In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel the heat from today’s topic: tidal heating!
Tidal heating happens when the gravity of an object is strong enough to flex and bend a planet or moon in orbit around it.
All orbiting object cause tides on each other. The most familiar example is the Moon, which raises ocean tides on the Earth. However, the Moon is too small to accomplish anything other than a gentle raising and lowering of our planet’s oceans. The Earth also causes tides on the Moon, and this effect is so strong that when the Moon was molten, it locked those tides in place. This forced the Moon to always present the same face towards the Earth for eternity.
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Tidal heating happens when a large object wants to tidally lock a smaller object but is prevented from doing so. The most famous examples occur in the four Galilean moons of Jupiter.
The innermost moon of Jupiter is Io, and in normal conditions it would have tidally locked ages ago. But the other moons also exert a gravitational influence on Io, constantly shifting its orbit. So Jupiter raises tides on Io in different places at different times during its orbit.
Imagine taking a ball of dough and stretching it out, then squeezing it. Then stretch it out in a different direction and squeeze it again. You are applying energy to the dough, and most of that energy goes to rearranging the shape of the dough. But some of it is lost to heat because of the internal friction of all the molecules of dough constantly rubbing up against each other.
This same process happens to Io, but with Jupiter’s gravity acting as the hands shaping the planet. The effect is so strong that Io is the only other world, besides the Earth, in the solar system to host active molten volcanoes.
The same process happens with the second moon of Jupiter, Europa. But the effect is smaller, so Europa’s crust remains frozen while only its interior melts, creating a world-spanning ocean of liquid water.