Barely landed, a little over a year ago, the NASA robot sent us the first audio ever captured on Mars thanks to a microphone, at frequencies audible to the human ear, which previous missions had tried without success.

Behind a shrill sound produced by the vehicle, one could clearly hear a gust of wind.

The red planet, whose interplanetary probes have been sending us thousands of images for 50 years, was finally coming out of its "sound nothingness", welcomes the CNRS on the occasion of the publication of a study Friday in Nature.

This first capture revealed previously unknown turbulence regimes, recalls its main author, Sylvestre Maurice, scientific co-manager of the rover's SuperCam instrument, where the microphone designed by Isae-Supaéro in Toulouse is installed.

But this "passive" listening was not telling enough.

And since Mars is most of the time very quiet, it was necessary to use two "active" sound sources on board from Earth, explains to AFP this astrophysicist from Irap (Institute for Research in Astrophysics and Planetology) of the University of Toulouse.

Photo provided by NASA on June 25, 2021 of the Ingenuity helicopter, taken by a Perseverance camera on Mars Handout NASA/AFP/Archives

His team used the flights of the small Ingenuity helicopter, companion of the Perseverance rover, and lasers fired at the rocks to probe their chemical composition (a "clack clack" sound).

"We had a very localized source of sound there, between two and five meters away from its target, and from which we knew exactly when it was going to shoot", explains the researcher.

"Difficult Talk"

The speed of sound could be measured in situ: 240 meters per second, against 340 meters per second on our planet.

Nothing abnormal that it is slower, given the composition of the Martian atmosphere (96% CO2 vs 0.004% on Earth) and its very low pressure (170 times more than on Earth).

The surprise came from the sound of the laser and its... 250 meters per second.

"There I panicked a little! I said to myself that one of the two measurements was wrong because on Earth, near the surface, there is only one speed of sound", recalls Sylvestre Maurice.

There are, however, two speeds: one for the treble (the laser), the other for the bass (the helicopter).

In addition, "the attenuation of sound is stronger on Mars than on Earth, particularly the treble which is lost very quickly, even at a short distance".

This would make "a difficult conversation between two people separated by only five meters", according to the CNRS.

The researcher even dares the analogy with a concert: "On Earth, the sounds of the orchestra reach you at the same speed, whether low or high. Let's imagine on Mars, if you are placed a little far from the stage ... you're going to have a hell of a time lag".

The "scientific bet", that of having equipped a space mission with a microphone, is in his eyes successful.

Even if this new tool is only in its infancy: continuing to listen to Mars should help to better understand its atmosphere, which in the past resembled that of Earth and could have favored the appearance of life.

The analysis of the sounds of turbulence, such as these vertical winds called + plumes of convection +, will in particular "allow us to refine our digital models for predicting the climate and the weather" of Mars, anticipates Thierry Fouchet, of the Paris Observatory - PSL, one of the authors.

And other atmospheres, such as that of Venus or Titan, a moon of Saturn, could be the subject of sound investigations with this same type of instrument.

© 2022 AFP