A scientist from the Southwest Research Institute set out to prove that Saturn's small, innermost moon was a frozen inert satellite and instead discovered compelling evidence that Mimas has a floating inland sea. In the waning days of NASA's Cassini mission, the spacecraft identified a strange release or oscillation in the moon's rotation, which often points to a geologically active body capable of supporting an inland ocean.
"If Mimas has an ocean, it represents a new class of small, 'stealth' ocean worlds with surfaces that do not betray the existence of the ocean," said SwRI's Dr. Alyssa Rhoden, specialist in geophysics of icy satellites, especially those containing oceans and the evolution of giant planetary satellites.
One of the most profound discoveries in planetary science over the last 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 Europe, Titan, and Enceladus, as well as distant planets such as Pluto. Worlds like Earth with surface oceans must reside within a narrow range of distances from their stars in order to maintain the temperatures that support floating oceans. However, inland water worlds (IWOWs) exist over a much wider range of distances, greatly expanding the number of habitable worlds that are likely to exist across the galaxy.
"Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice," Rhoden said. "IWOWs, such as Enceladus and Europe, tend to break and show other signs of geological activity. It turns out that Mimas' surface fooled us, and our new understanding has greatly expanded the definition of a potential habitable world in our solar system and beyond. "
Tide processes dissipate orbital and rotational energy as heat in a satellite. To match the internal structure derived from Mimas' release, tidal warming in the moon must be large enough to prevent the ocean from freezing out, but small enough to maintain a thick, icy shell. Using tidal warming models, the team developed numerical methods to create the most plausible explanation for a steady-state ice shell between 14 and 20 miles thick over a floating ocean.
"Most of the time, when we create these models, we have to fine-tune them to produce what we observe," Rhoden said. "This time, evidence of an inland sea emerged right out of the most realistic ice-shell stability scenarios and observed librations."
The team 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 example, the Juno spacecraft is scheduled to fly out of Europe and use its microwave radiometer to measure heat flows in this Jovian moon. These data will enable scientists to understand how the heat flow affects the icy shells of ocean worlds like Mimas, which are particularly interesting as NASA's Europe Clipper approaches its 2024 launch.
"Although our findings support a contemporary ocean in Mimas, it is challenging to reconcile the moon's orbital and geological characteristics with our current understanding of its thermal orbital evolution," Rhoden said. "Evaluating Mimas' status as a sea moon would benchmark models for its formation and evolution. This would help us better understand Saturn's small and medium-sized moons as well as the distribution of potentially habitable lunar moons, especially at Uranus. Mimas is a compelling measure of continued investigation."
Rhoden is a co-chair of NASA's Network for the Ocean Worlds Research Coordination Network and has previously been a member of the National Academies' Committee on Astrobiology and Planetary Science. A paper describing this research was published online in Icarus.
Citation: Uncovering evidence for an inland sea in small Saturn moon (2022, January 19) retrieved January 19, 2022 from https://phys.org/news/2022-01-uncovering-evidence-internal-ocean-small. html
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