Three of the four big moons of Jupiter appear to have something in common: oceans of liquid water below their crusts. For Europa, the ocean is considered a slam dunk. The case is also strong for Ganymede. But the case for the third moon, Callisto, is the weakest.

Callisto is about 3,000 miles in diameter – bigger than our own moon. And it’s more heavily cratered than any other large body in the solar system. That indicates that the surface of Callisto is pretty much dead. But things might be different far below the surface.

In the 1990s, the Galileo spacecraft flew near Callisto eight times. Its measurements of the magnetic field around the moon hinted that a salty ocean was sloshing around inside. But those observations also could be produced by an electrically charged layer of Callisto’s thin atmosphere.

In a recent study, though, scientists looked at all of Galileo’s observations, and used computer models to understand them. The work suggested that there is an ocean. It could be dozens of miles deep. But it’s buried beneath an icy crust that could be hundreds of miles thick.

Two spacecraft that are en route to Jupiter will fly close to Callisto many times. Their observations should tell us for sure whether an ocean is sloshing below Callisto’s battered surface.

Jupiter stands below our moon in the early evening twilight. It looks like a bright star.

Script by Damond Benningfield