Icy Moons Orbiting Saturn and Uranus May Hide Boiling Liquid Oceans

In the shadows of the gas and ice giants of our solar system hide small frozen moons. Some of these moons contain churning oceans of liquid water bound by a hard ice shell.

A new study, published in Natural astronomy, has cracked the surface of these moons and revealed what could be happening in the depths of their oceans, considered excellent candidates for hosting extraterrestrial life.

“Not all of these satellites are known to have oceans, but we know that some do. We are interested in the processes that shape their evolution over millions of years, and this allows us to think about what the surface expression of an ocean world would be,” Max Rudolph, an Earth and planetary scientist at the University of California, Davis, said in a press release.

Icy moons in the solar system

Rather than flowing magma, geological changes on icy moons are produced by melting ice. The ice shells covering these moons wax and wane in response to forces generated by the planet they orbit and by neighboring moons, which heat or cool these shells.

As Rudolph led the researchers, his team used mathematical models to analyze what happens when the ice sheet melts. The team found that warming ice sheets reduced pressure on the Moon, as dense ice melted into water.

Their calculations suggest that if the moon is small enough, this drop could be enough to reach a “triple point” at which ice, water and water vapor can all be present on the moon. In short, the frozen oceans will start to boil. Moons small enough for this to happen include Miranda, a moon of Uranus, and the Saturnian moons Mimas and Enceladus.

Learn more: Hidden heat on Saturn’s icy moon could help it sustain life

Probe images support ice analysis

Evidence from space probes also supports this theory. Voyager 2, which flew by Miranda in 1986, took images of the distinctive coronas, cliffs and valleys that mark the Moon’s surface. Boiling oceans could have formed these marks, the authors say.

On Mimas, a small cratered moon that is barely wider than the distance between New York and Boston, the surface appears geologically inactive. But this moon’s ice shell is unlikely to break as it gets thinner. Mimas could therefore have an arid surface with a boiling liquid ocean just below. The researchers noticed that the Moon “wobbled” slightly, confirming the presence of an ocean.

On larger moons, pressure drops would likely break the ice surface before the oceans could boil. These larger moons include Titania, which also orbits Uranus. Titania’s ridged surface, they write, could have been formed by periods of thinning and thickening of the ice shell.

The new publication is not Rudolph’s first article on icy moons. A previous study explored what might happen as icy moons cool and their surface layer thickens. This process could also have impacts on the Moon’s surface, forming features such as the fractured ridges commonly known as “tiger stripes” seen on Enceladus.

What we know about Earth’s geology can help us understand how our planet’s surface has changed over millions of years. Studying ice changes within these distant moons can also help us understand how their distinctive surfaces formed.

Learn more: NASA’s Europa Clipper will probe life in the plumes of icy moons

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