The Sounds of Mars, the Silent Planet, Making Some Noise

The Sounds of Mars, the Silent Planet, Making Some Noise

The mic of the Franco-American instrument SuperCam has picked up the sounds of the red planet which travel less quickly than on Earth. Listening also allows a lot of science.

If humans arrive on Mars one day, it will be difficult to welcome them with a brass band. The planet – and especially its atmosphere – does not lend itself to that. Pressure conditions (170 times lower than on Earth), as well as the composition of the atmosphere (96% CO2, 0.04% on Earth), have disastrous effects on acoustics. A brass band which would produce 95 dB on Earth would produce 20 dB less on Mars (if it could play outside), i.e. the equivalent of a loud conversation. What’s worse, the sound carries less and very quickly diminishes. If you were listening to the band from 8 m away, it would be as if you were 65 m away on Earth.

Different Speeds Depending on High and Low Notes

Finally, and surprisingly, sound does not travel at the same speed in the Martian atmosphere, depending on its frequency: high notes which diminish most quickly, travel faster than low notes. It’s as well to say that our Martian brass band would be inaudible, muffled and, above all, discordant!
This knowledge about the acoustic behaviour of the Martian atmosphere is derived from analysis of recordings made by the microphone placed on the American Perseverance rover of NASA. Developed by ISAE-Supaéro in Toulouse, this sensor was built into the SuperCam instrument, constructed in France under the authority of CNES (National Centre for Space Studies).

Positioning of the SuperCam mic on the Perseverance rover.

Positioning of the SuperCam mic on the Perseverance rover.
Credit: Cité de l’espace/NASA – JPL/ISAE-Supaéro

During the 60 years that artificial probes have been travelling through the Solar System, they have been sending back images, optical or radar, and measurements from a variety of sensors, most of them electromagnetic. However, capturing sound remains a rare endeavour. Since sound is only a pressure wave in an atmosphere, the bodies where it can be perceived are limited. Apart from Earth and the giant planets, only Venus, Mars and Titan have compatible atmospheres.

First sound recordings

The first attempt is already 40 years old. In March 1982, a mic on the Soviet probes Venera 13 and 14 enabled an extrapolation of the wind velocity on the surface of the planet Venus, but those few minutes of hyper-saturated sound that were collected do not represent the acoustic ambience which must prevail there. Almost 23 years later, on 14 January 2005, the European probe Huygens used a mic during its descent into the atmosphere of Titan. This was supposed to pick up the possible rumble of storms in the atmosphere of Saturn’s main moon, but it only transmitted the sound of the wind in the rigging of its parachute.

Two previous attempts on Mars came to nothing. The mic mounted on the Mars Polar Lander probe in 1999 was lost when it crashed without transmitting any data. The experiment was tried again on the Phoenix lander in 2007, but the mic was never activated because of risks of interference with another instrument.

This panorama of Mars by Perseverance shows the tracks of wheels left by the rover. Images have long been a primary source of scientific data, while helping the general public to take an interest in the exploration which produced them. Now, sound has an increasingly important role to play.

This panorama of Mars by Perseverance shows the tracks of wheels left by the rover. Images have long been a primary source of scientific data, while helping the general public to take an interest in the exploration which produced them. Now, sound has an increasingly important role to play.
Credit: NASA/JPL-Caltech/MSSS

“For a long time, the presence of a microphone on a probe was considered to be more of a communication operation,” explains Sylvestre Maurice from IRAP (Institut de recherche en astrophysique et planétologie de l’Université de Toulouse 3, Paul Sabatier), co-investigator of the Supercam instrument. “It was difficult to convince the people in charge that we could get real scientific results.”

Sound on Mars for Science

The five hours of sound recordings collected in a year on Mars enabled this perception to change. SuperCam’s mic was designed to pick up sounds audible to the human ear (from 20 Hz to 20 kHz) and helped to confirm what had been suspected: Mars is a very silent planet, to the point that at the beginning scientists thought their instrument was not working. It must be said that natural sound sources are rare: there is only the wind. The rover, on the other hand, is quite noisy when it is moving, as is its companion the Ingenuity drone-helicopter.
Above all, the clicks of the SuperCam laser on the rock offer a short and perfectly defined signal. The NASA video below has on its soundtrack several noises recorded on Mars, particularly those from the Perseverance rover’s operation and the crackling caused by SuperCam’s laser.

Among the results of these measurements, it appears that on Mars, the speed of sound is lower than on Earth, where it reaches 340 m/s, and depends on the frequency: 240 m/s for low notes, below 240 Hz and 250 m/s above that. On our planet, this differentiation only exists in the stratosphere, stresses Sylvestre Maurice.
Above all, the study of these sounds and their spread is very instructive for studying the low Martian atmosphere, which is characterised by a significant difference in temperature as we move away from the surface: up to 40°C of a difference between 0 and 2 m of altitude. There are therefore major turbulences which could, through the sound they make, be analysed with a precision 1000 times greater than up to now.

The evidence is now there of the value of these very miniaturised and low energy-intensive instruments. “The only thing they consume is the telemetry for sending back the data collected,” said Sylvestre Maurice. They will therefore proliferate on coming missions, on Mars, Venus and Titan, and let us learn about the sound ambience of these distant worlds, of which up to now we have only images.

From 5 April 2022, Cité de l'espace in Toulouse will offer its visitors to the Martian Ground a realistic overview of the rovers (reproduced life-size and mobile) which roam the red planet. The sounds on Mars will be shared with the public. The Martian Ground was produced in collaboration with CNES, CNRS, the University of Toulouse III Paul Sabatier, IRAP, OMP, with the support of Comat.

From 5 April 2022, Cité de l’espace in Toulouse will offer its visitors to the Martian Ground a realistic overview of the rovers (reproduced life-size and mobile) which roam the red planet. The sounds on Mars will be shared with the public. The Martian Ground was produced in collaboration with CNES, CNRS, the University of Toulouse III Paul Sabatier, IRAP, OMP, with the support of Comat.
Credit: Cité de l’espace / Olivier Sanguy