Astronomy
Tidal Heating and Its Role in Sustaining Subsurface Oceans
Quick fact
Jupiter's moon Io is not ocean-bearing, but it is the most volcanically active body in the solar system because of tidal heating—its neighbour Europa, with its subsurface ocean, is kept liquid by the same process.
Why this is interesting
You might think that living worlds need to be just the right distance from their star, but some of the most promising ocean worlds in our solar system are millions of kilometres away—kept warm by a force you can't see.
Read the full explanation
Understanding Tidal Heating and Its Role in Sustaining Subsurface Oceans
Imagine kneading a ball of dough: every time you press and fold, the dough warms up. Tidal heating works similarly on a planetary scale. When a moon orbits its planet in a slightly elliptical path, the planet's gravity pulls more strongly when the moon is closer and more weakly when farther. This changes the moon's shape—it stretches and relaxes with each orbit. This flexing creates friction inside the moon, and friction produces heat. For icy moons like Europa and Enceladus, this heat is enough to keep water liquid beneath a frozen crust, even far from the Sun. The gravitational pull also keeps the moon's orbit from becoming circular, so the flexing never stops.
A deeper explanation
The heat comes from the conversion of orbital kinetic energy into thermal energy through tidal friction. For a moon like Europa, its eccentric orbit is maintained by a resonance with the other Galilean moons—Io and Ganymede. As Europa speeds up and slows down, Jupiter's gravity stretches it, and the rocky interior bends. This internal friction releases about 10^13 joules per second—comparable to the heat output of many terrestrial volcanoes but spread over a moon, enough to melt ice and maintain a subsurface ocean. Tidal heating matters because it breaks the assumption that liquid water requires proximity to a star. It creates tidal flexing, which also drives geological activity, such as the geysers of Enceladus, which spray ocean material into space. Thus, tidal heating not only sustains oceans but also makes their contents accessible to spacecraft, turning these moons into prime targets for astrobiology.