They’d based their prediction on the orbital dance of Jupiter’s largest moons. For every four orbits that Io completes, Europa makes two and Ganymede one. This orbital configuration, known as a resonance, causes Io to wobble back and forth, making its orbit elliptical. When Io is closer to Jupiter, the planet’s gravity yanks on it more intensely. When it’s farther away, Jupiter’s tug is weaker. That never-ending gravitational tug-of-war makes the rocky surface of Io move up and down by 100 meters, the same height as a 30-story building. These are tides, like Earth’s — just in solid rock, not water.

Those tides create friction within the moon that generates heat. And that tidal heating is strong enough to melt the rock deep inside Io. “Io doesn’t have a water ocean, but it probably has a magma ocean,” Nimmo said. (Galileo picked up on a secondary magnetic field there too, generated by a global subterranean reservoir of molten rock.)

Europa also experiences some tidal heating. But how much those tides warm an ocean depends on where within the moon they occur; in other words, enough of that heat needs to get to the ocean to keep it liquid. “The tidal heating could be happening in the ice shell itself, or it could be happening in the rocky core underneath,” Nimmo said. Scientists don’t know which is correct — so they can’t say for sure how much tidal heating contributes to Europa’s liquid interior.

Enceladus, too, is stretched and squeezed by its gravitational tango with a neighboring moon called Dione. In theory this could produce tides that warm the moon’s interior. But the tides created by its resonance with Dione, at least on paper, do not seem to be sufficient to explain its ocean. The numbers don’t yet work, Sori said, and the amount of heat produced isn’t enough to maintain a global ocean for the billions of years since the solar system’s birth. Perhaps, as with Europa, scientists don’t quite know where the tides are creating heat within Enceladus.

Another confounding factor is that orbits aren’t fixed over astronomical time. As planetary systems evolve, moons migrate, and “tidal heating can turn on and off as things drift in and out of different resonances,” said David Rothery, a planetary scientist at the Open University in the United Kingdom. Scientists suspect this happened with Miranda and Ariel, two Uranian satellites that may be former dance partners; these moons look as if they were once geologically active but are now arguably frozen to their cores.

In a similar vein, Enceladus may not always have had Dione as its dance partner: Perhaps their Saturn-circling boogie kicked off more recently and warmed a previously solid moon. But that scenario is also troublesome to explain. “It’s easier to keep an ocean around and maintain it, rather than freeze and remelt it,” Sori said. Thus, if tidal heating is exclusively responsible for Enceladus’s ocean, then the moon is a veteran dancer that has bopped for several billion years.

For now, the only certainty about this moon’s ocean is that it exists. How it came to be, and how it’s still around today, “is one of the really big unsolved questions,” Sori said. “Enceladus is tough to figure out.”

Radioactive Renegades 

Fortunately, warm moony interiors don’t exclusively depend on tides.

Half of Earth’s internal heat came from its birth. The rest comes from decaying radioactive elements. Similarly, the rock-rich depths of icy moons should contain a decent amount of uranium, thorium and potassium — radioactive stores that can cook their surroundings for hundreds of millions, if not billions, of years before they decay into stable elements and stop releasing heat.

Bigger moons will have started out with more bountiful caches of radioactive matter. And perhaps that is all their oceans require. “For larger moons like Ganymede and Callisto and Titan, they’re sort of inevitable because of this radiogenic factor,” Vance said. Some scientists even argue that Pluto has a subsurface ocean. Like the three moons, this dwarf planet is likely insulated by a sufficiently thick crust that slows the leaking of its radioactive furnace into space.