(Kourou) Beneath their ice caps move vast oceans of liquid water, fertile ground for the emergence of life: the exploration of the icy moons of Jupiter, destination of the Juice mission, opens a new chapter in the quest for worlds habitable extraterrestrials.

These environments are so far from the Sun that astronomers have long excluded them from the zone of the solar system considered habitable, “which until recently stopped at Mars”, explains to AFP the astrophysicist Athéna Coustenis, the one of the scientific leaders of the European probe.

Discoveries by the Galileo (1995) probes around Jupiter, and Cassini (2004) around Saturn, have pushed back the frontiers of research. Which focuses not on these giant, gaseous and therefore unsustainable planets, but on their icy moons: Europa and Ganymede for Jupiter; Enceladus and Titan for Saturn.

“This is the first time that we will explore habitats beyond the frost line, where liquid water can no longer exist on the surface,” said Nicolas Altobelli, head of Juice for the space agency. European Union (ESA), in January at Airbus in Toulouse, where the probe was designed.

NASA’s future Europa Clipper mission will target Europe. Juice is betting on Ganymede: in 2034, it should be placed in orbit around this natural satellite, the largest in the solar system. It is also the only moon to have its own magnetic field protecting it from dangerous radiation.

So many characteristics suggesting a stabilized environment, another condition for the emergence of living things… and their maintenance. Because “the whole thing is not that life appears, but that it subsists”, underlines Athéna Coustenis, CNRS researcher at the LESIA Laboratory of the Paris-PSL Observatory.

Unlike missions to Mars, in search of traces of ancient life that has now disappeared, the exploration of icy moons seeks environments that are still habitable. What the Red Planet is no longer.

This phenomenon makes it possible to “dissipate the heat inside the moons and to maintain the water in a liquid state”, deciphers Francis Rocard, planetologist at the National Center for Space Studies (CNES).

Ganymede’s ocean is “gigantic”, describes Carole Larigauderie, head of the Juice project at CNES. Trapped between two thick layers of ice, it would be several tens of kilometers deep.

“On Earth, we manage to find life forms at the bottom of the abyss,” she notes. Some terrestrial ecosystems are indeed able to survive without light and are teeming with micro-organisms such as bacteria or archaea.

Such an ecosystem needs nutrients to maintain itself. “So the whole question is whether the ocean of Ganymede contains it,” according to Athena Coustenis. For example, the ocean would have to be able to absorb components deposited on the surface, to then be dissolved in water, continues the astrophysicist.

Juice’s instruments will inspect this ocean from every angle to gauge its depth, distance from the surface and, hopefully, its composition.

Devoid of a magnetosphere, its sister Europe is less hospitable for a spacecraft: the American probe Europa Clipper, which will reach its destination at the same time as Juice, will only be able to fly over its target. The data collected by the two missions will nevertheless be complementary, underline the scientists.

If Ganymede turns out to tick all the boxes for harboring life, the “logical next step” would be to send a lander there, says Cyril Cavel, Airbus chief scientist. “It’s part of the dream” although there are no plans at this stage.