Many small moons in our solar system have boiling oceans hidden beneath their ice

Several small worlds beyond Jupiter are not just frozen rubble. Beneath their shining skins, many hide oceans of liquid water. New research says those seas do not always stay calm. When the ice lids above them shrink, some oceans can start to boil. Others crush their own roofs instead.

A study published in Nature Astronomy explains how this split fate happens. It depends mostly on a moon’s size and gravity. The work was led by Max Rudolph, an associate professor of Earth and planetary sciences at the University of California, Davis.

“Not all of these satellites are known to have oceans, but we know that some do,” Rudolph said. “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.”

You might picture these moons as cold and quiet. They are not. Giant planets tug on them as they orbit. Moons also tug on one another. Those pulls stretch and squeeze their insides, making heat. More heat thins the ice. Less heat lets it grow thicker again. Over long cycles, the ice shell can swell or shrink by miles.

That change matters because water and ice have different sizes. When water freezes, it takes up more space and boosts pressure below. Past work by Rudolph and his team showed that thickening ice can put the surface in tension. That stress can split the crust and help launch jets like the famous “tiger stripes” on Saturn’s moon Enceladus.

This paper flips the question. What if the ice melts from below instead?

When ice melts, the volume drops and pressure falls in the ocean. On small moons with weak gravity, the drop can be steep enough to reach the “triple point” of water. That is the narrow mark where ice, liquid, and vapor can exist together. Get there, and parts of an ocean can begin to simmer.

The team tested this with two kinds of models. One used math to link ice loss to stresses in the shell. The other ran detailed computer simulations with heat flow and ice behavior that changes with temperature. The models agree on one big breakpoint. Below about 300 kilometers across, worlds are primed to boil. Above that, they tend to crack first.

For tiny moons such as Mimas and Enceladus at Saturn and Miranda at Uranus, the models show the ocean can hit boiling conditions before the ice breaks. For bigger ones like Titania and Iapetus, the ice fails under squeezing forces first. On the largest, such as Callisto, boiling never starts.

As thinning goes on, the cold upper ice behaves like a stiff shell. Warmer ice below can creep and ease stress. In the calculations, compressive forces rise to about 10 million pascals and still the ice holds. Meanwhile the ocean keeps losing pressure.

For Enceladus, boiling begins after about 14 kilometers of thinning. For Mimas, it takes only about 5 kilometers. That is not much on a world where the shell can be dozens of kilometers thick.

Once boiling starts, only a slim layer at the ocean’s top is involved. Pressure rises with depth, so the deeper water stays liquid. But the change has outsized effects. Gases that once dissolved in water suddenly pop free. Carbon dioxide, methane, ammonia, nitrogen and others collect with water vapor in a buoyant mix near the ice.

Water vapor would freeze if it rose into colder zones, but the gases would not. They stay light and push upward. The result can be fractures filled with gas, rising pockets of slushy ice, or plumes of mixed vapor and rock. These paths can punch through barriers that usually trap water below.

Mimas looks dead, pocked by craters and scarred by one huge pit that earned it a movie-style nickname. Yet slight wobbles in its spin hint at a hidden layer of liquid. If its ocean formed only recently and is now refreezing, thinning could still create a vapor sheet where water meets ice. That layer could someday stir up cracks, even if the surface looks quiet today.

Enceladus is already loud. Jets spray from its south pole in a shifting pattern. Over long cycles, its ice probably thickens and thins as its orbit changes. Thickening can open downward cracks. Thinning offers a different route upward, as underpressure and simmering help lift fluids and gases. Together, the two processes can keep the show going.

Miranda is stranger still. Voyager 2 revealed a patchwork of ridges and cliffs called coronae. Models suggest Miranda once held an ocean more than 100 kilometers deep. Only minor thinning may be enough to spark boiling there, jump-starting convection and building its rugged face.

Larger moons tell another story. On Titania and Iapetus, thinning by about 10 percent can crush the shell before water even flirts with vapor. The surface then shortens, making features that look like wrinkles and ridges. The same melting that stirs small worlds can buckle big ones.

The lesson is simple but striking. Size rules destiny. On low-gravity worlds, water can simmer under ice and help drive cracks and eruptions. On heavier worlds, pressure wins and the crust caves in.

When you study these frozen faces, you are not just seeing ice. You are reading a record of oceans at work. Some boiled. Some broke their shells. Others bent them. Each scar is a clue to a sea you will never see, except through the marks it left behind.

The work helps scientists decide where to search for life. Boiling zones can move gases and nutrients upward, making chemistry more active near the surface. That matters for future missions that sample plumes or drill ice.

The results also guide where to look for cracks that tap oceans and where to expect crushed terrain instead. In short, the study turns surface maps into ocean forecasts.

Research findings are available online in the journal Nature Astronomy.

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