What Lies Beneath the Ice of Jupiter's Moons? Two Missions Will Investigate

Jupiter's icy moons may soon reveal some of their secrets to Europe's JUICE and NASA's Europa Clipper spacecraft.

The icy moon Ganymede creates a round shadow over Jupiter's cloud tops. NASA's Juno spacecraft took ...
Data: NASA/JPL-Caltech/SwRI/MSSS; Image processing: Thomas Thomopoulos © CC BY

Jupiter is like an ogre, with one arm made of gravity, and the other made of radiation. These metaphorical arms stretch, squeeze, and bathe the menagerie of objects orbiting the giant planet. A pair of missions will soon go towards this planet to study ice-encrusted moons within its grasp that may harbor oceans where life might be huddled away — but will each use different strategies to avoid the ogre arms.

First up is the European Space Agency (ESA) JUICE mission, short for Jupiter Icy Moons Explorer, which launches Thursday. After a seven-year-long trip to the outer Solar System, it will characterize the moons Callisto and Europa, and put its major focus on Ganymede, the largest moon in the Solar System. Then next year, NASA’s Europa Clipper will launch toward the system to learn about Europa, the best Jovian candidate for life.

The missions will be able to complement each other in ways that will help us understand the origins of life on Earth, and if those conditions persist elsewhere.

This NASA animation shows the space agency’s Europa Clipper spacecraft performing a flyby of Jupiter's moon Europa. The spacecraft will launch next year.


JUICE and Clipper, Ganymede and Europa

With the launch of JUICE just days away, Claire Vallat, operations scientist of the soon-to-fly JUICE mission and former scientist on Europe’s unprecedented 2014 comet mission Rosetta, eagerly awaits takeoff.

She describes JUICE’s two big goals. The first is putting Jupiter and its worlds in a broader perspective of the rest of the Milky Way, and, in turn, the Universe at large. Jupiter is a “mini-Solar System” that scientists can use as a laboratory to see how planetary formation works, she says. “With all the exoplanets that have been discovered in the past 20 or 30 years, there are a lot of Jupiter-like planets, so understanding how the Jovian system works will help us also to understand how other exoplanet systems might work,” Vallat tells Inverse.

JUICE’s other objective is related: honing in on “the emergence of habitable worlds” in particular, Vallat says. They’re gathering data on Europa, Ganymede, and Callisto to compare them to one another.

“Despite being born in the same kind of neighborhood, they have evolved completely differently,” Vallat says. Callisto is a battered shell, and Europa is relatively smooth. Ganymede is somewhere in between, meaning it has an intermediate level of activity, which shows in its so-so ability to fix superficial deformities. The surface features correspond, potentially, to the ability of any subsurface ocean to “repair” the surface.

This makes Ganymede valuable to science, according to Vallat. Europa likely has an ocean about 10 to 15 miles under its surface, while Ganymede likely has one 95 miles down. Callisto is believed to either have no ocean or one very, very far down. “Ganymede can give you an open book to the different stages that have taken place from the Jovian system formation to quite recently,” she says.

Ganymede stands out in the planetary rolodex in another major way: It is the only moon in the Solar System with its own magnetic field, capable of making auroras like Earth. But when it comes to resembling Earth for its most exceptional characteristic, the ability to host life, Europa comes closest. It’s why Clipper has dedicated its mission to this moon.

“Europa is super special,” Don Blankenship, senior research scientist at the Institute for Geophysics at the University of Texas, tells Inverse. He is principal investigator of Clipper’s Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument, and a science team member of JUICE’s Radar for Icy Moon Exploration (RIME) instrument.

Blankenship says Europa has the right conditions to support anaerobic life, the sort that lives in oxygen-free environments. “What people are not really picking up on is how close Europa really is to things that exist on Earth now,” Blankenship says. According to him, there are ice shelves in Antarctica with pressure, temperature, and salt environments that are similar to what scientists predict exists below Europa’s frozen surface. Ganymede is less likely to harbor life. But it's a robust world that might help explain exoplanets around other stars.

These illustrations offer two possibilities for Europa’s ice thickness. In both, the subsurface ocean is depicted in direct contact with the silicate layer, allowing heat to move through the water through hydrothermal vents, and cause changes to Europa’s icy surface.

NASA/Michael Carroll

The Perils of Jupiter

To learn about Europa’s possible lifeforms, the spacecraft must survive the most hazardous radiation belts in the Solar System, which are found around Jupiter.

NASA’s Juno is already there. The spacecraft survives the choppy space waters behind a titanium radiation vault, which Heidi Becker, Juno’s radiation-monitoring lead, described to Popular Mechanics as a “bulletproof vest.” First demonstrated by Juno, protective shields will also be used to keep the JUICE and protect the Europa Clipper spacecraft safe.

