Your mind’s eye can already paint a picture of what Mars looks like. You will probably conjure up the image of a barren landscape of scattered boulders and pebbles under a reddish-brown sky. It is familiar by now.
But what about our other senses?
Thanks to recent work by scientists, we now have a better idea of how we would hear the red planet. The Perseverance rover made sound recordings using one microphone on its SuperCam, and a second microphone mounted on its chassis. A team then analyzed these recordings and released the results in Nature.
Silence of the birds
One thing that stands out about Mars is its silence. Mars is really quiet.
The red planet’s silence flows from the quality of its atmosphere. To understand how an atmosphere affects sound, we need to recall how a sound wave propagates.
Sound is actually a pressure wave. Unlike light, sound waves must travel through a medium. They cause the atoms and molecules of the medium to vibrate back and forth as the sound passes. This compresses the medium.
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We know a few things about Mars’ atmosphere in comparison to our own on Earth. First of all, it is thin — just over half a percent the pressure of the Earth’s atmosphere at sea level. But it is also colder, and it is made up mostly of carbon dioxide, whereas nitrogen is the principal ingredient of the Earth’s atmosphere.
As a sound wave travels, it needs to shake the molecules within the atmosphere in order to keep itself going. Because Mars’ atmosphere is colder, these molecules are already moving slower. Because the atmosphere is less dense, there are fewer molecules available to collide with and transfer the sound wave. And because molecules of carbon dioxide have more mass than molecules of nitrogen or oxygen, the atmosphere is heavy. All of these factors work together to make sound drop off faster with distance. On Earth, sounds drop off at about 65 meters of distance. On Mars, they drop off after just eight meters.
The atmosphere will also affect the pitch of the sound. Certain high-pitched noises such as the chirping of birds would almost completely vanish. This is mostly due to the special properties of a carbon dioxide-rich atmosphere: It would attenuate, or absorb, sounds with a higher pitch.
A concert on Mars
When Perseverance’s microphones were listening, they did not hear much — just a bit of Martian wind.
However, when the little Ingenuity helicopter took off, Perseverance could hear the whir of its wings beating the air. Also recorded were the pings of the rover’s Gaseous Dust Removal Tool, which cleans off dust and rock shavings once the rover has examined them. These sounds are useful to analyze, because the team knows exactly when they were emitted and how far away they were.
Their analysis led to some surprises.
At first, the team thought something was wrong — the sound from the laser was reaching the microphone much quicker than expected. Was something broken?
The real reason surprised everyone — sounds travel at different speeds on Mars. High-frequency sounds travel faster than sounds at lower frequencies. This means that the high-pitched ping of the laser traveled faster than the helicopter’s lower-pitched whir.
On Earth, sounds travel at the same speed. On Mars, the difference in speed between these sounds was about ten meters per second. Carbon dioxide was previously thought to affect the speed of sound, but to actually hear its effects was surprising.
These unique properties of Mars’ atmosphere would make it sound quite foreign. Imagine listening to a concert on Mars, sitting in a row a bit far from the stage. The timing of the music would sound wrong. The high pitches from the flute and the harp would reach your ears before the notes from the tuba and the cello. In addition, the flute and harp would sound a lot quieter than they should, since those high-pitched sounds are more attenuated than the low-pitched ones.
In the really cheap seats, far away from the orchestra, you’ll have to use your imagination, because you might not hear anything at all.