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.