The moon Mimas may hide an ocean resulting from melting ice

April 16 () –

Saturn's moon Mimas could have developed a huge subterranean ocean as its orbital eccentricity decreased to its current value and caused its icy shell to thin.

“In our previous work, we discovered that for Mimas to be an ocean world today, it must have had a much thicker ice shell in the past. But because Mimas' eccentricity would have been even greater in the past, the path from thick ice to thinner ice was less clear“said Planetary Science Institute senior scientist Matthew E. Walker. “In this work we show that there is a pathway for the ice sheet to be currently thinning even as the eccentricity is decreasing due to tidal warming. but the ocean must be very young, geologically speaking“.

Walker is co-author of the paper “The evolution of a young ocean within Mimas,” which appears in Earth and Planetary Science Letters. Alyssa Rose Rhoden of the Southwest Research Institute is the lead author.

“Eccentricity is what drives the tidal heating. Right now it is very high compared to other active ocean moons, such as neighboring Enceladus. We believe that tidal heating is the heat source responsible for the current thinning of the cape,” Walker said it's a statement. “However, tidal heating is not free energy, so as it melts the shell, it draws energy from the orbit, reducing that eccentricity until it eventually circularizes it and shuts everything down.”

The start of the merger had to occur when the Mimas eccentricity was two or three times greater than the current value. The thinning of the ice sheet during the last 10 million years of Mimas evolution is consistent with its geology.

“Generally when we think about oceanic worlds we don't see many craters because the environment resurfaces and ends up erasing them, like Europa or the south pole of Enceladus. The shape, the central peak and the intact interior of the Herschel crater require that the shell have been thicker in the past, when Herschel formed. To get the crater morphology we observed, the projectile must have been at least 55 kilometers away when it was impacted,” Walker said. “Craters can provide clues to the presence of an ocean and the thickness of the ice sheet through their morphology, such as the relationship between the diameter of the crater and its depth and the existence of a central peak.”

Mimas has a radius of just under 200 kilometers. It is estimated that the thickness of the outer hydrosphere, made up of ice and liquid, is about 70 kilometers. Current estimates of ice sheet thickness are 20 to 30 kilometers, based on precession (the rotational motion of the axis of a rotating body), or a narrower range of 24 to 31 kilometers from libration (a slight oscillation in the speed of rotation) of the moon causing it to appear to sway back and forth), leaving an ocean that is between 40 and 45 kilometers deep before hitting the rock.

“We may be seeing Mimas at a particularly interesting time. To match current eccentricity and thickness limitations based on libration information, we think all of this must have started no more than 25 million years ago. In other words , We believe that Mimas was completely frozen until between 10 and 25 million years ago, at which point its ice cap began to melt. “What changed to start that era of melting is still under investigation,” Walker said.