The icy moons at the outer reaches of our solar system are hiding a secret—and it could have important implications for life beyond our planet.
New research published in Nature Astronomy examines the surface features of the frigid moons of Saturn and Uranus, modeling the activity beneath their frozen shells. On Earth, tectonic forces are governed by molten rock. Moons like Enceladus, Mimas, and Iapetus (of Saturn) and Miranda and Titania (of Uranus), which get tugged at by the gravitational pull of their parent planet and neighboring moons, generate a heat from within as they squish and expand. But the new work finds that even more immediately in oceans themselves, water and ice may be responsible for heat as well. And it turns out there could be a lot more to this action than previously thought.
“Not all of these satellites are known to have oceans, but we know that some do,” lead author Max Rudolph of the University of California, Davis said in a statement. “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.”
Read more: “Intelligent Life May Be Hiding in the Moons”
Water, believed to be a key component in the evolution of life, has some other unique characteristics. While most things expand when they heat up and contract when they cool down, water’s unique hydrogen bonds force the molecules apart when they get too close together, somewhat paradoxically making ice take up more volume than liquid water. When the layer of ice surrounding a liquid mantle melts, this drop in pressure can actually cause the water beneath to boil.
That’s because the freezing point and boiling point of substances are as much a function of pressure as they are a function of temperature. Apply a great enough vacuum and you can even make water boil at its freezing point here on Earth. If you graph the pressure vs. temperature of water, there’s a point called the “triple point” where all three phases of matter can exist simultaneously.
This new research suggests that some icy moons have conditions where water can exist in this turbulent state—but only the small ones. Moons with a radius of less than about 186 miles (Enceladus, Mimas, and Miranda), are able to maintain the integrity of their icy shells when the pressure drops, the researchers say. Any larger than that (Titania and Iapetus), and the drop in pressure causes the shell to buckle.
Rather than frozen satellites quietly orbiting in the dark, this new research suggests these small, icy moons are actually churning with activity beneath their icy surfaces. It’s that churn that could provide more favorable conditions for life, as the movement of water also transports materials and nutrients from the rocky core to the surface.
In other words, if you want to find life on a frozen planet, it’s best to look at a smaller one. There could be a groundswell of activity hiding just beneath the surface. 
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Lead image: NASA/JPL/Space Science Institute
