Sciency Words: Orbital Resonance

Sciency Words MATH

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:


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.

Galilean moon Laplace resonance animation 2.gif

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.


Meet a Moon: Io

For the month of August, I’ve decided to forego my regular Molecular Monday posts. Instead, each Monday this month, we’re going to meet a moon, specifically a moon of Jupiter, starting with:


I’d always assumed that space and everything in it is beautiful. The constellations are beautiful. Saturn is beautiful. The Andromeda Galaxy is beautiful. Then I saw a true color photograph of Io.

Ag01 Io

Io doesn’t look much like a celestial body. It looks more like something I might find growing in the back of my fridge. So what the heck happened to this poor moon?

Of Jupiter’s four largest moons (collectively known as the Galilean moons) Io is the closest to Jupiter. With Jupiter’s enormous gravity on one side and the gravities of the other three Galilean moons on the other, Io is trapped in an endless gravitational tug-of-war. Who wouldn’t feel queasy being pulled in so many different directions at once?

As a result, Io ends up constantly puking its guts out. By the word “puking,” I refer volcanic activity, and by “guts” I mean warm interior materials. These interior materials have a high percentage of sulfur and sulfur compounds. All that sulfur, which has been spewed liberally across Io’s surface, gives the moon its nasty, yellow-green complexion.

Months ago, we talked about the remote possibility that sulfur-based microorganisms could exist in the soupy atmosphere of Venus. I suppose a similar biochemistry could be possible on Io. It’s a stretch, but at least some astrobiologists think the possibility is worth investigating.

Next week, we’re going to me another of Jupiter’s moons, a moon with a much, much higher likelihood of supporting alien life.

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Today’s post is part of Jupiter month for the 2015 Mission to the Solar System. Click here for more about this series.

Sciency Words: Kirkwood Gaps

Sciency Words PHYS copy

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:


Some of the asteroids in the asteroid belt have gone missing.

Jy13 Milk CartonThe asteroid belt begins at a distance of 2.1 AU (astronomical units) from the Sun and stretches all the way out to 3.5 AU from the Sun. But there are empty regions at approximately 2.5 AU, 2.8 AU, 3 AU, and 3.3 AU. These empty regions are called Kirkwood gaps.

Jy13 Kirkwood Gaps

Kirkwood gaps are named after Daniel Kirkwood, the astronomer who first discovered them and correctly deduced what caused them.

Any asteroid orbiting the Sun at a distance of 2.5 AU would happen to be in a 3:1 orbital resonance with the planet Jupiter. This means Jupiter would complete exactly one orbit for every three orbits the asteroid completed. The 2.8, 3, and 3.3 AU distances happen to correspond to other orbital resonances with Jupiter.

Asteroids in these resonant orbits would experience nagging, persistent gravitational tugs by the Solar System’s largest planet. This would slowly drag them away from their original circular paths around the Sun and throw them into new, highly eccentric orbits.

Many of the asteroids that cross Earth’s orbital path are probably former residents of the Kirkwood gaps. So the next time an asteroid comes along and wipes out the dinosaurs, don’t get mad at the asteroid. It might not be the asteroid’s fault.

Jy13 Jupiter the Bully

Get mad at Jupiter.

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Today’s post marks the end of asteroid belt month for the 2015 Mission to the Solar System… and the beginning of Jupiter month!