Earlier this year, researchers discovered a vast liquid ocean lurking beneath the icy crust of Saturn’s tiny moon Mimas. Now the same team may have discovered how it was created.
New research suggests that Mimas’ icy crust is melting and thinning as its orbit around Saturn becomes less flat, or less “eccentric,” due to the ringed planet’s gravitational pull. This would have created a vast ocean between 2 and 25 million years ago, making this subsurface sea relatively young for a solar system feature.
The tiny moon has already redefined what ocean worlds could be because such small moons were not expected to host subsurface oceans. This previous discovery, and the new discovery of how this ocean may have arisen, could ultimately impact our search for life elsewhere in the solar system.
Relating to: Saturn’s ‘Death Star’ moon Mimas may have an ocean scientists never believed could exist
“In our previous work, we found that Mimas would have had to have a much thicker icy crust in the past to be an ocean world today. But because Mimas’ eccentricity was much higher in the past, the way out is team leader and Planetary Science Institute Senior Scientist Matthew “Thick ice was less clear than thin ice,” E. Walker said in a statement.
“In this study, we show that there is a pathway for the ice shell to be thinning now, even as eccentricity decreases due to tidal heating. But geologically speaking, the ocean must be very young.”
A small moon with a big ocean
Mimas is nicknamed the “Death Star” for its Herschel crater, which gives the moon an appearance resembling the Empire’s moon-sized space station in Star Wars. This large impact scar was formed by the impact of Mimas approximately 4.1 billion years ago.
Mimas is about 148 miles (400 kilometers) in diameter, which is quite small compared to the 2,159 miles (3,475 kilometers) diameter of Earth’s moon.
The Mimas ocean is estimated to lie approximately 12 to 18 miles (20 to 30 kilometers) below the surface of the ice shell of this moon of Saturn. Mimas’ outer hydrosphere of ice and water is estimated to be 43 miles (70 kilometers) deep, and the lunar ocean is estimated to be 25 miles to 28 miles (40 to 45 kilometers) deep. This means that oceans make up half the volume of Mimas.
This new research shed new light on a process that may have created these vast subsurface seas, known as tidal heating. This occurs when a body such as the moon becomes distorted or stretched due to changes in the gravitational forces it experiences in an elliptical or oval-shaped orbit.
“The eccentricity drives tidal heating. It’s currently very high compared to other active ocean moons like neighboring Enceladus,” Walker said. “We currently think that the heat source responsible for the thinning of the crust is tidal heating.”
Walker added that the issue here is that tidal heating is not free energy, meaning that as it melts Mimas’ crust, tidal heating draws energy from the Moon’s orbit around Saturn. This will further reduce orbital eccentricity until Mimas’ orbit eventually becomes circular and the entire process is completely shut down, Walker said.
Orbital eccentricity is measured in values from 0 to 1; 0 represents a perfect circle and 1 represents a parabola. Any value between this is an ellipse. The team estimates that the onset of ice melt must have begun when Mimas’ orbital eccentricity was about two to three times the present value.
This represents the last 10 million years of Mimas history and presents an evolution consistent with the geology we see today on Saturn’s moon.
“When we think of ocean worlds in general, we don’t see a lot of craters because the environment resurfaces and the environment like Europa or the south pole of Enceladus erases them,” Walker said. “The shape of Herschel crater, its central peak and continuous interior, require that the crust was thicker in the past when Herschel formed.”
He added that to achieve the crater morphology observed at Mimas, the crust of Saturn’s moon would have to be at least 34 miles (55 kilometers) thick when multiplied by the body that formed the Herschel crater.
“Craters can provide clues about the presence of an ocean and the thickness of the ice shell through their morphology, such as the ratio between crater diameter and depth and the presence of a central peak,” Walker said.
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Walker added that to match the current eccentricity of Mimas’ orbit and the thickness constraints based on slight wobble or “relaxation” in its rotation, they estimate that this whole ocean birth process must have begun no more than about 25 million years ago. Years ago.
“In other words, we think Mimas was completely frozen until 10 to 25 million years ago, at which point the ice crust began to melt. What initiated this melting period is still being investigated,” Walker concluded. “We may be seeing Mimas at a particularly interesting time.”
The team’s research appears in the journal Planetary Science Letters.