Moons with oceans

Far from the Sun everything should be frozen solid. Yet some moons out there have oceans.

The water is hidden. On top is a shell of ice, kilometres thick and as hard as rock. Underneath is a sea of salty liquid water, in the dark.

Europa, a moon of Jupiter a little smaller than ours, is thought to hold about twice as much water as all of Earth’s oceans put together.

At Saturn, the small moon Enceladus gives its ocean away. Jets of water vapour and ice grains spray out of cracks near its south pole and into space. A spacecraft has flown straight through them and tasted what is inside.

Where there is liquid water there might be life, so these moons are some of the most interesting places we know.

Where the heat comes from. Sunlight is no help: at Jupiter it is 27 times weaker than at Earth, and at Saturn about 90 times weaker. The warmth comes from inside, and most of it comes from tides.

A moon on a slightly oval orbit is squeezed harder when it is near its planet and less when it is far. That steady flexing rubs the inside of the moon against itself and warms it, the way a paper clip warms when you bend it back and forth. Left alone, an orbit would settle into a circle and the heating would stop. What keeps it going is a neighbour: another moon whose regular tugs hold the orbit oval. The lesson on moons in step explains how.

How we know the water is there. Nobody has seen any of these oceans. The evidence is indirect, and it differs from moon to moon.

Moon	Planet	The evidence
Europa	Jupiter	A young, cracked surface with few craters, and a magnetic signal of the kind salt water gives
Ganymede	Jupiter	Its auroras rock back and forth less than they should, as a salty ocean would make them
Enceladus	Saturn	Jets of water and ice sampled directly, and a wobble that shows the crust floats free of the core
Titan	Saturn	The way its shape flexes during each orbit

Europa is the best studied. Its ocean is thought to hold about twice the water of all Earth’s oceans, under a shell of ice. NASA’s Europa Clipper, launched in October 2024, is on its way to fly past it dozens of times.

Enceladus is only about 500 km across, and should have frozen through long ago. Instead its south pole is split by four long warm cracks, and jets of water vapour, ice grains, salts and simple carbon compounds come out of them. The ice there may be as little as 1 to 5 km thick.

Ganymede is the largest moon in the Solar System and the only one with a magnetic field of its own. Its ocean may hold more water than all of Earth’s surface, perhaps in several layers with ice between them.

Titan has two kinds of sea. On the surface there are lakes and seas of liquid methane and ethane, under a thick nitrogen atmosphere. It is the only place besides the Earth known to have liquid on its surface. Far below, under the ice, there is water as well.

Why it matters. Life as we know it needs liquid water, a source of energy and some chemistry to work with. These moons appear to have all three. None of that means anything lives there. It means they are worth a close look, and two spacecraft are on their way to take one.