I am getting excited about the upcoming Perservance landing on Feb. 18, 2021. Not just because of the helicopter, 25 cameras, and the focus on searching for evidence of ancient microbial life; but also because I have learned that Perservance is also carrying two microphones so we can “hear” Mars.
With such a thin atmosphere on Mars, I began to wonder if we can even hear sound on Mars. Sound needs a medium to travel through from the source to our ears. But the density of Mars’s atmosphere is only about 1% the density of Earth’s atmosphere. Is there enough matter for the sound waves to transmit? And Mars’s atmosphere is about 95% carbon dioxide; ours is 78% nitrogen, 21% oxygen, and the rest various gases. Does the high percentage of carbon dioxide have any effect on sound, assuming it can travel through the thin atmosphere? Finally, does temperature have an effect? It’s extremely cold on Mars. Nights can be as cold as –90 °C at Jezero Crater, Perseverance’s landing site.
(Note: Light is a combination of waves and particles. But we can see light, and we can send radio waves through the vacuum of space to Mars for communication purposes because light and its components are electromagnetic radiation, which doesn’t need particles to travel.)
How Do We Hear Sound?
When we hear something, our eardrums are detecting vibrations. Eventually, the vibrations get to the brain. And the brain interprets the vibrations and tells us that we are hearing something; it will identify the sound if possible. The vibrations need a medium to travel through. By definition, a vacuum is an absence of matter—no matter, no particles, no sound waves, and no sound.
Are Sounds on Mars the Same as on Earth?
Turns out, we can definitely hear sound on Mars. It will just be different than what we hear on Earth.
- The effect of the thin atmosphere on sound quality is a muffled, lower-frequency sound.
- The carbon dioxide atmosphere has the effect of absorbing the higher frequencies in a sound, so any sounds with high frequencies would arrive at our ears without those high frequencies.
- The temperature definitely has an effect: sound travels slower in colder temperatures. The average speed of sound through air on Earth is about 330 metres/second. On Mars, the average speed is about 240 metres/second. So a slower speed means sounds generated on Mars will take a little longer to reach our ears.
At https://mars.nasa.gov/mars2020/participate/sounds/ , you can listen to a few Earth sounds and then hear how they would sound on Mars.
Left: the SuperCam microphone; right: the Entry, Descent, and Landing (EDL) microphone
One of the microphones on Perseverance (the EDL microphone) will hopefully pick up the sounds as Perseverance enters the Martian atmosphere and lands. The other microphone is part of an instrument called SuperCam. SuperCam will fire a laser at rocks, and should generate a sound when it hits a rock. The sound it makes could help the team at home understand more about the target rock. The microphone could also pick up any background noise on Mars, such as wind.
Here’s a video of a demonstration: Mars’s thin atmosphere is a partial vacuum. So I created a (temporary) partial vacuum in a mason jar, added a sound source, and lit the tea light to get a change in atmospheric pressure. The loud pop from the jar means the pressure has changed. After the change, you can still hear sound quite clearly!