EUROPA
Europa is Jupiter’s fourth-largest moon and the smoothest of all the celestial bodies. There are almost no craters, and despite a dense network of cracks and ridges covering this moon, none are higher or deeper than a few hundred metres. This suggests that Europa’s surface is geologically young and possibly floating on a liquid mantle. The Hubble Space Telescope has also spotted plumes of water vapour spewing 200 kilometres (124 miles) into the air from the south pole. This lends weight to the idea that Europa has a subsurface saltwater ocean covered by a layer of ice that may be just a few kilometres thick in places.
SIZE: 25 per cent the diameter of Earth
DISTANCE FROM THE SUN: 4.9 to 5.4 astronomical units
BIOLOGICAL POTENTIAL: Possible
TYPE OF OCEAN: Active
SIZE OF OCEAN: Twice as big as Earth’s
Tidal flexing and friction from gravitational interactions with Jupiter generate enough heat to keep the interior ocean liquid, but because it’s so far from the Sun, the surface remains frozen. Europa also has a very thin oxygen atmosphere, generated when radiation splits water molecules in the surface ice. A tiny fraction of this could become trapped within the ice and would eventually be carried down to the subsurface ocean by tectonic subduction. A 2007 study at Stanford University in California calculated that it was possible for the oxygen levels in Europa’s ocean to equal that of Earth’s own deep seas, further bolstering Europa’s chances of harbouring life.
SIZE: 41 per cent the diameter of Earth
DISTANCE FROM THE SUN: 4.9 to 5.4 astronomical units
BIOLOGICAL POTENTIAL: Unknown
TYPE OF OCEAN: Trapped
SIZE OF OCEAN: One to six times Earth’s
GANYMEDE
Ganymede, Jupiter’s largest moon, is eight per cent
