Could vast, long-hidden oceans be teeming with alien life in our own solar system? A new chapter in humanity’s search for extraterrestrial life opens Thursday, when Europe’s JUICE spacecraft begins its mission to explore Jupiter’s icy moons. First discovered over 400 years ago by Italian astronomer Galileo Galilei, these ice-covered moons are so far from the Sun that they have long been dismissed as possible candidates for hosting life in our backyard.

Until recently, the solar system’s habitable zone was thought to “end on Mars”, French astrophysicist Athena Coustenis, one of the scientific leaders of the European Space Agency’s (ESA) JUICE mission, told AFP. But NASA’s 1995 Galileo probe to Jupiter and the Cassini spacecraft’s recent trip to Saturn have allowed scientists to broaden their horizons.

The gas giant planets themselves were rightly excluded, but their icy moons, including Jupiter’s Europa and Ganymede, as well as Saturn’s Enceladus and Titan, have offered new hope for life nearby.

Beneath their icy surfaces are thought to be vast oceans of liquid water, an essential ingredient for life as we know it. ESA’s JUICE project scientist Nicolas Altobelli said it would be “the first time we’ve discovered habitats beyond freezing point” between Mars and Jupiter. Beyond this limit, the temperature drops significantly and “liquid water can no longer be found at the surface,” Altobelli told AFP earlier this year.

“Giant” ocean

The Jupiter Icy Moons Explorer (JUICE) mission will launch Thursday from the European Spaceport in Kourou, French Guiana, on an eight-year journey through space. It will orbit Jupiter in July 2031, from where it will study Ganymede, Europa, and another icy moon, Callisto. Then, in 2034, JUICE will orbit Ganymede, the first time a spacecraft has ever made it around a moon other than ours.

In addition to being the largest satellite in the Solar System, Ganymede is the only moon with its own magnetic field that shields it from harmful radiation. This is just one of several indications that Ganymede’s hidden ocean can provide a stable environment for life. Unlike similar missions to Mars, which focus on finding signs of ancient life long gone, scientists hope that Jupiter’s icy moons will host living organisms, even if they are only tiny or single-celled.

Such habitability requires a power source. Because no energy comes from the Sun, the moons may instead use the gravity Jupiter exerts on its moons. This force creates a process called tidal heating, which heats the moons interior and keeps the water in a liquid state.

Carole Larigoderi, head of the JUICE project at the French space agency CNES, said Ganymede’s “huge” liquid ocean is compressed between two thick ice sheets tens of kilometers below the surface.

“On Earth, we still find life forms at the bottom of the abyss,” he added.

Tiny microbes such as bacteria and archaea have been found to be able to survive on Earth without sunlight, raising hopes that life elsewhere could do the same. In addition to water and energy, life needs nutrients.

“So the real question is whether Ganymede’s ocean contains the necessary chemical elements,” Koustenis said.

He added that the ocean must be able to absorb nutrients from anything that falls to the moon’s surface, such as those that are then dissolved in water.