Mimas appears inactive on the surface, so an ocean there could mean a new class of "stealth" ocean worlds.
NASA is studying several moons of Saturn and Jupiter that carry oceans of liquid water deep beneath their surfaces, where alien life could thrive. Another moon, previously assumed to be a barren ice rock, might join their ranks.
Mimas is a small moon of Saturn, often compared to the Death Star from Star Wars, thanks to its large, distinct crater. Scientists have long believed that Mimas is an inert ball of ice because of its heavily cratered surface. Icy worlds with oceans are usually smooth, since changes in their surface ice pave over craters, or cracked. Tidal forces stretch and relax these moons, which both cracks the surface ice and heats the moons' insides, sustaining internal oceans.
But NASA's Cassini mission, which orbited and studied Saturn for over a decade, detected an unexpected oscillation in Mimas's rotation. As it spins on its axis, Mimas wobbles slightly. Such oscillations may point to an ocean deep beneath the moon's ice, according to a new analysis.
"If Mimas has an ocean, it represents a new class of small, 'stealth' ocean worlds with surfaces that do not betray the ocean's existence," Alyssa Rhoden, a geophysicist who led the analysis at the Southwest Research Institute, said in a press release.
Rhoden's finding was published online in the journal Icarus this week, in a paper co-authored with Matthew Walker of the Planetary Science Institute in Arizona.
It's still no guarantee of a secret Mimas ocean. Researchers need to investigate the moon for further evidence.
"The work doesn't prove that there is a subsurface ocean on Mimas, but it does show that an ocean is perfectly consistent with the available data and our understanding of the physics, and the authors are appropriately cautious about this," Michael Bland, a space scientist who studies icy worlds at the US Geological Survey, and previously spoke with Rhoden about the research, told Insider in an email. "I think the study also opens as many questions as it answers."
Underground oceans on distant moons could mean alien life in our solar system
Mimas's oscillation could indicate that Saturn's gravity stretches and relaxes the planet as it spins — similar to the moon's gravity pulling Earth's water back and forth during tides. This process could build up energy deep inside Mimas, which is released as heat, warming its internal ice and creating an underground ocean of liquid water.
Rhoden and Walker developed computer models to simulate that stretching process for Mimas, including how it would affect the stability of the moon's ice shell, and how that stretching fits in with Cassini's measurements of Mimas's oscillation. The model indicated that Saturn's tidal pull could maintain an ocean lying beneath 14 to 20 miles of ice on Mimas.
"This result is really intriguing because it implies that we can't tell which moons in the outer solar system may have subsurface oceans just from their surface geology," Bland said.
Some moons — like Jupiter's Europa and Saturn's Enceladus — have plumes of water shooting up from their internal oceans, through their surface ice, and into space. In 2020, scientists also discovered evidence of an underground ocean on a dwarf planet called Ceres, which orbits the sun between Mars and Jupiter. Salt deposits on Ceres's surface appear to have percolated up from liquid water below.
These ocean worlds could have deep-sea hydrothermal vents that produce energy for living organisms. Such vents serve as an energy source for life at the bottom Earth's oceans, where there is no sunlight. If Mimas has an ocean, it might be able to host such ecosystems, too.
But the study isn't enough evidence to prove that. If the researchers' models are off — if Mimas's ice cools faster than they assumed, for example — there may be no Mimas ocean at all.
"If the Cassini spacecraft had been able to fly closer to Mimas, it might have sensed a magnetic field generated by the Mimas ocean, which would be a direct confirmation that an ocean is present today," Steve Vance, who studies icy worlds at NASA's Jet Propulsion Laboratory, told Insider in an email. "Hopefully a future mission will be able to look for an ocean in Mimas."
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