Are Gas Giant Moons Tidally Locked?

The other day I was wondering what the tides would be like on a moon orbiting a gas giant. But then it occured to me that most large moons would be tidally locked, meaning that one face would always face the planet, just like Luna.

If that were the case, then there wouldn’t be any severe tides, just sloshes, and unpredictable tides from the other moons, and the slight effect of the sun.

So: are the major moons of Jupiter and Saturn tidally locked? Or not?

All the large moons of Jupiter are tidally locked to their primary, but they get tides from each other and (I think) from the equatorial bulge of Jupiter.

As an extreme case, Io, a moon of Jupiter, gets heated up so much from tidal flexing that it’s by far the most volcanically active body in the solar system. Not quite as extreme is Europa which gets enough heating from tides that it has a large liquid ocean under the surface ice.


Europa’s liquid ocean is still just a hypothesis. While there are good indications that the ocean may exist, it has not yet been proven to exist. That is why the next NASA mission to the Jovian system will include a probe that will attempt to get beneath Europa’s ice.

This summer I spent about 2 weeks at the ASU campus for an Earthwatch project on Europa. We measured landforms (lineaments, craters, ridges) in a longitudal region. We found that over time the various landforms that were of the same type rotated. This would show that the crust is moving at a different rate than the moon is rotating, thus showing that Europa probably has a liquid ocean underneath the crust to allow for this non-Geosynchronous rotation.

my wag is that the tidal bulge would point towards and away from the planet. The ‘moon’ could still rotate and is not nes. locked in like our moon to the same side always pointing to earth.

All of the major moons of the gas giants are tidally locked, and the tidal bulge does, indeed, point towards and away from the planet, as it does for all cases of tidal lock. The equitorial bulge of the primary could cause tidal effects, except that all of the major satellites orbit in a plane very close to their primary’s equator. There’s also effects from an eccentricity in the satellite’s orbit: If the orbit isn’t a pefect circle, then you’ll get libration, so the bulge won’t always point directly towards the primary.

As to the Europan ocean, all evidence of any sort concerning the moon since the Voyager missions has consistently pointed towards the probable existence of a liquid ocean. If we somehow find that there’s not, there’s going to be a lot of surprised planetologists out there. The main question, anymore, is what if anything we might find in that ocean.

All the missing Mars probes. :slight_smile: