This article first appeared in Issue 9 of our free digital magazine CURIOUS.
To quote Edwin Hubble, "Equipped with his five senses, man explores the universe around him and calls the adventure Science." Of course, we enhance our senses as much as we can with technology, but it is here we often start. How would the sky look if we could see in infrared? Depending on the wavelength, you might only see the water vapor in the air. What does the Moon smell like? Of gunpowder, according to astronauts. And truly a fan favorite: is there sound in space?
The answer, as always, is a lot more interesting than a simple yes and no. In interplanetary, interstellar, or intergalactic space there is no sound that we can hear. Thanks to the Alien movies’ tagline, everyone knows that in space no one can hear you scream. However, if we are talking about sound in general, beyond the senses of people who can hear, then outer space definitely has sounds – and some of them are absolutely mind-boggling.
How can sound travel in space?
But first of all, let’s discuss what sound is and how it travels. Sound is a vibration propagating through a medium as a wave. It cannot propagate through a perfect vacuum – a truly empty void absent of anything, including particles – because there is nothing to travel through. Outer space is certainly a vacuum, but not a perfect one. The Sun is constantly releasing particles. This stream is known as the solar wind and it has an extremely low density. While there is an ebb and flow, depending on the activity of the Sun, scientists estimate between 3 and 10 particles per cubic centimeter at Earth's orbit. Even on top of Mount Everest, you would have a density of billions of trillions of particles.
While the density is low in interplanetary space, waves still propagate throughout. The Voyager space missions have now left the Solar System but their decade-spanning travel among the planets allowed them to measure waves propagating through the plasma released by the Sun as the solar wind.
When the solar wind is released by the Sun, it’s extremely hot and moves at high speed. As it expands outwards, it cools down and has a lower density. As mentioned, sound waves are just waves that move through a medium and interplanetary plasma can be such a medium. At Earth’s orbit, 150 million kilometers (93 million miles) from the Sun, the speed of sound is about 50 kilometers (31 miles) per second.
Plasma and the speed of sound
Plasma is where the conventional wisdom about sound waves pretty much goes out the window. You might have learned in school that sound travels faster in a liquid than in a gas and faster in solids than in liquids. A classic example is to compare air (about 340 meters per second), water (1,480 meters per second), and iron (5,120 meters per second). But compared to plasma, those speeds are tiny.
The reason for the difference is that you can interpret sound waves as pressure disturbances traveling in a medium. Without getting into the details of the math, the speed of sound will depend on pressure (and due to the laws of gases, temperature) and it will be inversely proportional to the density. So you have something that is big (hot) divided by something that is small (density) making the speed of sound through plasma a much bigger number.
But despite the speed of sound in the plasma being a big number, the solar wind moves faster than that. The particles in the wind move at a variety of speeds from about 200 to 750 kilometers (124 to 466 miles) per second. So the solar wind is intrinsically supersonic, which ends up creating some fun effects throughout the Solar System. Something pretty neat is that the plasma waves that arrive at Earth from the Sun have a frequency in the audible spectrum, which goes from about 20 Hertz to 20 kiloHertz.
Would that mean that we could hear the sound of these waves? Well not exactly. “There is too little plasma for us to hear the sound directly,” Dr Nigel Meredith, Space Weather Research Scientist with the British Antarctic Survey, told IFLScience. But these waves have an effect on Earth that allow us to hear them.
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“It's interesting because when they come down to Earth, they're guided by the Earth's magnetic field in the ionosphere high up, and they get converted to radio waves,” Dr Meredith explained.“So you have this conversion from the plasma to radio waves and then you're converting them back to sound.” The frequencies of these waves do not change, just the medium in which they travel.
Hearing A Star Being Born
Plasma waves are found everywhere in the universe where there is plasma. And given that plasma is the most abundant state of matter in the universe, this means they are everywhere. The speed of sound in the different aggregations of the interstellar medium (the gas that exists in the space between stars) and the plasma turbulences present in this gas have huge implications for the birth of stars.
Stars are born in large molecular clouds. Gas in the clouds cools down over time. The regions where it is cooler are denser and when they cross a certain density, they collapse under their own gravity into a star. The star formation rate in large molecular clouds is pretty inefficient. Just 1 percent of the gas turns into stars, and astronomers believe it is due to turbulence – the chaotic changes in the pressures and velocities of the medium in question. By measuring these movements and the speed of sound in the plasma, scientists can estimate how many new stars are being born.
But sounds don’t stop once the stars are born. The interiors of stars are full of waves moving about at sonic speed, just like quakes propagate through the Earth. The field of asteroseismology actually employs these oscillations to study the interior of stars. A violin sounds like a violin because of its shape and so stars have unique sounds too. Astronomers can measure these sounds by looking at small changes in the star's brightness. Resonant oscillations produce small but significant changes in the amount of light we get from a star.
This approach can be used to measure the mass and age of a star. This is extremely useful as a star’s age and mass matters both in the context of an individual star and for how we consider the properties of stars as a group. Asteroseismology is a tool that can literally go even deeper: different vibrations can reach layers at different depths, providing a way to understand the internal properties of stars. Similar techniques can even be applied to the Sun.
The deepest note in the cosmos
If those Sun vibrations are not enough like sound for you, worry not. There is something out there that can produce actual musical notes: supermassive black holes. Both the Virgo Cluster and the Perseus Cluster have a central galaxy with an active supermassive black hole that has generated bubbles of plasma moving close to the speed of light. These ripples are concentric, spaced over millions of years. In the case of the Perseus Cluster, the note is a B flat. Unfortunately, the ripples are every 9.6 million years making this the deepest note in the universe, 57 octaves under middle C, and way below what humans can hear.
We can’t mention black holes without mentioning mergers and the emission of gravitational waves. These are also vibrations (of space-time itself rather than a medium), which physicists have sonified as the iconic chirp. Different types of merging events have different chirps.
Can we hear sounds in space?
At the end of the day, the most intriguing aspect of sounds in space are those that we could potentially hear ourselves. It is a matter of location after all, but also of atmospheric composition. Assuming for a second that we could survive the hellish temperatures, acidic conditions, or incredible pressures, this gives us a wide range of places in the Solar System where we could go and find extraterrestrial sounds. All the planets (except for Mercury) and Saturn’s moon Titan have a significant atmosphere. If we consider the giant planets alone, Uranus and Neptune will have a slower speed of sound than Saturn and Jupiter.
That’s because they are colder. Researchers think that equipping a probe with a microphone and sending it down into the planets might actually provide insights into the different layers due to changes in the speed of sound. No such mission is planned, but microphones have been employed on terrestrial planets.
The Soviet-era missions Venera 13 and 14 had instruments to measure sound waves on Venus in the early 1980s, producing a measurement of wind speed on the planet. NASA’s Perseverance Mars rover has also got a mic that has been used to measure the sound of its lasers and even the first-ever sound of a dust devil (and thus its properties) on another world. Interestingly, especially in relation to this discussion, thanks to the instrument, researchers were able to estimate the speed of sound on Mars, and it is slightly lower on average than on Earth.
Sounds there move at about 240 meters per second (540 miles per hour, or 787.4 feet per second). But there is a peculiar effect taking place on Mars. Due to its atmosphere being made almost exclusively of carbon dioxide and at low pressure, something weird happens if sounds are above 240 hertz (just below a piano middle C). The CO2 molecules can’t relax their vibrational modes and the speed of sound is 10 meters per second (32.8 feet per second) faster for those noises.
Sound may not be the most useful of our senses when it comes to space exploration, but the universe is filled with these waves, whether we can hear them or not.