Astronomers compare it to the Death Star from “Star Wars”, but Mimas, a small moon of Saturn, harbors beneath its icy surface an unlikely ocean suitable for the emergence of life, according to a study published this Wednesday ( 7).
Mimas thus joins the family of rare moons in the Solar System that contain liquid water beneath their ice layer: Europa and Ganymede, from Jupiter; Enceladus and Titan, from Saturn.
“If there is any place in the Universe where we would not expect to find favorable conditions for life, it is Mimas,” said Valéry Lainey, lead author of the study published in Nature, at a press conference.
The ringed planet’s satellite, discovered in 1789 by astronomer William Herschel, did not have “the right appearance”, according to the astronomer at the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) at the Paris Observatory – PSL.
The celestial body, just 400 kilometers in diameter, was nicknamed the “moon of death” because it appeared cold, inert and therefore uninhabited.
This was due to its surface full of craters, including a huge one that gave it an appearance similar to the Death Star, the Empire’s station in the “Star Wars” saga.
Its ice shell appeared to be frozen, with no sign of internal geological activity that could modify it. In contrast, the smooth surface of its larger sister, Enceladus, is regularly reshaped by the activity of its internal ocean and geysers.
However, scientists had a hunch that “something was going on inside” Mimas, Lainey said. They studied the moon’s rotation around itself and its small oscillations, called librations, which can vary according to the internal structure of the star.
A young ocean
His first works, published in 2014, failed to demonstrate the existence of a liquid ocean. Most researchers tended to believe in the existence of a rocky core.
“We could have left it like that, but we were frustrated,” Lainey said.
His team compiled dozens of images taken by NASA’s Cassini probe (2004-2017) to expand their search to the entire Saturn system and 19 of its moons.
This data allowed us to analyze the orbital movement of Mimas around Saturn and how this affects its librations. The detection of minute variations in these librations, on the order of hundreds of meters, revealed the presence of a liquid ocean beneath the entire surface.
“It’s the only viable conclusion,” highlighted Matija Cuk, from the SETI Institute for Searching for Extraterrestrial Intelligence, in California, and Alyssa Rose Rhoden, from the Southwest Research Institute, in Colorado, in a comment attached to the Nature papers.
The ocean moves beneath an ice sheet 20 to 30 kilometers thick, comparable to that on Enceladus, the study describes. It arose under the influence of the gravity of Saturn’s other moons: these are “tidal effects” that agitate the star and generate heat, preventing its ocean from freezing.
Calculations suggest a sea formed recently, just between 5 and 15 million years ago, which would explain why no geological sign of its existence has yet been detected on the surface.
The moon “meets all the conditions for habitability: liquid water, maintained by a heat source, in contact with rock, which favors the development of chemical exchanges” essential for life, summarized Nicolas Rambaux, from IMCCE, one of the authors .
Could Mimas harbor primitive life forms such as bacteria or archaea? “The issue will be addressed in space missions in the decades to come,” said Lainey.
“One thing is certain: if you are looking for more recent conditions of habitability in the Solar System, it is to Mimas that you should look”, concluded the astronomer.