A new study reveals how shifting ice shells on icy moons can cause dramatic pressure changes, sometimes enough to boil their hidden oceans.
The outer planets of our Solar System are accompanied by numerous moons covered in thick layers of ice. A few of these moons, including Saturn’s Enceladus, contain oceans of liquid water trapped between their frozen surfaces and rocky interiors. These hidden seas make them some of the most promising locations in our solar system for searching for extraterrestrial life.
According to a study published in Nature Astronomy, researchers are beginning to uncover what may be happening beneath the icy exteriors of these distant worlds and how the wide range of geological structures seen on their surfaces might have developed.
“Not all of these satellites are known to have oceans, but we know that some do,” said Max Rudolph, associate professor of earth and planetary sciences at the University of California, Davis and lead author on the paper. “We’re 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.”
Geologic Forces Driven by Ice and Water
Earth’s mountains, earthquakes, and other geological activity arise from the shifting and melting of rock far beneath its surface. In contrast, the geology of icy moons is shaped by the behavior of water and frozen layers.
These distant worlds gain internal heat from tidal forces created by the planets they orbit. Interactions among multiple moons can change the strength of this heating over time. When tidal heating becomes stronger, the ice shell can melt and become thinner; when the heating drops, the ice layer thickens again.
Rudolph and colleagues had previously looked at what happens when the ice shell gets thicker. They found that because ice has a greater volume than liquid water, freezing would put pressure on the ice shell, which could cause features such as the “tiger stripes” of Enceladus.
But what happens when the opposite happens and the ice shell melts from the bottom? That could actually cause the ocean to boil, the researchers conclude.
That’s because as ice melts into less-dense liquid water, pressure drops. Rudolph and colleagues calculated that at least on the smallest icy moons, such as Saturn’s Mimas and Enceladus, or Miranda, a moon of Uranus, the pressure could drop low enough to reach the triple point at which ice, liquid water, and water vapor can all co-exist.
Boiling Oceans and Strange Terrains
Images of Miranda from the Voyager 2 space probe show distinct areas of ridges and cliffs called coronae. Ocean boiling could explain how these features formed.
Mimas is less than 250 miles across and pitted with craters, including a very large crater earning it the nickname “Death Star.” It appears to be geologically dead, Rudolph said, but a wobble in its movement suggests an ocean is present. Because Mimas’ ice shell is not expected to break as a result of ice shell thinning, the presence of an ocean can be reconciled with a geologically dead surface.
The size of these moons is important. On larger ice moons, such as Titania, another moon of Uranus, the drop in pressure from melting ice would cause the ice shell to crack before the triple point for water is reached, the team calculated. The authors find that Titania’s geology could be the product of a period of ice shell thinning followed by re-thickening.
Just as geology on Earth helps us understand why our planet looks the way it does after billions of years of change, understanding geological processes on these moons can help us see why they have the features that they do, Rudolph said.
Reference: “Boiling oceans and compressional tectonics on emerging ocean worlds” by Maxwell L. Rudolph, Michael Manga, Alyssa R. Rhoden and Matthew Walker, 24 November 2025, Nature Astronomy.
DOI: 10.1038/s41550-025-02713-5
The work was supported in part by NASA.
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