Welcome to Mimas, Saturn’s icy moon with a life-friendly ocean

“If there is one place in the universe where we would not expect to find favorable conditions for life, it is Mimas,” Valerie Laney, lead author of the study published in the journal Nature, explained at a press conference.

The ringed planet’s moon, discovered in 1789 by astronomer William Herschel, “had no profile at all,” says the astronomer at the IMCCE (Institute of Celestial Mechanics and Calculus of Ephemerides) at the Paris Observatory.

The star, barely 400 kilometers in diameter, has been nicknamed the “moon of death” for its cold, inert and therefore uninhabitable appearance. Its surface is riddled with craters, including a huge one reminiscent of the Death Star, the space station of the Galactic Empire from the Star Wars saga.

Its ice cap appeared motionless, with no sign of internal geological activity that could change it, unlike its older sister, Enceladus, whose smooth surface regularly changes due to the activity of the internal ocean and geysers, the source of heat needed to maintain ice cap liquid water.

But scientists had a hunch that “something was going on inside” Mimas, Laney explains, so they studied the moon’s spin itself and its small fluctuations called librations, which can vary depending on the star’s internal structure.

Their first studies, published in 2014, found no evidence of a liquid ocean. Most scientists were more inclined to the rocky core hypothesis.

“We could have left it as is, but we were disappointed,” recalls Valerie Laney. His team then reconstructed dozens of images taken by NASA’s Cassini probe (2004–2017) to expand the study to the entire Saturn system and its 19 moons.

These data made it possible to analyze the orbital motion of Mimas around Saturn and how it affects its librations, as well as to detect minute changes in them, several hundred meters, that indicate the presence of a liquid ocean beneath the entire surface. .

“This is the only viable conclusion,” said Matija Cook of the SETI Institute in California and Alyssa Rose Rhode of the Southwest Research Institute in Boulder, Colorado, in a commentary associated with the journal Nature. study.

The ocean moves under a layer of ice 20 to 30 kilometers thick, comparable to the thickness of the ice on Enceladus, the study says. It is thought to have been formed by the gravity of Saturn’s other moons: “tidal effects” that churned up the star and created heat that kept the ocean from freezing.

Calculations show that the sea formed recently, between 5 and 15 million years ago, which explains why geological signals on the surface have not yet been detected.

The moon “meets all the conditions for habitation: liquid water, supported by a heat source, in contact with rocks so that the chemical exchange necessary for life can develop,” summarizes Nicolas Rambaud, another of the authors.

Is it possible that Mimas is home to primitive life forms such as bacteria or archaea? “This problem will be solved in future space missions in the coming decades,” predicts Valerie Laney.

“One thing is clear: if you are looking for the latest habitat in the solar system, you need to look at Mimas,” the astronomer concludes.