Out in the cold, distant reaches of our solar system, many moons are wrapped in thick layers of ice.
At first glance, these worlds look frozen and lifeless.
But scientists are discovering that beneath these icy shells, some of the smallest moons may hide warm, active oceans — and in some places, these oceans could even be close to boiling.
Moons like Saturn’s Enceladus have already captured scientists’ attention because they are known to contain liquid water beneath their icy crusts.
These underground oceans are considered some of the most promising places in the solar system to search for signs of extraterrestrial life.
Now, a new study published in Nature Astronomy is helping researchers understand what might be happening under the ice on other small moons as well.
On Earth, mountains, volcanoes, and earthquakes are driven by hot, moving rock deep inside the planet.
On icy moons, however, geology works very differently. Instead of molten rock, the main players are ice and water.
These moons are heated not from within like Earth, but by powerful gravitational forces from the giant planets they orbit.
As they move through space, they are constantly pulled and squeezed, a process called tidal heating. This heating can cause the ice shell to partially melt and become thinner. When the heating weakens, the ice can freeze and thicken again.
In past research, scientists explored what happens when an ice shell grows thicker. As water freezes, it expands.
This expansion can squeeze and crack the surface, possibly explaining the deep fractures seen on Enceladus, known as “tiger stripes.” But the new study focused on the opposite situation: what happens when the ice melts from below.
Surprisingly, when ice turns into liquid water, the pressure in the ocean underneath the surface can drop.
On very small moons, such as Enceladus, Mimas (another moon of Saturn), and Miranda (a moon of Uranus), this drop in pressure could be so great that the ocean reaches a special point where ice, water, and water vapor can exist at the same time.
In other words, parts of the ocean may actually begin to boil, even though the temperature is still extremely cold by Earth standards.
This strange process could help explain unusual surface features. Miranda, for example, has enormous ridges and steep cliffs arranged in circular patterns. These formations might have been created by boiling or bubbling beneath the surface, changing the shape of the ice above.
Mimas is famous for its huge crater, which makes it look like the “Death Star” from Star Wars. It appears dull and inactive on the outside, yet slight movements suggest it may contain an ocean inside. Because its ice shell may be too strong to break apart easily, it could hide its inner activity without leaving obvious surface signs.
Larger icy moons, like Titania, behave differently. When their ice begins to melt, the pressure changes are more likely to crack the surface before boiling can occur. This means each moon’s size plays an important role in determining how its surface is shaped over time.
Just as Earth’s geological history explains its mountains and oceans, understanding how icy moons change helps scientists uncover the hidden stories written in their frozen shells.
