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Jupiter’s fourth largest moon, EuropaJupiter’s fourth largest moon, Europa For astronomers searching for life on other worlds in the Solar System, Europa, one of Jupiter’s moons, is of special interest. Europa measures 3140 kilometres (1950 miles) across. It has a smooth, icy surface covered with a maze of lines, thought to be cracks in the ice. There are very few craters. Both these features are taken to be signs that Europa’s icy crust is continually melting and re-solidifying, which in turn suggests the presence of a warm-water ocean lying just beneath. It is just possible that this ocean might be home to living things. In December 2013 the Hubble Space Telescope detected water vapour plumes erupting from Europa which are probably ice bursting through cracks in the moon’s thin crust.

Europa's surfaceEuropa's surface

Plate tectonics

Just as on Earth, whose outer layer is split into a number of giant tectonic plates slowly drifting around its globe, so Europa’s crust is known to be constantly on the move. Earth’s plates float like rafts on a semi-liquid underlayer, and the movement of Europa’s outer layer strongly indicates that it, too, is not solidly attached to its internal core. The cracks in it may have been created by eruptions of liquid water from underneath the crust along lines of weakness.

Europa’s surface with part of its crust stripped awayEuropa’s surface with part of its crust stripped awayEuropa’s geology may thus resemble that on Earth, where the tectonic plates are being eased apart at the mid-oceanic ridges by magma pushing up from beneath. On Europa, these crustal movements could well be caused by the gravitational pull of its giant parent planet, Jupiter, which tugs on its moon’s crust in a similar way to the Moon’s (much weaker) pull on Earth’s oceans, causing our tides.

The Jupiter Icy Moon Explorer (JUICE) is a space probe that will visit three of Jupiter's moons: Ganymede, Callisto and Europa. Launch is planned for 2022. It will attempt to discover for sure whether Europa has a liquid ocean beneath its icy crust.

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