Sounds of Mars

By Fred Watson, Australia’s Astronomer-at-Large 5 July 2022
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NASA’s Perseverance rover, accompanied by a helicopter drone called Ingenuity, is seeking geological evidence of past life on Mars.

Together, these space probes have been a formidable team, with Ingenuity’s airborne camera allowing mission scientists to see beyond Perseverance’s immediate horizon and map a course to the most interesting rock samples.

It’s amazing Ingenuity can fly at all, given that Mars’s atmosphere is so different from Earth’s. Composed mainly of carbon dioxide, Martian air has a pressure that’s only 0.6 per cent of Earth’s, and an average temperature of –65oC. How can Ingenuity’s rotors grip such thin air? Because they are relatively large and rotate at high speed. We’ve heard their humming sound thanks to Ingenuity’s onboard microphones. 

Those mics have also captured ambient sounds such as gusting wind, the metal wheels of Perseverance squealing on Mars’s dry soil and the clicks made by the rover’s laser as it zaps rocks to study their chemical composition. Those zaps can be timed very precisely by Perseverance’s onboard clocks, as can the sound of the clicks in the microphones. Even though the distance they travel through Martian air is no more than 5m, this allows the speed of sound on the planet to be accurately determined. That has revealed some extraordinary results to the international team leading the research.

The speed of sound on Mars, for example, is 240m/second compared with Earth’s 340m/s. Sounds are also fainter on Mars than on our home planet because the atmosphere is so thin. But the biggest surprise concerns the so-called Mars idiosyncrasy, an odd feature predicted by atmospheric models, and now confirmed. 

Imagine playing piano on Mars, starting with the lowest notes and working up the keyboard. On Mars, at just below middle C the speed of sound suddenly switches from 240m/s to 250m/s for all higher notes. It means a listener would hear higher frequency sounds before lower frequencies, making music – and speech – difficult to interpret. 

Of course, astronauts on Mars will always have to wear pressurised spacesuits and communicate by radio… which is just as well.

Related: Do mirages exist on Mars?