Jupiter’s icy ocean worlds could be cool travel destinations in the future

Callisto and Ganymede are among the biggest moons in the solar system. NASA’s JUICE mission will reveal some of their secrets.
Callisto Jupiter moon in colorized NASA Galileo image
The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material. NASA/JPL/DLR

It’s time for JUICE to get to work. The European Space Agency’s JUpiter ICy moons Explorer blasted off on an Ariane 5 rocket yesterday to begin its eight-year journey to the Jovian system to study Europa, Ganymede, and Callisto, three of the largest moons in the entire solar system.

Together with NASA’s Europa Clipper, which will launch in October 2024 but arrive at its destination a year earlier than JUICE, the missions will get the first close-ups of Jupiter’s icy moons since NASA’s Galileo probe visited the gas giant from 1995 and 2003.

“We learned about Europa having a subsurface ocean as a result of the Galileo mission,” says Emily Martin, a research geologist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air And Space Museum. The Galileo finding ignited interest in so-called  “ocean worlds” that have liquid water under their thick surface ice and might be the best place to look for alien life in our solar system. Ganymede and Callisto are likely ocean worlds too.

[Related: Astronomers find 12 more moons orbiting Jupiter]

While Galileo captured some images of the lesser-known siblings, it couldn’t analyze their surfaces as well as originally plannedspacecraft was hamstrung from the beginning, when its high-gain antenna, necessary for sending back large amounts of data, failed to fully deploy. Consequently, when JUICE arrives at Jupiter in 2031, it will begin providing the first truly high-resolution studies of Ganymede and Callisto, and add to the data on Europa collected by the Europa Clipper. JUICE will use its laser altimeter to build detailed topographic maps of all three moons and use measurements of their magnetic and gravitational fields, along with radar, to probe their internal structures.

“Galileo did the reconnaissance,” Martin says, “and now JUICE gets to go back and really dig deep.”

Is there water on Jupiter’s moons?

If people know one Jovian moon, it’s likely Europa: The icy moon’s subsurface ocean has been the focus of science fiction books and movies. But Martin is particularly excited about what JUICE might find at Callisto. Jupiter’s second largest moon, it orbits farther out than Europa or Ganymede. It appears to be geologically inactive and may not be differentiated, meaning Callisto’s insides haven’t separated into the crust-mantle-core layers seen in other planets and moons.

Despite the low-key profile, data from the Galileo mission suggests Callisto could contain a liquid ocean like Europa and Ganymede. Understanding just how that could be possible, and getting a look at what Callisto’s interior really looks like, could help space researchers better understand how all of Jupiter’s moons evolved.

“In some ways, Callisto is a proto-Ganymede,” Martin says.

What comes after Mars?

It’s not just Callisto’s interior that is interesting, according to Scott Sheppard, an astronomer at the Carnegie Institution for Science. It’s the only large moon that orbits outside the belts of intense radiation trapped in Jupiter’s colossal magnetic field—radiation that can fry spacecraft electrics and human explorers alike. “If humanity is to build a base on one of the Jupiter moons, Callisto would be by far the first choice,” Sheppard says. “It could be the gateway moon to the outer solar system.”      

JUICE will fly by Europa, then Callisto, and then enter orbit around Ganymede, the largest moon in the solar system. With a diameter of around 3,270 miles, it’s larger than the planet Mercury, which comes in at 2,578 miles in diameter.

Jupiter moon Ganymede closeup
This image of the Jovian moon Ganymede was obtained by the JunoCam imager aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. At the time of closest approach, Juno was within 645 miles of its surface, closer to Jupiter’s largest moon than any other spacecraft has come in more than two decades. NASA/JPL-Caltech/SwRI/MSSS

Geoffrey Collins, a professor of geology, physics and astronomy at Wheaton College, says he’s most excited about the Ganymede leg of the mission. “It will be the first time we’ve orbited a world like this, and I know we will be surprised by what we find.” 

If Ganymede hosts a liquid water ocean beneath its frozen shell how deep its crust is, and whether its suspected subsurface ocean is one vast cistern or consists of liquid layered with an icy or rocky mantle. JUICE will be the first mission to give scientists some real answers about to those questions.

“Even if JUICE just lets us reach a level of understanding of Ganymede like we had for Mars 20 or 30 years ago, it would be a massive leap forward from what we know now,” Collins says. “This will be the kind of thing that rewrites textbooks.”

[Related: A mysterious magma ocean could fuel our solar system’s most volcanic world]

Any clues that JUICE gathers from Ganymede and Callisto could apply to more than just Jupiter and its icy moons. They can tell us more about what to expect when we look further out from our own solar system, according to Martin.

“It contextualizes different kinds of ocean world systems and that has even broader implications to exoplanet systems,” she says. “The more we can understand the differences and the similarities between the ocean world systems that we have here in our solar system, the more prepared we’re going to be for understanding the planetary systems that we’re continuing to discover in other solar systems.”