Old NASA mission reveals an unlikely world harbors an ocean

A moon of Jupiter about the size of Mercury likely has a several-miles-deep ocean under its icy shell, according to a new study. 

This Jovian moon, Callisto, is the outermost of 95 circling the gas giant planet. Back in the 1990s, NASA's Galileo spacecraft discovered something strange about it: The moon seemed to react to Jupiter’s changing magnetic field. Knowing that saltwater can conduct electricity, scientists suspected the world might have a layer of liquid ocean deep down. 

But it wasn't an open-and-shut case. Callisto also seemed to have an ionosphere, a layer of electrically charged particles high in its atmosphere. That meant scientists couldn't rule out the possibility it was the ionosphere — not a subsurface ocean — causing the magnetic reactions. 

Now researchers are more optimistic than ever that Callisto is among the pantheon of water worlds in the solar system. And where there's water, there's the chance for life as we know it. 

From left, moons Ganymede, Callisto, and Io are seen orbiting Jupiter in a Hubble Space Telescope image. Credit: NASA / ESA / Hubble Heritage Team (STScI / AURA)

Callisto, the third largest moon in the entire solar system, orbits Jupiter from about 1.2 million miles away. The rocky world, discovered in 1610 by Italian Renaissance man Galileo Galilei, has earned a reputation as the most heavily cratered moon in our solar system.

The new evidence regarding Callisto comes from a somewhat surprising place: old data. NASA's Galileo mission, which launched into space in 1989, was designed to study Jupiter, its moons, and its surrounding invisible magnetic bubble, called the magnetosphere. The spacecraft had 35 encounters with Jupiter's major moons, including eight with Callisto, before the mission ended in 2003. 

Recently, a team of scientists took another look at Galileo's data, using advanced computer models and statistical techniques. Unlike prior studies, the team analyzed all the magnetic measurements from the close flybys of Callisto. Their findings revealed that Callisto’s ionosphere alone could not explain the magnetic reactions — and when they factored in an underground ocean, the data made more sense. The research, published in the journal AGU Advances, helps substantiate the ocean theory. 

The team predicts the ocean is locked beneath a solid ice shell that could be tens to hundreds of miles thick. The flowing water likely wraps around a solid rocky core. If true, that would make Callisto a close sibling to Europa, another moon of Jupiter.

Though the existence of an ocean remains inconclusive, future spacecraft measurements should be able to confirm once and for all whether Callisto is hiding liquid water. NASA's Europa Clipper and the European Space Agency's JUICE, a wonky acronym for Jupiter Icy Moons Explorer, are expected to capture data when they make flybys. China's planned Tianwen-4 mission may also observe Callisto.

After a successful October 2024 launch, Europa Clipper has been hurtling through space. It's on schedule to make its first flyby of Mars on Feb. 27, where it will get a gravity assist to continue its journey. The craft won't arrive at Europa until 2030. JUICE, which launched in 2023, will have a Venus flyby this August. The spacecraft isn't expected to reach Jupiter to begin its science mission until 2031. 

Whether the moon could harbor life is unknown, but it's "on the list" of possible places NASA would like to look. Not only does it probably have a salty ocean, but a layer of rocks more than 150 miles beneath the surface. Both are key conditions known for leading to life on Earth. Oxygen and hydrogen have also been detected in Callisto's extremely thin atmosphere.