The ocean that hides Mimas, Saturn’s icy moon

The astronomers They compare it to the Death Star from Star Wars, but Mimas, a small moon of Saturn, harbors beneath its icy surface an unlikely liquid ocean conducive to the emergence of life, according to a study published Wednesday.

Mimas thus joins the family of rare moons in the solar system that harbor liquid water under its ice shell: Europa and Ganymede (around Jupiter), Enceladus and Titan (around Saturn).

“If there is any place in the universe where we did not expect to find favorable conditions for life, that place is Mimas”explained Valéry Lainey, lead author of the study published in Nature, at a press conference.

The satellite of the ringed planet, discovered in 1789 by astronomer William Herschel, had no “not at all the right appearance”, according to this astronomer from the IMCCE (Institute of Celestial Mechanics and Ephemeris Calculation) of the Paris Observatory-PSL.

The celestial body, with only 400 kilometers in diameter, was nicknamed “the moon of death” because it seemed cold, inert and, therefore, uninhabited.

This was due to its surface full of craters, including an immense one that gave it an appearance similar to the Death Star, the Empire station in the Star Wars saga.

Its ice shell appeared to be frozen, with no trace of internal geological activity that could modify it. In contrast, the smooth surface of its older sister, Enceladus, is regularly reshaped by the activity of its internal ocean and its geysers.

However, scientists had the intuition that “something was happening inside” from Mimas, said Valéry Lainey. They studied the rotation of the satellite on itself and its small oscillations, called librations, which can vary depending on 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 scientists tended to believe in the existence of a rocky core.

“We could have left things like that, but we felt frustrated”said Valéry Lainey.

His team collected dozens of images taken by NASA’s Cassini probe (2004-2017) to expand their research to the entire Saturn system and 19 of its moons.

These data allowed us to analyze the orbital movement of Mimas around Saturn and how it 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 is the only viable conclusion,” highlighted Matija Cuk, from the SETI Institute for the Search for Extraterrestrial Intelligence (California), and Alyssa Rose Rhoden, from the Southwest Research Institute in Boulder (Colorado), in a commentary attached to the Nature works.

The ocean moves under a layer of ice 20 to 30 kilometers thick, comparable to that of Enceladus, the study describes. It arose under the influence of the gravity of other moons of Saturn: they are “tidal effects” that shake the star and generate heat, preventing its ocean from freezing.

Calculations suggest a sea formed recently, only between 5 and 15 million years ago, which would explain why no geological sign of its existence has yet been detected on the surface.

Moon “it 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 of the IMCCE, one of the authors.

Can Mimas harbor primitive life forms, such as bacteria or archaea? “The issue will be addressed in upcoming space missions in the decades to come”anticipated Valéry Lainey.

“One thing is certain: if you are looking for the latest habitability conditions in the solar system, Mimas is where you should look,” concluded the astronomer.