Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:
ORBITAL RESONANCE
Three of Jupiter’s moons, Io, Europa, and Ganymede, have a special relationship with each other. For every complete orbit of Ganymede, Europa completes exactly two orbits, and Io completes exactly four. This relationship is known as an orbital resonance.
To be more specific, Io, Europa, and Ganymede’s 4:2:1 relationship is called a Laplace resonance in honor of Pierre-Simon Laplace, the astronomer who first noticed it.
As the moons pass each other, they pull on each other gravitationally. This would happen with or without the resonance, but the resonance means these gravitational interactions are more regular and repetitive than similar interactions between other passing objects in space.
The persistent gravitational tug-of-wars between these three moons helps keep their interiors warm through a process called “tidal heating.” As a result, Ganymede and Europa appear to have oceans of liquid water beneath their surfaces. Meanwhile, poor Io keeps spewing sulfur all over itself.
Resonances can play an important role in shaping the rest of the Solar System as well. We’ve already seen how resonances with Jupiter created the Kirkwood gaps in the asteroid belt. A similar process created the gaps in Saturn’s rings, and a 3:2 resonance between Neptune and Pluto ensures that the two planets celestial objects won’t crash into each other.
It’s also worth noting that the Solar System is full of not-quite-perfect resonances. Earth and Mars almost have a 2:1 orbital resonance, as do Uranus and Neptune. Jupiter and Saturn almost have a 5:2 resonance. And Callisto (another of Jupiter’s moons) is so close to joining the resonance party. So close! It almost has a 7:3 resonance with Ganymede. (This list could go on for a while.)
Maybe some of these resonances and near-resonances are pure coincidence. But it’s hard to believe they all are. There’s something about gravity that makes planets and moons want to resonate with each other. Science fiction writers might want to keep that in mind while designing new star systems.
P.S.: It’s sometimes mistakenly assumed that Io, Europa, and Ganymede routinely “meet up” on the same side of Jupiter. In reality, whenever two of these moons line up with each other on the same side of their host planet, the third is always somewhere else—frequently the exact opposite side of the planet.
Planet Pailly 
Interesting, as always.
“Maybe some of these resonances and near-resonances are pure coincidence. ”
I wonder if there might be a form of natural selection at work here. Maybe the resonance orbits are more stable, and the less stable arrangements all disappeared a long time ago.
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That’s certainly true for Neptune and Pluto, and I read somewhere (I can’t find the source) that Io, Europa, and Ganymede might have spiraled into Jupiter one by one without their resonance. Perhaps other Jovian moons did precisely that long ago.
But I suspect there’s another factor at work too. I didn’t want to go too far into this because I don’t really understand the math, but apparently the resonance between Io, Europa, and Ganymede is self-correcting. Meaning (I think) that if one of them starts straying out of sync, the other two pull it back.
My guess is that most of the resonances and near-resonances we see are caused by planets/moons trying to pull each other into sync. But that’s only my guess. Again, the math sort of went over my head for this one.
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