When, in 1979, the American probes Voyager-1 and Voyager-2 flew over the Jovian system, the photographs they returned from the Europa satellite astounded astronomers. “A cracked ball of ice”, summarizes Cécile Ferrari, professor at the University of Paris Cité. The images show a flat moon, without mountains or volcanoes, as if covered with a multi-fractured pack ice. All stitched with fine streaks and large scars bordered by deposits of a brown verging on red.
Very quickly the idea emerges that, under the icy crust, there is an immense ocean of salt water, with its possible promises of life. Under the effect of the tidal forces created by the gigantic mass of Jupiter, only 670,000 kilometers away, Europa is constantly kneaded and seems to crack everywhere, like the shell of a hard-boiled egg that is kneaded before peel. “It’s an active moon, explains Olivier Witasse, scientific manager of the Juice mission at the European Space Agency (ESA). Its surface has practically no craters, so it is young. The age of this surface is estimated at a few tens of millions of years, which is the equivalent of yesterday in the time scale of the Solar System. »
Track the appearance of life
Everything in Europe makes astrobiologists wriggle, those researchers who track down the environments where life is likely to appear. The activity of this moon and the reshaping of its surface are encouraging. To this we must, of course, add the virtual certainty that there is an immense ocean under the ice crust, which, on its own, could contain twice as much water as there is on Earth. The presence of geysers on the surface is strongly suspected and many believe that the brownish deposits along the fractures are due to resurgences of salt water. “We have the signature of magnesium sulphates, which are analogous to the salts found in our oceans”, says Cécile Ferrari.
Finally, Olivier Witasse evokes a final exciting point for astrobiologists: “We deduce from the internal structure of Europa that the ocean would be in contact, at the bottom, with rocks”while on Ganymede, the satellite that will be mainly inspected by Juice, an ice floor probably separates the ocean from the rocky mantle. “This interface with the rock is very interestingcontinues Olivier Witassebecause it can be quite similar with what is found at the bottom of our terrestrial oceans”in particular the hydrothermal vents around which original ecosystems have developed.
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