The mystery relates to Saturn’s tiny ice moon Enceladus. Until relatively recently, very little had been known about Enceladus, however scientists expected it to be a cold and dead place given its physical characteristics.

We knew from the prior Voyager missions that Enceladus might have a complex geology, but most people thought that was in the past. Yet it turns out this 500km-across ball of ice is one of the most active moons or planets in the solar system.

‘The pent-up heat – enough to melt the interior, and possibly sustain a liquid water ocean under the ice – would be released as one catastrophic event around every billion years or so. Cassini just happened to fly into it,’ O’Neill said.
‘Eventually you reach a critical point, and the whole thing just blows,’ he said.

The ice sheets would flow like glaciers, the heat causing geysers to pop up all over the active surface, he added.