How have the moons Ganymede, Callisto and Europa sustained a subsurface ocean despite being so far away from the Sun?
They are called ‘icy moons’ because they are made of frozen ice. But there is a mechanism inside the moons which generates heat, and then the ice can melt and become liquid. The question is, how is the heat generated inside? Here we have two possible reasons. One answer is that because the moons are actually large, there are – like on Earth – some radioactive elements. Inside Earth, for example, there is heat generation coming from the interior from radioactive elements. But the other process, which is quite interesting, is that because the moons are relatively close to the very big planet Jupiter, and because of the characteristics of their orbits, there is a gravitational struggle. In fact, these moons don’t have the shape of a normal ball – they are more like a rugby ball. They change shape because of the gravitational attraction between the moon and Jupiter. That makes the moon change shape and keeps the interior moving. That also generates some heat, and this heat allows the ice to melt and become liquid.
Dr Olivier Witasse, planetary scientist, European Space Agency