Astronomers compare it to the Death Star from Star Wars, but it turns out to be more hospitable than expected: Mimas, a small moon of Saturn, contains an unlikely liquid ocean beneath its icy surface that is conducive to the emergence of life, according to a study published Wednesday.
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Mimas completes the family of rare moons in the solar system that harbor liquid water beneath 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 favorable conditions for life, it is Mimas,” said 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, is “not at all suitable for this task,” this astronomer tells the IMCCE (Institute of Celestial Mechanics and Ephemeris Computation) of the Paris-PSL Observatory.
The star, only 400 kilometers across, was nicknamed the “Moon of Death” because it appeared cold, sluggish and therefore uninhabitable. The reason: Its surface is littered with craters, including a huge crater that gives it the false appearance of the Death Star, the station of the Galactic Empire in the Star Wars saga.
Its icy shell appeared to be frozen, and there was no trace of internal geological activity to alter it. Unlike its big brother Enceladus, whose smooth surface regularly reshapes thanks to the activity of its inner ocean and its geysers – a source of heat necessary to keep the water in a liquid state.
The scientists still had an inkling that “something was happening in Mimas,” says Valéry Lainey. They then examined the satellite's rotation around itself and its small oscillations, so-called librations, which can vary depending on the star's internal structure.
AFP
A young ocean
Their first paper, published in 2014, failed because of the decision to use a liquid ocean. The majority of scientists lean towards the hypothesis of a rock core.
“We could have left it at that, but we were frustrated,” remembers Valéry Lainey. His team then recovered several dozen images from NASA's Cassini spacecraft (2004-2017) to expand their research to the entire Saturn system and 19 of its moons.
These data enabled the analysis of Mimas' orbital motion around Saturn and its effects on its librations. And to detect tiny fluctuations in these librations, on the order of a few hundred meters, which indicate the presence of a liquid ocean beneath the entire surface.
“That 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, the study describes. It would have formed under the influence of the gravity of Saturn's other moons: “tidal effects” that shake the star and generate heat that prevents its ocean from freezing.
Calculations suggest that the sea formed recently, between 5 and 15 million years ago, which would explain why no geological signs have yet been discovered 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 the chemical exchange vital for life develops,” summarizes Nicolas Rambaux from IMCCE, one of the authors .
Could Mimas harbor forms of primitive life such as bacteria or archaea? “The question will be addressed in the next decades with the next space missions,” expects Valéry Lainey.
“One thing is certain: if you are looking for the most up-to-date habitability conditions in the solar system, Mimas is the place to be,” concludes the astronomer.