2024-02-10 14:45:16
Published on: 02/10/2024 – 3:45 p.m.
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Mimas completes the family of rare moons in the solar system sheltering liquid water under their ice floes: Europa and Ganymede (around Jupiter), Enceladus and Titan (around Saturn).
“If there is one place in the Universe where we did not expect to find conditions favorable to life, it is Mimas,” explained Valéry Lainey, lead author, at a press conference. of the study published in Nature.
The satellite of the ringed planet, discovered in 1789 by the astronomer William Herschel, was “not at all suitable for the job”, this astronomer tells the IMCCE (Institute of Celestial Mechanics and Calculation of Ephemeris ) from the Paris-PSL Observatory.
The star, only 400 kilometers in diameter, was nicknamed the “death moon” as it seemed cold, inert and therefore uninhabitable. The reason: its surface riddled with craters, including a huge one giving it the false appearance of the Death Star, the station of the Galactic Empire in the Star Wars saga.
Its icy shell appeared frozen, with no trace of internal geological activity likely to modify it. Unlike its big sister Enceladus, whose smooth surface is regularly remodeled thanks to the activity of its internal ocean and its geysers – a source of heat necessary to maintain the water in a liquid state.
The scientists nevertheless had the intuition that “something was happening inside” Mimas, says Valéry Lainey. They then studied the rotation of the satellite on itself and its small oscillations, called librations, which can vary depending on the internal structure of the star.
A young ocean
Their first work, published in 2014, failed to decide in favor of a liquid ocean. A majority of scientists lean more towards the hypothesis of a rocky core.
“We might have left it there, but we were frustrated,” remembers Valéry Lainey. His team then recovered several dozen images taken by NASA’s Cassini probe (2004-2017), in order to broaden its research to the entire Saturnian system and 19 of its moons.
These data made it possible to analyze the orbital motion of Mimas around Saturn and how it affects its librations. And to detect tiny variations in these librations, of the order of a few hundred meters, betraying the presence of a liquid ocean under the entire surface.
“This is the only viable conclusion,” emphasize Matija Cuk, of the SETI Institute for the Search for Extra-Terrestrial Intelligence (California), and Alyssa Rose Rhoden, of the Southwest Research Institute in Boulder (Colorado), in an accompanying commentary to the works of Nature.
The ocean moves under an ice thickness of 20 to 30 kilometers, comparable to that of Enceladus, describes the study. It would have been born under the influence of the gravity of other moons of Saturn: “tidal effects” which shake the star and create heat preventing its ocean from freezing.
Calculations suggest a sea formed recently, only between 5 and 15 million years ago, which would explain why no geological signs have yet been detected on the surface.
The moon “brings together all the conditions for habitability: liquid water, maintained by a heat source, in contact with rock so that chemical exchanges develop” essential to life, summarizes Nicolas Rambaux of the IMCCE, one of the authors.
Could Mimas harbor forms of primitive life, such as bacteria or archaea? “The question will be addressed to the next space missions in the decades to come,” anticipates Valéry Lainey.
“One thing is certain: if you are looking for the most recent conditions of habitability in the solar system, Mimas is the place to look,” concludes the astronomer.
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