Poor Ganymede. Unlike its other Galilean moons – well, apart from Callisto – it doesn’t get much attention. You’ve got flashy, volcanic, paradoxical Io on one hand, and ocean-hiding, icy, potentially life-containing Europa on the other, all taking up everyone’s attention.
Thanks to a stunning new Nature Communications study, though, it looks like Ganymede is about to take the limelight.
Turns out that a team, led by the GFZ German Research Centre for Geosciences, have been listening in for some time now via the Galileo space probe. They’ve been assessing Ganymede's chorus waves and found out that the moon’s “whistle” is 1 million times more intense than would be expected around other moons and planets.
At this point, you’re understandably asking how moons can, well, whistle, so allow me to explain.
Although predominantly a vacuum, space contains charged particles, and when they interact with the magnetic fields of planets or stars, they move about fairly sharpish. As noted by NASA, they often get violently jostled about by plasma waves – clumps of electrons and ions – and in some instances, their motions can be somewhat rhythmic.
That proves to be rather useful, as these motions occur as very low-frequency radio waves. Using some clever tech here on Earth, we can “hear” them.
Whistler mode waves (WMWs), one type of particularly musical plasma waves, make different “noises” depending on the type of plasma they are passing through. This means that we can ascertain plasma properties based on the music they make.
The more we know about this, the more we know about the planet or star’s properties. At the same time, they aid us in spaceflight: Earth generates its own chorus waves, and the electrons that it launches out into space can potentially damage spacecraft, which is useful to be aware of.
This new study describes the detection of particularly strong WMWs in the vicinity of Jupiter’s moons, particularly Ganymede. Europa, unable to resist a cameo, is also noted in the paper as having WMWs 100 times above the average level.
Although it’s not entirely clear why the WMWs for these two satellites are so powerful, it’s suspected that it is partly because both run through the extremely powerful magnetic field of Jupiter.
Jupiter’s magnetic field, which is 20,000 times stronger than Earth’s, is weird enough as it is. It’s not generated by a convecting, liquid, iron-rich outer core, but by a rapidly rotating highly compressed hydrogen core that acts like a metal.
Its magnetic field line tendrils reach out to the orbits of both Ganymede and Europa, which provides the region with plenty of energetic particles.
Ganymede – the largest satellite in the Solar System – has a potent magnetic field of its own. Europa also possesses one, but it’s far weaker, suggesting it’s probably induced by the movement of salty water, not via a metallic core like Ganymede’s.
The authors note that objects without a magnetic field could achieve the same effect, but powerful magnetic fields create large traps of obstacles for particles streaming nearby. This results in the generation of stronger WMWs – something, among other aspects of the waves, that the ongoing Juno mission will have a closer look at.
So there you have it: Ganymede's party trick is essentially some damn fine whistling.
Incidentally, Io’s sulfurous eruption columns do provide the region with plenty of plasma to play with, but there’s actually so much of it that conditions for generating waves aren’t really there. It’s essentially trying too hard to perform the same magic trick, which serves it right for being such a showoff in the first place.