Europa

The surface of Europa, here imaged by the Voyager 2 probe in 1979, is criss-crossed by a complex network of low ridges.

In astronomy, fourth-largest moon of the planet Jupiter, diameter 3,140 km/1,950 mi, orbiting 671,000 km/417,000 mi from the planet every 3.55 days. It is a rocky body, probably with an iron core, covered by ice.

NASA's Galileo spacecraft began circling Europa in February 1997. One of the first discoveries from the data it sent back was that the ‘cracks’ covering the surface of the moon are in fact low ridges. These features lend credence to the idea that Europa possesses a hidden subsurface ocean.