Frédéric Durillon/Animea Studio/Observatoire de Paris – PSL, IMCCE
An artist’s depiction shows Mimas orbiting around Saturn, resembling the Death Star from “Star Wars” films due to a massive crater.
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Astronomers have found further evidence that one of Saturn’s smallest moons, Mimas, has an underground ocean beneath its icy surface. Establishing a stronger argument for the presence of water — essential for life as we know it — could help scientists better understand where to look for habitable worlds in the vastness of deep space.
Previously, scientists thought Mimas was mainly composed of ice before NASA’s Cassini mission studied Saturn and some of its 146 moons during its orbit around the ringed planet from 2004 to 2017.
Discovered in 1789 by English astronomer William Herschel as a pinpoint near Saturn, Mimas was first photographed from space by the Voyager probes in 1980. The moon’s surface is covered with craters, with the largest one measuring 80 miles (approximately 130 kilometers) across, giving the moon the appearance of the Death Star from the “Star Wars” movies.
Data obtained during Cassini flybys of Mimas intrigued astronomers. The moon takes slightly over 22 hours to orbit Saturn and is only about 115,000 miles (186,000 kilometers) from the planet. The Cassini data revealed changes in Mimas’ rotation and orbital motion caused by the moon’s interior.
In 2014, a team of European researchers determined that either a rigid, elongated, and rocky core or a subsurface ocean caused the moon’s rotation and motion.
To follow up on the previous study, Observatoire de Paris astronomer Dr. Valéry Lainey and his colleagues analyzed the orbital motion data to see which scenario was most likely. The findings were published Wednesday in the journal Nature.
The team concluded that the moon’s spin and orbital motion didn’t match up with the Cassini observations if Mimas had a pancake-shaped rocky core. Instead, the evolution of Mimas’ orbit over time suggested that an internal ocean has influenced its motion, according to Lainey.
“This discovery adds Mimas to an exclusive club of moons with internal oceans, including Enceladus and Europa, but with a unique difference: its ocean is remarkably young, estimated to be only 5 (million) to 15 million years old,” said study coauthor Dr. Nick Cooper, honorary research fellow in the astronomy unit of the School of Physical and Chemical Sciences at Queen Mary University of London, in a statement.
The research team determined the origin and age of Mimas’ ocean by studying how the moon, roughly 249 miles (400 kilometers) in diameter, responded to the gravitational forces that Saturn exerted on it.
“Internal heating must come from the tides raised by Saturn on Mimas,” Lainey said. “These tidal effects have induced friction inside the satellite, providing heat.”
The team suspects the ocean is about 12.4 miles to 18.6 miles (20 kilometers to 30 kilometers) deep beneath the moon’s ice shell. With the ocean being relatively young, astronomically speaking, there wouldn’t be any obvious signs of activity on the surface hinting at the presence of a subsurface ocean.
Enceladus appears younger because active geysers have contributed to resurfacing, or depositing of new, fresh material on its surface, unlike Mimas, which has craters across its surface, suggesting it has an old surface.
The ocean is still evolving, so Mimas may provide a unique insight into the processes behind how subsurface oceans have formed on other icy moons, according to the researchers.
The discovery could alter the way astronomers perceive moons across our solar system.
“If Mimas hides a global ocean, this means that liquid water could lie almost anywhere,” Lainey said. “We already have serious candidates for global oceans (on moons such as) Callisto, Dione and Triton.”
In 2017, NASA suggested that ocean worlds might be the most promising places for finding life beyond Earth, and missions such as the European Space Agency’s Juice and NASA’s Europa Clipper and Dragonfly spacecraft will investigate the potential habitability of Jupiter’s moons Europa, Ganymede, and Callisto, as well as Saturn’s moon Titan.
“The existence of a recently formed liquid water ocean makes Mimas a prime candidate for study, for researchers investigating the origin of life,” Cooper said.
It may be time to observe other apparently quiet moons in the solar system that could potentially support life, the study authors suggested.
“Lainey and colleagues’ findings will motivate a thorough examination of mid-sized icy moons throughout the Solar System,” wrote Drs. Matija Ćuk and Alyssa Rose Rhoden in an article accompanying the study. Ćuk is a research scientist at the SETI Institute in California, and Rhoden is a principal scientist at the Southwest Research Institute’s Planetary Science Directorate in Colorado.
Though not involved in the study, Rhoden authored research about the potential for a “stealth” ocean on Mimas.
“Basically, the difference between our 2022 paper and this new paper is that we found an ocean could not be ruled out by Mimas’ geology, whereas they are actually detecting the signature of the ocean within Mimas’ orbit,” Rhoden said. “It is the strongest evidence we have, so far, that Mimas really does have an ocean today.”
Since the 2022 report, Rhoden and her research group have continued their study of Mimas, and they agree with the new study’s conclusion about the relatively young age of the moon’s ocean.
“Mimas certainly demonstrates that moons with old surfaces can be hiding young oceans, which is pretty exciting,” Rhoden said. “I do think we can speculate as to moons having developed oceans much more recently than we often assume.”
