You’d think moons would be quiet compared to their host planets, but that’s not entirely true — if you know how to listen. The principal investigator for NASA’s Juno mission, Scott Bolton, has produced an audio recording of magnetic field activity around Jupiter’s moon Ganymede as the Juno spacecraft flew past on June 7th, 2021. The 50-second clip reveals a sharp change in activity as the probe entered a different part of Ganymede’s magnetosphere, possibly as it left the night side to enter the daylight.
The audio came from shifting electric and magnetic frequencies into the audible range. Jupiter’s magnetosphere dominates that of its moons and is present in the recording, but Ganymede is the only moon in the Solar System to have a magnetic field (likely due to its liquid iron core). This isn’t a feat you could replicate elsewhere in the near future.
The soundtrack was part of a larger Juno briefing where the mission team revealed the most detailed map yet of Jupiter’s magnetic field. The data showed how long it would take the Great Red Spot and the equatorial Great Blue Spot to move around the planet (roughly 4.5 years and 350 years respectively). The findings also showed that east-west jetstreams are ripping the Great Blue Spot apart, and that polar cyclones behave much like ocean vortices on Earth.
You wouldn’t hear these sounds if you could visit Ganymede yourself. However, they’re a reminder that even seemingly dead worlds are frequently brimming with activity you can detect using the right instruments. It’s just a question of how easy it is to notice that activity.