Exploring the possibility of life on Jupiter’s moons: Two missions set to investigate their hidden subsurface oceans.

On April 13, 2023, the European Space Agency launched a rocket carrying a spacecraft destined for Jupiter. The Jupiter Icy Moons Explorer — or JUICE — will spend at least three years on Jupiter’s moons after arriving in 2031. In October 2024, NASA also plans to launch a robotic spacecraft called Europe Clippers to the Jovian moons, indicating an increased interest in these distant but fascinating places in the solar system.

I am a planetary scientist who studies the structure and evolution of fixed planets and moons in the solar system.

There are many reasons why my colleagues and I look forward to the data that JUICE and Europa Clipper will hopefully send back to Earth in the 2030s. But perhaps the most exciting information has to do with water. Three of Jupiter’s moons — Europa, Ganymede and Callisto — harbor large, subsurface oceans of liquid water that could support life.

Meet Io, Europa, Ganymede and Callisto

Jupiter has dozens of moons. Four of these are of particular interest to planetary scientists.

Io, Europa, Ganymede, and Callisto, like Earth’s Moon, are relatively large, spherical complex worlds. Two previous NASA missions have sent spacecraft to orbit the Jupiter system and collected data on these moons. The Galileo mission orbited Jupiter from 1995 to 2003 and led to geological discoveries on all four major moons. The Juno mission still orbits Jupiter and has given scientists an unprecedented view of Jupiter’s composition, structure and space environment.

These missions and other observations revealed that Io is the closest of the four to its host planet effervescent geological activity, including lava lakes, volcanic eruptions and tectonically formed mountains. But it is not home to large amounts of water.

Europe, Ganymede and Callisto, on the other hand, have icy landscapes. The surface of Europe is a frozen wonderland with a young but complex history. possibly including icy analogues of plate tectonics and volcanoes. Ganymede, the largest moon in the entire solar system, is larger than Mercury and has its own magnetic field generated internally from a liquid metal core. Callisto appears somewhat inert compared to the others, but serves as a valuable time capsule of an ancient past no longer accessible on the youthful surfaces of Europa and Io.

Most exciting of all, Europa, Ganymede and Callisto almost certainly all possess subsurface oceans of liquid water.

Ocean worlds

Europa, Ganymede and Callisto have chilly surfaces hundreds of degrees below zero. At these temperatures, ice behaves like solid rock.

But just like Earth, the deeper you go underground on these moons, the hotter it gets. Go down far enough and eventually you’ll reach the temperature where ice melts into water. Exactly how far down this transition occurs on each of the moons is a subject of discussion which scientists hope to solve with JUICE and Europa Clipper. While the exact depths are still uncertain, scientists are confident that these oceans exist.

The best evidence of these oceans comes from Jupiter’s magnetic field. Salt water is electrically conductive. So as these moons travel through Jupiter’s magnetic field, they generate a secondary, smaller magnetic field that signals to researchers the presence of a subsurface ocean. Using this technique, planetary scientists have been able to demonstrate that the three moons contain subsurface oceans. And these oceans are not small – Europe’s ocean alone could have more than them double the water of all the Earth’s oceans combined.

An obvious and tantalizing next question is whether these oceans can support extraterrestrial life. Liquid water is an important part of what makes for a habitable world, but it is far from the only requirement for life. Life also needs energy and certain chemical compounds next to water to bloom. Because these oceans are hidden under miles of ice, sunlight and photosynthesis are off. But it is possible that other sources can provide the necessary ingredients.

On Europa, for example, the ocean with liquid water covers a rocky interior. That rocky seabed could provide energy and chemicals through underwater volcanoes that could make Europa’s ocean habitable. But it’s also possible that Europa’s ocean is a sterile, inhospitable place — scientists need more data to answer these questions.

Upcoming ESA and NASA missions

JUICE and Europa Clipper were set up to provide scientists with groundbreaking information about the potential habitability of Jupiter’s moons. While both missions will collect data across multiple moons, JUICE will spend time orbiting and focusing on Ganymede, and Europa Clipper will make dozens of close flybys of Europa.

Both spacecraft will carry an array of science instruments built specifically to explore the oceans. JUICE and Europa Clipper are possible on board the radar to examine the moons outer layers of solid ice. Radar could reveal small pockets of liquid water in the ice, or, in the case of Europa, which has a thinner outer ice cover than Ganymede and Callisto, hopefully discovering the greater ocean.

Magnetometers will be too on both missions. These tools will allow scientists to study in detail the secondary magnetic fields produced by the interaction of conducting oceans with Jupiter’s field and will hopefully provide researchers with clues about the salinity and volumes of the oceans.

Scientists will also observe slight variations in the moons’ gravitational pull by tracking subtle motions in the orbits of both spacecraft, which could help determine whether Europa’s seafloor has volcanoes that provide the necessary energy and chemistry for the ocean to support life.

Finally, both craft will carry a host of cameras and light sensors that will provide unprecedented images of the geology and composition of the moons’ icy surfaces.

Perhaps one day a spacecraft will be able to drill through the miles of solid ice on Europa, Ganymede or Callisto and explore the oceans directly. Until then, observations from spacecraft like JUICE and Europa Clipper are scientists’ best bet for learning more about these ocean worlds.

When Galileo discovered these moons in 1609, they were the first objects known to directly orbit another planet. Their discovery was the final nail in the coffin of the theory that the Earth – and humanity – is at the center of the universe. Perhaps these worlds have another humiliating surprise in store.