Orbital changes suggest recent formation of underground ocean on Saturn's Mimas

"Previously, we established that Mimas could currently host an ocean world only if it previously had a thicker icy shell. However, the process transitioning from thick to thinner ice was uncertain given Mimas' higher past eccentricity," explained Matthew E. Walker, Senior Scientist at the Planetary Science Institute. "Our latest findings indicate that the ice shell might still be thinning as the moon's eccentricity decreases due to tidal heating. This suggests the underground ocean is quite young from a geological perspective."

Walker, who coauthored the study "The evolution of a young ocean within Mimas" published in Earth and Planetary Science Letters, noted the significant role of eccentricity in driving tidal heating, which is currently thinning the ice shell.

"Tidal heating, acting as a heat source, is extracting energy from Mimas' orbit as it melts the ice, leading to a decrease in eccentricity that will eventually stabilize the moon's orbit," he added.

The hypothesis is supported by the ice shell's thinning over the past 10 million years, aligning with the geological data from Mimas. Observations of Herschel crater, with its distinctive shape and central peak, suggest the ice shell was much thicker in the past when the crater was formed.

"The outer hydrosphere of Mimas, composed of ice and liquid, is estimated to be around 70 kilometers thick, with the current ice shell ranging from 20 to 30 kilometers in thickness, based on precession data, and 24 to 31 kilometers from libration measurements," detailed Walker.

This recent phase in Mimas' history, possibly beginning 10 to 25 million years ago, marks a significant shift from a fully frozen state to its current condition with an active ocean layer.

"This presents Mimas during a particularly intriguing phase of its evolution. Matching current eccentricity and thickness constraints suggests this process started relatively recently, geologically speaking," Walker concluded.

Research Report:The evolution of a young ocean within Mimas