Photo of Mimas taken by the Cassini probe, provided by NASA on March 13, 2017
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Astronomers compared Mimas to the Death Star from Star Wars, but this small moon of Saturn might be cozier than imagined. Its icy surface hides an unsuspected liquid ocean, conducive to the emergence of life, according to a study published on Wednesday.
Mimas joins the family of the few moons in the solar system with liquid water under their ice sheets: Europa and Ganymede (around Jupiter), Enceladus and Titan (around Saturn).
“If there is a place in the universe where we did not expect to find favorable conditions for life, it is Mimas,” explained Valéry Lainey, main author of the study published in the journal Nature, in a press conference.
The satellite of the ringed planet, discovered in 1789 by astronomer William Herschel, did not have “the profile at all,” says the astronomer from the IMCCE (Institute of Celestial Mechanics and Ephemeris Calculation) of the Paris Observatory.
The star, barely 400 kilometers in diameter, was nicknamed the “moon of death” for its cold, inert and, therefore, uninhabitable appearance. Its surface is riddled with craters, including a huge one that resembles the Death Star, the space station of the Galactic Empire in the Star Wars saga.
Its ice cap seemed motionless, with no trace of internal geological activity that could alter it, unlike its older sister, Enceladus, whose smooth surface changes regularly due to the activity of its internal ocean and its geysers, a source of heat necessary to maintain the water in liquid state.
But scientists had a hunch that “something was happening inside” Mimas, explains Lainey, so they studied the satellite’s rotation on itself and its small oscillations, called librations, which can vary depending on the internal structure of the star.
Their first studies, published in 2014, found no evidence of the existence of a liquid ocean. Most scientists leaned more towards the hypothesis of a rocky core.
“We could have left it at that, but we were frustrated,” Valéry Lainey recalls. His team then recovered several dozen images taken by NASA’s Cassini probe (2004-2017), in order to expand the research to the entire Saturn system and 19 of its moons.
These data made it possible to analyze the orbital movement of Mimas around Saturn and the way in which it affects its librations, and to detect minute variations in these, of a few hundred meters, which reveal the presence of a liquid ocean under the entire surface.
“It is the only viable conclusion,” indicate Matija Cuk, from the SETI Institute for the Search for Extraterrestrial Intelligence (California), and Alyssa Rose Rhode, from the Southwest Research Institute in Boulder (Colorado), in a comment associated with the Nature study.
The ocean moves under an ice thickness of between 20 and 30 kilometers, comparable to that of Enceladus, the study describes. It is believed that it formed under the influence of the gravity of other moons of Saturn: “tidal effects” that agitate the star and create heat that prevents the ocean from freezing.
Calculations suggest that the sea was formed recently, between 5 and 15 million years ago, which would explain why geological signals have not yet been detected on the surface.
The moon “meets all the conditions for habitability: liquid water, maintained by a heat source, in contact with rocks so that the chemical exchanges essential for life can develop,” summarizes Nicolas Rambaux, another of the authors.
Is it possible that Mimas harbors primitive life forms, such as bacteria or archaea? “This issue will be addressed in future space missions in the coming decades,” predicts Valéry Lainey.
“One thing is clear: if you are looking for the most recent habitable conditions in the solar system, you have to look at Mimas,” concludes the astronomer.