THE SOLAR SYSTEM’S

Pack your swimming costume – the Solar System is much wetter than you think

Written by Luis Villazon

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 larger than Mercury but only half its mass. Such a low density suggests that it should be made of equal parts rock and water. In the 1990s, the Galileo spacecraft found that Ganymede has its own magnetic field, which means that it must have a molten iron core. The heat from this core would be enough to melt ice and create an enormous subterranean ocean. This ocean could be a 100-kilometre (62-mile) thick layer sandwiched between an icy crust on the surface and another layer of ice below, kept solid by the enormous pressures within the moon. Other models have suggested that there might be several different oceans, arranged in concentric rings like an onion, with different phases of solid ice separating them. Ganymede’s ocean is trapped a long way underground, so we don’t see any water plumes spewing at the surface like on other moons, but there are other observations that provide direct evidence of its ocean.