Astronomers compare it to the “Death Star” from the “Star Wars” movie franchise, but it turned out to be more hospitable than scientists imagined until now: Mimas, a small moon of the planet Saturn, hides an unexpected liquid ocean under its frozen surface which is conducive to the emergence of life, according to a study published on Wednesday, informs AFP.
Mimas has a diameter of only 400 kilometers. Photo: NASA / JPL-Caltech / Space Science Institute
Mimas thus completes the family of the few natural satellites in the Solar System that harbor liquid water under their ice caps: Europa and Ganymede (around the planet Jupiter), Enceladus and Titan (around the planet Saturn), writes Agerpres.
“If there is a place in the Universe where we did not expect to find conditions favorable to life, it is Mimas”said Valery Lainey, the lead author of the study published in the journal Nature, in a press conference.
This satellite of the planet Saturn, discovered in 1789 by the astronomer William Herschel, “it did not seem at all suitable for this purpose“, said the same astronomer from the Institute for Celestial Mechanics and the Calculation of Ephemeris (IMCCE) within the Paris-PSL Observatory.
The Mimas satellite, with a diameter of only 400 kilometers, was nicknamed the “Moon of Death” because it appeared cold, inert and therefore uninhabitable. The reason: its surface is pockmarked with craters, including a huge crater that makes it look like “Death Star” (“Death Star”), the station of the Galactic Empire from the “Star Wars” movie franchise (“Star Wars”).
“The Only Viable Conclusion”
Its ice sheet appeared immobile, with no trace of internal geological activity to alter it. unlike its bigger “sister” Enceladus, whose smooth surface is regularly reshaped by the activity of its internal ocean and its geysers – a source of heat needed to keep water in a liquid state.
Scientists, however, had the intuition that “something was happening inside” to Mimas, says Valéry Lainey. They studied the rotation of the satellite around its own axis and its small oscillations, called librations, which can vary depending on the internal structure of the celestial body.
Their first research, published in 2014, failed to support the theory of the existence of a liquid ocean. Most scientists have leaned more towards the hypothesis of a telluric core.
“We could have stopped there, but we were frustrated” of those results, remembers Valery Lainey. His team then recovered dozens of images taken by the Cassini space probe, managed by NASA in the period 2004-2017, in order to expand its research field to the level of the entire Saturnian system, which includes 19 natural satellites.
These data allowed the French researchers to analyze the orbital movement of the Mimas satellite around the planet Saturn and the way it affects its librations. They thus detected minute variations in the librations, of the order of a few hundred meters, which thus betrayed the presence of a liquid ocean under the entire surface of the satellite.
“It is the only viable conclusion”Matija Cuk of the California-based SETI Institute for Extraterrestrial Intelligence Research and Alyssa Rose Rhoden of the Southwest Research Institute in Boulder, Colorado, pointed out in an accompanying commentary published alongside the study in the journal Nature.
“It meets all the conditions of habitability”
The ocean on Mimas moves under a layer of ice with a thickness of 20-30 kilometers, comparable to that on Enceladus, the authors of the study state. It is believed that it was born under the influence of the gravity of the other moons of Saturn: “tidal effects” which shakes the celestial body and creates heat preventing the total freezing of its ocean.
Calculations suggest a recently formed ocean, only 5 million to 15 million years ago, which would explain why no geological formations have yet been detected on the surface.
Moon Mimas “brings together all the conditions for habitability: liquid water, maintained by a heat source, in contact with rocks, so that chemical exchanges can develop” indispensable for life, summarized Nicolas Rambaux, researcher at IMCCE and also one of the authors of the study.
Could Mimas harbor primitive life forms such as bacteria and archaea? “This question will be addressed by future space missions in the coming decades“, anticipates Valery Lainey.
“One thing's for sure: if you're looking for the latest data on habitability in the Solar System, then Mimas is the place to look.”concluded the French astronomer.