Image provided in 2003 by Nasa showing Jupiter’s four Galilean moons: Io, Europa, Ganymede, and Callisto (NASA/NASA)
Beneath their pack ice 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 extra-terrestrial worlds habitable.
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. Who focuses not on these giant, gaseous planets therefore not conducive to life, but their icy moons: Europa and Ganymede for Jupiter; Enceladus and Titan for Saturn.
Their major asset: to shelter under their surface of ice oceans of liquid water – only water in the liquid state making life possible.
Diagram representing the ESA’s Juice (Jupiter icy moons explorer) probe, whose launch on board an Ariane 5 rocket is scheduled for April 13, 2023 in Kourou, French Guiana (AFP / )
“This is the first time that we are going to explore habitats beyond the frost line, where liquid water can no longer exist on the surface”, welcomed Nicolas Altobelli, head of Juice for the space agency. European Union (ESA), in January at Airbus in Toulouse, where the probe was designed.
The future NASA Europa Clipper mission will target Europe. Juice, she bets on Ganymede: in 2034, she 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.
– Gigantic ocean –
So many characteristics suggesting a stabilized environment, another condition for the emergence of life… and its maintenance. Because “the whole thing is not that life appears, but that it remains”, underlines Athéna Coustenis, CNRS researcher at the LESIA Laboratory of the Paris-PSL Observatory.
Hubble Telescope image provided by Nasa, December 2008, showing Jupiter and its moon Ganymede (NASA/HO)
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.
Habitability also requires a power source. Under the freezing temperatures of the Jovian system, it does not come from the Sun but from the gravity that Jupiter exerts on its satellites: “tidal effects” similar to what happens on Earth with its moon.
This phenomenon makes it possible to “dissipate the heat inside the moons and to maintain the water in a liquid state”, deciphers Francis Rocard, planetary scientist 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 remarks. Some terrestrial ecosystems are indeed able to survive without light and are teeming with micro-organisms such as bacteria or archaea.
– Complementary missions –
Such an ecosystem needs nutrients to maintain itself. “The whole question is therefore whether the ocean of Ganymede contains it”, according to Athéna 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.
Artist’s impression provided by Nasa and ESA in 2015, depicting Ganymede orbiting Jupiter (NASA/ESA/Handout)
Juice’s instruments will inspect this ocean from every angle to gauge its depth, distance from the surface and, hopefully, its composition.
The spacecraft will orbit about eight months around Ganymede, which it will be able to approach up to 200 km in altitude, sheltered from radiation.
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 it turns out that Ganymede ticks all the boxes to host life, the “logical next step” would be to send a lander there, says Cyril Cavel, Airbus scientific manager. “It’s part of the dream” although there are no plans at this stage.