Mimas, the smallest and innermost of Saturn’s eight main moons, may be warm enough to harbor a global, liquid water ocean beneath a 24-31-km (15-19-mile) thick ice shell, according to a new analysis of data from NASA’s Cassini mission.
Mimas’ surface, like the surfaces of most of the other major Saturnian moons without atmospheres, is not pure ice but contains some dark impurities; the relatively dark markings appear along the lower portion of the walls of the 130-km-wide Herschel Crater (the peak in the middle of the crater is about as tall as Mount Everest; the impact that made it likely almost destroyed the moon) and some of the smaller craters; scientists interpret this darkening as evidence for the gradual concentration of impurities from evaporating icy materials in areas where the dark impurities slide slowly down the crater wall. Image credit: NASA / JPL / Space Science Institute.
One of the most profound discoveries in planetary science over the past 25 years is that worlds with oceans beneath layers of rock and ice are common in our Solar System.
Such worlds include the icy satellites of the giant planets, such as Europa, Titan and Enceladus, as well as distant planets like Pluto.
Worlds like Earth with surface oceans must reside within a narrow range of distances from their stars to maintain the temperatures that support liquid oceans.
Interior water ocean worlds, however, are found over a much wider range of distances, greatly expanding the number of habitable worlds likely to exist across the galaxy.
“Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice,” said Dr. Alyssa Rhoden, a researcher with the Southwest Research Institute.
“Interior water ocean worlds, such as Enceladus and Europa, tend to be fractured and show other signs of geologic activity.”
“Turns out, Mimas’ surface was tricking us, and our new understanding has greatly expanded the definition of a potentially habitable world in our Solar System and beyond.”
Tidal processes dissipate orbital and rotational energy as heat in a satellite.
To match the interior structure inferred from Mimas’ libration, tidal heating within the moon must be large enough to keep the ocean from freezing out but small enough to maintain a thick icy shell.
Using tidal heating models, Dr. Rhoden and Planetary Science Institute’s Dr. Matthew Walker developed numerical methods to create the most plausible explanation for a steady-state ice shell between 24 and 31 km thick over a liquid ocean.
“Most of the time when we create these models, we have to fine tune them to produce what we observe,” Dr. Rhoden said.
“This time evidence for an internal ocean just popped out of the most realistic ice shell stability scenarios and observed librations.”
The scientists also found that the heat flow from the surface was very sensitive to the thickness of the ice shell, something a spacecraft could verify.
For instance, NASA’s Juno spacecraft is scheduled to fly by Europa and use its microwave radiometer to measure heat flows in this Jovian moon.
These data will allow planetary researchers to understand how heat flow affects the icy shells of ocean worlds such as Mimas, which are particularly interesting as NASA’s Europa Clipper approaches its 2024 launch.
“Although our results support a present-day ocean within Mimas, it is challenging to reconcile the moon’s orbital and geologic characteristics with our current understanding of its thermal-orbital evolution,” Dr. Rhoden said.
“Evaluating Mimas’ status as an ocean moon would benchmark models of its formation and evolution.”
“This would help us better understand Saturn’s rings and mid-sized moons as well as the prevalence of potentially habitable ocean moons, particularly at Uranus.”
“Mimas is a compelling target for continued investigation.”
The study appears in the journal Icarus.
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Alyssa Rose Rhoden & Matthew E. Walker. 2022. The case for an ocean-bearing Mimas from tidal heating analysis. Icarus 376: 114872; doi: 10.1016/j.icarus.2021.114872
