[♪ INTRO] When it comes to exploring the universe, we usually only get to use one of our senses: our sight.

We can’t smell stars or taste comets or touch exoplanets. But every now and then, we can use sound to probe other worlds.

Scientists have only sent microphones into the solar system a few times so far. But it turns out there’s plenty to hear out there! And by listening in on the whooshes, gurgles, or clunks of other worlds, we can learn all kinds of new things about them.

Now, sound waves always need some sort of medium to travel through, like an atmosphere. So space itself is dead quiet. So is the Moon.

But plenty of the bodies in the solar system do have some sort of atmosphere. And that means we can listen to them. Astronomers first attempted this in 1982.

The Soviet spacecraft Venera 13 carried a microphone with it, all the way down to the blistering surface of Venus. The lander only lasted 127 minutes before succumbing to, you know, Venus, but in that time it recorded a few minutes of sound. Mostly it picked up its own noises as instruments shifted around and carried out their tasks.

But it also picked up some sounds believed to be the wind. And using this data, scientists were able to measure the speed of the gusts blowing across the surface. It was a while before scientists pulled off anything like that again.

But in 2005, the Huygens space probe carried a microphone that listened in on its whole descent onto Saturn’s moon Titan. Originally, NASA hoped the microphone might pick up sounds of thunder in Titan’s atmosphere. There was some speculation at the time that lightning strikes on Titan might help produce organic compounds.

So the team behind the mission wanted to detect this lightning by listening for thunder. They knew the odds were small that this would actually happen, though, and it didn’t. By now we have learned that Titan either doesn’t have thunderstorms at all, or if it does they’re extremely rare.

But the team’s efforts didn’t go to waste! Huygens still recorded the sound of the wind during its two-and-a-half-hour descent. [AUDIO OF HUYGENS DESCENT] And okay, it pretty much sounds like static, but that’s alien wind blowing past a spacecraft from Earth! Huygens also recorded the sound of the spacecraft as it landed with a clunk. [AUDIO OF HUYGENS LANDING] That might not sound very exciting, but the nature of the clunk does tell us something about what the probe hit.

If Huygens had landed with a splash or a crunch, or if it had picked up the sound of gurgling liquid, it would have revealed something else about what lay under Titan’s hazy clouds. So these recordings hinted at the potential for sound to offer clues about the surface of an alien world. Sound can also uncover information about the atmosphere of other worlds.

And scientists recently demonstrated that on Mars. In 2021, two microphones onboard the Perseverance rover began tuning in to the sounds of the Red Planet. They picked up dust devils and gusts of wind, along with the squeak of the rover’s wheels as it rolled across the ground. [AUDIO OF MARTIAN DUST DEVIL] And these recordings revealed that sound on Mars is different from sound on Earth.

For one, it travels slower overall because Mars is so cold. Even on Earth, cold air molecules transfer sound slower than warm air molecules, because they vibrate more slowly. But there are actually two different speeds of sound on Mars.

And that has to do with the fact that the atmosphere is mostly made of CO2. Because of some quirks of its structure, CO2 interacts differently with sounds below and above 240 Hertz. As a result, high-pitched sounds travel about 10 meters per second faster than low-pitched sounds.

So, if you were listening to a Martian orchestra, you’d hear the flutes and violins before you heard the tubas and trombones. Although there’s one other quirk about sound on Mars. The CO2 in the atmosphere actually absorbs a lot of high-pitched sounds, so you can mostly just hear low frequencies.

Basically that means sounds are more muted, like if you were listening to them from behind a closed door. Scientists had predicted some of these quirks before anyone ever recorded sound on Mars, but these microphones were the first to let us hear what Mars sounds like for ourselves. Today, scientists are constantly pushing the boundaries of what we can learn by recording sound on other worlds.

One idea researchers are looking into is using a sound recorder to detect venusquakes. Scientists have long wondered whether or not Venus has seismic activity. But getting down to the surface of Venus with a seismometer is just not happening anytime soon.

The good news is, it might not be necessary. Here on Earth, seismic waves in the ground regularly produce acoustic waves in the atmosphere. These are too low-pitched for us to hear, but they get picked up by monitoring stations around the world.

That made scientists wonder if acoustic recorders in Venus’s atmosphere could detect shaking on the ground. Because while things aren’t exactly balmy up in Venus’s clouds, scientific instruments have survived for months up there. So, it all seemed promising, and as a first step, in 2019, scientists gave it a whirl on Earth.

They put infrasound recorders in four balloons… and floated them up above California. Shortly after, one of the balloons that was nearly 5 kilometers above the ground registered an earthquake. Amazingly, the acoustic signal lined up with the seismic wave measurements from the ground, making it the first earthquake ever detected from a balloon.

Next, scientists hope to try the same thing with a balloon over Venus. Over the last few decades, our eavesdropping on other worlds has shown us that there’s plenty to hear out there. The whooshes, thuds, and rattles of other planets and moons can reveal so much about these worlds.

They can let us in on the secrets of hidden surfaces, alien atmospheres, and even sound itself. Thanks for listening to this episode of SciShow Space, which was supported as always by our generous patrons on Patreon. We were trying something a little different here with a moody exploration of sound, so if you liked it and you want to help us make more, you can get started at patreon.com/scishowspace. [♪ OUTRO]