Mars, it falters at just 8 m, with high-pitched sounds being lost completely at that distance.
The recordings from SuperCam’s microphone also reveal previously unobserved pressure variations produced by turbulence in the Martian atmosphere as its energy changes at tiny scales.
Martian wind gusts at very short timescales also were measured for the first time.
Maurice et al. find that the speed of sound is slower on Mars than on Earth and that, mostly, a deep silence prevails. Image credit: Maurice et al., doi: 10.1038/s41586-022-04679-0.
“One of the most striking features of the sound recordings is the silence that seems to prevail on Mars,” Dr. Maurice said.
“At some point, we thought the microphone was broken, it was so quiet. That, too, is a consequence of Mars having such a thin atmosphere.”
“Mars is very quiet because of low atmospheric pressure. But the pressure changes with the seasons on Mars,” said Dr. Baptiste Chide, a researcher at Los Alamos National Laboratory.
“That means, in the Martian autumn months to come, Mars might get noisier — and provide even more insights into its otherworldly air and weather.”
“We are entering a high-pressure season. Maybe the acoustic environment on Mars will be less quiet than it was when we landed.”
The team also studied what the SuperCam microphone picked up from the spinning double rotors of Ingenuity.
“Spinning at 2,500 revolutions per minute, the rotors produce a distinctive, low-pitched sound at 84 Hz,” Dr. Maurice said.
“On the other hand, when SuperCam’s laser, which vaporizes bits of rock from a distance to study their composition, strikes a target, it makes sparks that create a high-pitched noise above 2 kHz.”
The results appear in the journal Nature.
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S. Maurice et al. In situ recording of Mars soundscape. Nature, published online April 1, 2022; doi: 10.1038/s41586-022-04679-0
