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The speed of sound is slower on Mars than on Earth, and most of the time, a profound silence prevails there. This is the main conclusion of the first analysis of acoustics on the Red Planet, carried out by an international team with Spanish participation based on recordings made by the rover Perseverance from NASA.
Most of the sounds in the study, published Friday in the journal Nature, were recorded with the microphone on Perseverance’s SuperCam, mounted on the rover’s masthead. Another microphone on the chassis recorded the beeps from the gDRT tool, which ejects chips from rocks that the rover, dedicated to studying the planet’s geology and searching for traces of past life, scrapes to examine.
[En el vídeo de Youtube sobre estas líneas puede escuchar los sonidos de Marte grabados por el Perseverance: el gemido mecánico del rover y el clic en un ligero viento marciano; el zumbido de los rotores del helicóptero Ingenuity y el golpe crepitante de un láser que rompe rocas].
«On the one hand, some aspects that were already predicted by the models have been confirmed experimentally, such as that the sound is weaker on Mars than on Earth -20dB weaker, that is, for the same equivalent source, it would be heard more faintly-; which dims much more with distance; or that the different frequencies have different speeds”, summarizes for this newspaper José Antonio Rodríguez-Manfredi, an engineer at the Astrobiology Center (INTA-CSIC) and head of MEDA (Mars Environmental Dynamics Analyzer), a meteorological station that will characterize the atmosphere and the Martian dust and traveling aboard the Perseverance.
The violin is ahead of the double bass
On Earth, sounds travel at about 343 meters per second. But on Mars, the low-frequency ones move at about 240 meters per second, while the higher-frequency ones move faster, at 250 meters per second.
“It was very interesting to see how, at a few meters, the highest frequencies go almost 20 meters per second faster than the lowest. Imagine what it would be like to hear a violin and double bass concert, in which we perceived the notes of the violin before those of the double bass, when they were played at the same time”, says the researcher.
As explained by NASA, these different speeds are an effect of the cold and extremely thin Martian atmosphere, composed mainly of carbon dioxide. Another effect is that sounds are carried over a short distance and higher tones are hardly carried at all. On Earth, the sound can diminish after about 65 meters; on Mars, it fails at only 8 meters, and the treble is completely lost at that distance.
The whistling of the wind
For Rodríguez Manfredi, “the most interesting thing is that these data allow us to analyze turbulence at scales below the centimeter (which perfectly complements the analyzes of turbulence at larger scales that we do with MEDA), which has led to the observation of turbulence phenomena which we did not expect.” For the first time, “it has been possible to hear how the wind sounds on silent Mars. Together with those high resolution images and videos that we have seen since we landed in February last year, we have more and more the feeling of ‘being there’, “he says.
But one of the most striking characteristics of the recordings was, precisely, the silence. “At some point, we thought the microphone was broken, it was so quiet,” says Sylvestre Maurice, an astrophysicist at the University of Toulouse in France and lead author of the study. That’s also a consequence of Mars having such a thin atmosphere.
“Mars is very calm due to low atmospheric pressure,” says Baptiste Chide of Los Alamos National Laboratory in New Mexico, also a co-author of the study. “But the pressure changes with the seasons on Mars.” That means that in the coming Martian fall months, Mars could get louder and provide even more information about its air and weather.
The drone of the helicopter
The SuperCam microphone also captured the twin spinning rotors of Ingenuity, the first powered aircraft to fly in a controlled fashion on another world. Spinning at 2,500 revolutions per minute, the rotors produce “a distinctive, low-pitched sound at 84 hertz,” says Maurice. The microphone works so well that they were even able to record the drone of the helicopter from a long distance. On the other hand, when SuperCam’s laser, which vaporizes chunks of rock from a distance to study their composition, hits a target, it produces sparks that create a high-pitched noise above 2 kilohertz.
These sounds also help scientists and engineers assess the health and function of the rover’s systems, in the same way that one might notice an annoying noise while driving a car. “It’s a new sense of investigation that we’ve never used before on Mars,” says Maurice. “I expect many discoveries to come, using the atmosphere as a sound source and propagation medium,” he adds.
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