The speed of sound on Mars is constantly changing, bizarre study finds

Researchers have teased out the details of how sound behaves at various times and places on Mars — and the results are very different from what we are used to on Earth.

NASA's Perseverance rover on Mars carries several microphones. These devices, intended to study the properties of materials on the Red Planet, have picked up all sorts of additional sounds, including the eerie spluttering of Martian dust devils. 

Recordings have already shown that sound behaves peculiarly on Mars. For instance, noises below 240 hertz — roughly a piano's middle C — travel about 30 feet per second (10 meters per second) slower than higher-pitched sounds do. This is because carbon dioxide molecules, which absorb some of sound's energy at low frequencies, make up 95% of Mars' atmosphere. Such bizarre properties, if unaccounted for, could compromise communications on future Mars missions, particularly crewed ones. 

With this in mind, a team of scientists from French and U.S. institutions set out to study the speed of sound and its attenuation — its tendency to die down over distance — within the first 60 feet (20 m) of Mars' atmosphere. 

To begin, the team collated values of different parameters — including atmospheric pressure, temperature and chemical composition — at various spots on the Red Planet from the Mars Climate Database. Changes in these parameters can stretch or shrink sound waves, making these factors essential in predicting sound's properties. 

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The team calculated sound speed and attenuation at different points of time in the planet's year (which is about 687 Earth days) and in various spots across the Martian landscape, including mountain peaks and valleys. This approach was necessary because the underlying factors vary massively over space and time.  In the polar regions, for example, midday temperatures can fluctuate by 108 degrees Fahrenheit (60 degrees Celsius), and carbon dioxide levels by 30%, across seasons.