Another way Juno sails as unscathed as possible is that it “threads the needle” just a few thousand kilometers above the gas giant’s clouds, principal Juno mission investigator Scott Bolton tells Inverse. Juno circles Jupiter’s poles rather than its middle section, a beneficial tactic because it evades the strongest radiation blows near Jupiter’s equator, Bolton says.

Juno may help guide both missions. “We're going through the regions now that both of those spacecraft will visit … that may also inform them, help them understand the radiation environment that they're going to experience, and how they might need to adjust science observation strategies or orbits to accommodate radiation as a formidable foe,” Bolton says.

Blankenship says Europa Clipper gets its name from the 50 radiation-defying dives it’ll take past Europa — effectively clipping the moon — as the spacecraft orbits Jupiter. Without this approach, “the radiation at Europa can be pretty dramatic and it's very difficult for spacecraft to survive for long periods,” Blankenship says.

JUICE, however, will endeavor to enter into a Ganymede orbit. Jupiter’s gravity will warp this path over time, according to ESA. But JUICE will make history when it does: the mission will become the first spacecraft to orbit another planet’s moon. JUICE saved the most radiation-intense segments of the trip — when it orbits Ganymede — for the end. It will fly an ambitious mission, and for that reason, the mission will last just four years. Justin Byrne, the head of ESA’s Science Programs at Airbus Defence and Space (which led the consortium building the JUICE spacecraft), told that JUICE will not survive 10 years like many other missions do.

A family portrait of Jupiter and the four Galilean moons. Distances are not to scale. At the top is the volcanic moon Io. Below it, to the right of Jupiter’s Great Red Spot, is Europa. Bottom right shows Ganymede, the largest moon in the Solar System and where JUICE will orbit next decade. Bottom left shows the battered and unhealed surface of the icy moon Callisto.


Maybe Life Found a Way on Europa

Jupiter is harsh, but this could be what makes Europa habitable.

Its gravity, for instance, could be exerting so much damage on a nearby moon — the barren, volcanically active moon Io — that it spouts ingredients for life that reach other moons.

“Tidal forces are responsible for small deformations in the shape of the [moons], particularly for Io, which is so heated by this deformation that it generated interior heat … and multiple sets of sulfur-spewing volcanoes all over Io’s surface,” Glenn Orton, Juno co-investigator at NASA’s Jet Propulsion Laboratory in California, tells Inverse.

After the volcanoes release the sulfur, a chemical element that anaerobic microbes metabolized before Earth’s atmosphere became oxygenated, the particles travel through space and “get pasted all over the surface of Europa,” Blankenship says.

The ice of Europa creates a haven. Not only does it act like a barrier against Jupiter’s radiation, but it might be delivering these nutrients to the ocean below it. Clipper and JUICE may help to characterize the ice and ocean interplay.

Jupiter’s hefty influence could induce hydrothermal vents in Europa's silicate rock below the ocean, too, not unlike the ones found on our planet. “Learning more about oceans on the moons of Jupiter will provide a different perspective on vents here on Earth,” Brendan Smith, a Ph.D. candidate in acoustical oceanography at Dalhousie University in Canada, tells Inverse.

Smith says that Earth’s deep ocean is home to hydrothermal vents. It’s here that bacteria have developed the ability to make energy without sunlight. The process is called chemosynthesis, akin to photosynthesis, but substituting chemicals for light.

“It is hypothesized that these types of chemosynthetic bacteria may have been some of the first life to develop on Earth, and that this could have originated at hydrothermal vents,” Smith says. “As a result, the prospect of hydrothermal vents elsewhere in the Solar System provides an opportunity to investigate whether there could be similar chemosynthetic life outside Earth.”

Imagery from NASA’s Juno spacecraft are put to music and in sequence. This data comes from Juno’s flyby of Ganymede and Jupiter in June 2021.

When they launch

JUICE will fly into space aboard an Ariane 5 rocket from Europe’s Spaceport, which is based near Kourou, French Guiana. The launch is scheduled for 8:15 a.m. Eastern on Thursday, April 13. It will arrive at Jupiter in 2031.

Europa Clipper will leave later, currently set to launch in October 2024, but will reach Jupiter earlier, in 2030. JUICE will cruise past several inner Solar System planets to gain the necessary gravity assists to reach Jupiter. Clipper will do a Mars-Earth gravity assist, skipping JUICE’s Venus flyby, to reach Jupiter.

To watch the JUICE launch, you can tune in to the ESA Web TV YouTube channel. The broadcast begins at 7:15 a.m. Eastern. On Twitter, ESA will share updates using the #ESAJuice hashtag and the @ESA_JUICE @esaoperations @ESA_STS and @esascience accounts, too.