Tidal Heating - crunchy on the outside, liquid goodness on the inside
Tidal heating
We are of course familiar with tides on Earth. The gravitational interaction of the mass of the moon raises a large wave of water that circles the planet. When a large satellite orbits a giant planet, gravitational interactions due to eccentricity of the orbit (variation in the distance between the two bodies), deforms the surface of the satellite by raising a large tidal bulge on the surface that varies in height during the orbit. The bulge is higher when the satellite is closer to the large planet. Io, Jupiter's innermost moon is generally considered to be the poster-child of tidal heating, because its tidal bulge is thought to vary by as much as 100 m. This continued squeezing of the satellite causes friction within the satellite and heats it up. This has transformed Io into a hot rocky world with many volcanoes, but the other satellites experience it as well as the animation on the right demonstrates. If Jupiter had only one satellite, its orbit would become circular over geological time and the variation in tidal bulge would disappear. But the outer large planets all have multiple large satellites that prevent this from happening. Hence tidal heating remains a viable heat source for these satellites for billions of years.For a more in-depth explanation of why the tidal heating continues to work, read this excellent explanation. If you prefer to hear a 1 min explanation, here you go:
Water in the interior of the icy satellites
Tidal heating provides enough energy to the small icy satellites to melt parts of their interior and keep it in liquid form. Please note that there is still some discussion about this heat source as some issues remain. Since this initial discovery, we have now identified a number of worlds which we believe have interiors that contain liquid water. Please recall the special properties of water discussed previously, you will see below that these are indeed very important. What these worlds have in common is that each world is covered by an icy crust, is believed to have some kind of rocky core and has liquid or at least a slushy mantle. This gives an overview of our current understanding of which worlds may contain liquid water.We will only examine the arguably two most interesting and best studied examples of the role of water on these satellites, namely the icy geysers of Enceladus and the liquid ocean of Europa. To get a better idea of how large these worlds are, here are size comparisons for both.
To give you a sense of scale, one can compare Enceladus to the British isles:
And here is Europa with respect to our moon:
