All the Galilean moons are hotter than they should be at their distance from the Sun, and researchers have been trying to understand why. The explanation up until now has been that Jupiter’s tidal pull on the moons is strong enough to create a flex and release that heats up the cores of the planets. This reaction is called tidal heating.

I always explain this phenomenon by taking a rubber ball and squeezing it repeatedly until the surface heats up and cracks a little. The squeezing motion causes the heat.

In a new study in the Geophysical Research Letters, research shows that it may be the moon-moon interactions that are responsible for heating the moons more than it is Jupiter’s gravitational influence. 

Lead author Hamish Hay remarked: It’s surprising because the moons are so much smaller than Jupiter. You wouldn’t expect them to be able to create such a large tidal response.

So how do the moons exert this influence on one another? Tidal resonance. Hay goes on to explain: Basically, if you push any object or system and let go, it will wobble at its own natural frequency. If you keep on pushing the system at the right frequency, those oscillations get bigger and bigger, just like when you’re pushing a swing. If you push the swing at the right time, it goes higher, but get the timing wrong and the swing’s motion is dampened.

To get the right resonance, you need both Jupiter’s influence and the influence of the other moons. This is due to the depth of the oceans on these moons, or in the case of Io, it’s magma ocean. The oceans are too deep for just Jupiter alone to cause all the heating of ice and rock that we find.

More Information

The University of Arizona press release 

“Powering the Galilean Satellites with Moon-Moon Tides,” Hamish C. F. C. Hay, Antony Trinh & Isamu Matsuyama, 2020 July 19, Geophysical Research Letters