Tiny icy moons may hide boiling water under their frozen shells

Some of the smallest moons in the outer solar system may hide water that behaves in a very strange way, far from the Sun.

New research suggests that on worlds like Saturn’s moon Mimas, parts of the buried ocean can actually start to boil under miles of ice.

This boiling does not come from a hot surface at all. It comes from how the ice shell slowly grows thinner over time and changes the pressure on the water below.

Boiling water inside icy moons

Many outer moons have a subsurface ocean – a layer of liquid water, sometimes salty, trapped between a rocky core and a frozen surface.

This hidden water never sees sunlight, and the ice above crushes it under enormous pressure every day.

The study was led by Maxwell Rudolph, associate professor of earth and planetary sciences at the University of California, Davis (UCD).

Professor Rudolph’s research focuses on how water and ice move inside planets and moons over long periods of time, shaping their interiors.

The team used computer models to follow how an ice crust slowly thins and relaxes under changing pressure.

As the ice turns back into water, the change in volume lowers the pressure in the ocean instead of raising it.

If that pressure drop becomes large enough near the top of the ocean, water there reaches its “triple point.” This refers to the pressure and temperature where liquid, ice, and vapor coexist at once.

Why size matters for icy moons

These moons are heated by tidal forces, repeated stretching and squeezing by a giant planet’s gravity.

That heating can thin the ice enough for boiling to start on some of the smallest bodies orbiting these planets.

On somewhat larger ice covered worlds, the crust responds in another way. Instead of allowing the ocean to boil, pressure in the stiff ice produces compressional tectonics, deformation of the crust when it is squeezed.

“Not all of these satellites are known to have oceans, but we know that some do. We’re interested in the processes that shape their evolution over millions of years,” said Professor Rudolph.

For small moons, models show that their ice shells can remain intact even under compressive stress – the inward-pushing force that stiff ice distributes throughout the shell.

This allows the ocean pressure to keep dropping until boiling begins near the top of the water layer.

The moons of Saturn

Mimas is less than 250 miles across and looks like a battered ball of ice with a giant crater. Yet its rotation hints at a liquid layer inside, deep below the crust.

A recent analysis suggests that this hidden water formed as the ice shell thinned over time under continued tidal heating.

Saturn’s moon Enceladus hosts an ocean beneath its ice, and the Cassini spacecraft saw jets of water and ice shooting from its pole.

Grains of ice from those plumes hold organic molecules and building blocks for life, according to a NASA report from the Cassini team.

Ocean pressure on small worlds

Interior modeling indicates that Miranda and Ariel, two of Uranus’s moons, may conceal water layers that previously powered intense tectonic change.

Voyager 2 images show that Miranda’s surface hosts three regions called coronae, made of ridges and valleys.

Professor Rudolph’s team found that on small worlds like Mimas, Enceladus, and Miranda, ocean pressure can fall until boiling begins before the ice fails.

The resulting pockets of vapor and exsolved gas can rise through the shell, reshaping ice flow. This may help explain Miranda’s coronae and the absence of fractures on Mimas.

Looking for signs of life on icy moons

Boiling inside a buried ocean does not mean the whole moon melts or explodes any time soon. Instead, it can build a thin layer of vapor and gases at the top of the water that slowly pushes upward into the ice.

When that buoyant material moves, it can trigger convection, circulating motion where warm material rises and cool material sinks.

Rising blobs of gas-rich ice may carve paths through the shell or widen existing cracks so that water and vapor reach the surface.

For bigger moons with stronger gravity, the same process tends to stop once the ice begins to fail under compression.

In those cases the surface is more likely to record squeezed ridges and cliffs than the subtle signs of a gently growing ocean.

Knowing which moons may hide boiling oceans can help scientists decide where to look for chemical energy and liquid water that might support life.

The findings also give mission planners a clear set of targets to map, from broad ridges on larger moons to patchy terrain on the smallest ocean worlds.

The study is published in the journal Nature Astronomy.

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