An ocean discovered beneath the surface of Mimas, Saturn’s small satellite

2024-02-07 16:00:12
Recomposed image of Mimas, a satellite of Saturn which shelters an ocean under its icy crust. FRÉDÉRIC DURILLON/ANIMEA STUDIO/OBSERVATORY DE PARIS-PSL/IMCCE

Even though it bears the name of a giant from Greek mythology, Mimas is only a fairly small satellite of Saturn with a radius of 200 kilometers. In the images we have of the star, a ball of extremely cratered ice appears, whose pockmarked surface seems not to have changed since the dawn of time. Apparently a dead star, but appearances are sometimes deceiving. Because, under the frozen crust, in the dark depths of this Saturnian moon, a complex activity led to the appearance of a subterranean ocean less than twenty-five million years ago – i.e. recently, on the scale astronomical times, since the Solar System is more than 4.5 billion years old.

Published in the weekly magazine Nature, Wednesday February 7, this discovery is the fruit of the long analysis work of an international team led by researchers from the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE), an entity belonging to the Paris Observatory -PSL. More than a discovery, language purists would speak of a demonstration, of a complex demonstration in two stages separated by a decade.

Let’s go back ten years to refer to an article published by the journal Science in 2014, from what was more or less the same team. At the time, these researchers wondered how to determine the internal structure of Mimas. To achieve this, “we went back to basics: celestial mechanics and Newton’s laws”remembers Valéry Lainey, astronomer at IMCCE and first author of the study of Nature. The only clues available to scientists were the photos taken by NASA’s Cassini space probe which, between 2004 and 2017, tirelessly explored the Saturnian system.

Devilish dynamism

This was enough to carefully study the rotation of Mimas and the small oscillations that we detect there. To explain them, the 2014 article put forward two hypotheses: either the satellite had a rocky core in the shape of a rugby ball, or its ice shell slid on an internal ocean.

“We were frustrated not knowingadmet M. Lainey. The overwhelming majority of scientists were convinced that Mimas was cold, frozen, uninhabitable, that there was no liquid ocean. » But a conviction does not make a demonstration. “We looked for a solution to discriminate between the two scenarios”, continues the IMCCE astronomer. By deciding to no longer study the rotation of the moon but the subtleties of its orbital movement, again thanks to Cassini data, in this case tens of thousands of images.

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