In Gustav Holst’s “The Planets,” a sweeping, seven-part composition inspired by Earth’s neighbors in the solar system, the song of Saturn begins softly, with the gentle hum of flutes. The melody, solemn and nostalgic, marches slowly forward. Then the woodwinds subside, and there’s an explosion of sound, a frenzy of horns and clanging bells. Melancholy seems to morph into menace. The roar is brief, and the movement returns to its opening softness, closing on the dreamy whisper of violins.
The movement, first performed in late 1918, is enchanting and unsettling at the same time—just like the real music around Saturn.
And by music, I mean these noises from the space between Saturn and its icy moon Enceladus:
The source of this ethereal chorus is the movement of plasma waves between Saturn and Enceladus, recorded by the Cassini spacecraft and then converted into sound the human ear can register.
Plasma—the fourth state of matter, alongside solid, liquid, and gas—is an extremely hot gas composed of charged particles. It exists throughout the cosmos, including in Saturn’s magnetic field, the protective bubble that surrounds the planet.
Of course, space, despite its many cool attributes, is missing the main ingredient for producing sound: air. When an object vibrates—a bell, for example—the movements cause molecules in the air to vibrate, too. The air molecules bump into other air molecules, which bump into other air molecules, and so on, producing waves that keep going until the molecules run out of energy and stop bouncing around. If you’re within earshot of this interaction, you’ll hear the sound. Ding ding ding.
Similarly, when plasma particles get jostled, they generate waves. We can’t hear these waves, but we can build spacecraft—like Cassini—to measure them. Back on Earth, scientists can translate the waves into audible sound, much like the way radio waves get converted into the Top 40.
The plasma waves were recorded in September 2017, two weeks before NASA plunged Cassini into Saturn’s atmosphere, ending its 13-year orbit around the planet. The data shows the waves result from an electric-current-like interaction between Saturn and Enceladus, its sixth-largest moon, according to the space agency. Enceladus, which is thought to harbor an ocean, spews water vapor from cracks in its icy crust toward Saturn. In space, these particles can become charged and jostle the surrounding plasma. Saturn reacts by emitting signals in the form of plasma waves, NASA scientists say.
The eerie clip is a reminder that space can be noisier than one would expect for a cold, dark void—you just have to listen. Scientists have spent years translating observations of inaudible phenomena into familiar noises. In our own magnetic field, plasma waves send highly charged particles bouncing around. To us, the action sounds like a summer evening on a lakeshore, alive with the chirps of crickets and frogs.
When the Juno spacecraft approached Jupiter in 2016, its instruments detected changes in the particles of the planet’s magnetic field. The translated waves sound like a roar of static. Lightning on Jupiter, captured by the Voyager spacecraft, sounds like fireworks shooting into the sky.
The soundtrack extends beyond our solar system. As Voyager 1 pierced the fuzzy boundary between our system and interstellar space in 2012, the spacecraft’s instruments recorded plasma waves. To our ears, the ripples sound like high-pitched whistles.
We human beings have even managed to hear the sounds of distant black holes, the mysterious cosmic objects from which nothing can escape, not even light. In 2015, astronomers observed, for the first time, gravitational waves produced by the collision of two black holes 1.3 billion light-years away. When scientists placed this signal into the audible range, they heard a little chirp.
We can even hear the very beginnings of the universe, thanks to the leftover radiation that, 13.8 billion years later, fills the cosmos in every direction to this day. This radiation, known as the cosmic microwave background, is essentially the foggy residual heat of the Big Bang itself. It appears “slightly patchy,” according to a listening guide from Mark Whittle, an astronomer at the University of Virginia, in part because of the presence of sound waves. “The extraordinary truth is that we can see the sound waves, exactly as they were, just 400,000 years after the Big Bang,” Whittle writes.
Using computer simulations, Whittle can excavate these sound waves and convert them into something we can hear. The first million years of the cosmos, when compressed into just 10 seconds, sound like a zap, then a low rumble, and then a heavy, intensifying hiss. It’s not exactly music, but it’s beautiful nonetheless. That growing hiss, according to Whittle, is the sound of the cosmos building clumps of matter that eventually gave rise to the very first stars.