The bizarre acoustics of Mars

Shortly after NASA’s Mars Perseverance rover landed and unlimbered its instruments, scientists turned on one of the more unusual of them – a microphone system – and for the first time listened in on an alien world: first to the wind, then to the sounds of the rover driving, and later yet to the Ingenuity helicopter on one of its early flights.

It was captivating. But it also seemed to be of limited scientific value – the type of thing you’d do because today’s microphones are so lightweight that there’s no real cost to including a couple on the rover (and being able to listen to it might help diagnose mechanical problems if they arose).

But it turns out there are a lot of other things you can do with microphones, once you have them.

The simplest is to measure the speed of sound. On Earth, says Baptiste Chide, a postdoctoral researcher at Los Alamos National Laboratory, New Mexico, US, that’s about 340 metres per second. In the thin Martian air, it was expected to be more like 240 m/s.

Sounds on Mars were also expected to be about 20 decibels lower than on Earth, Chide said at this year’s Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas. The Red Planet, he says, is also the “quiet planet.”

The difference, he adds, is particularly pronounced at higher frequencies – something borne out by the muted sounds first released by NASA.

To a large extent, Chide says, “you can only hear bass sounds on Mars”.

Weirder yet, due to strange properties of its thin carbon dioxide atmosphere, sounds on Mars travel at different speeds depending on their frequency, with those above 400 hertz (about that of the G above middle C) travelling 4% faster than those of lower pitch.

“Let’s imagine that we have this talk on Mars,” Chide said at the LPSC meeting. “My voice is between 200 hertz and 1000 hertz. You would receive all of the high-frequency tones before the low-frequency tones, so it might [be] distort[ed], not understandable.”

But things like that are mere curiosities. The true scientific value of the microphones came into play when Chide’s team realised they could use them to measure rapid fluctuations in the air temperature in the vicinity of the rover.

It works, he says, because air temperature is another factor that affects the speed of sound. And one way the rover produces sound is by firing its mast-mounted laser at nearby rocks.

The primary purpose of the laser is to vaporise tiny puffs of rock so that other instruments can determine their composition from spectroscopic analysis of the escaping vapor.

But the vaporisation process produces an audible pop when the laser hits the rock, and by timing how quickly that is heard after the laser is fired, Chide says, it’s possible to determine the temperature of the intervening air.

That makes the microphone a useful complement to the rover’s onboard weather station, because it gives much more rapid readings, repeating each time a laser pulse is fired, which can be as often as three times a second.