For astronomers searching for life on other worlds in the Solar System, Europa, one of Jupiter’s Galilean moons, is of special interest. Europa measures 3140 kilometres (1950 miles) across on average. It has a smooth, icy surface covered with reddish streaks called lineae, thought to be cracks in the crust. Scientists are fairly sure that below the ice there is a warm-water ocean. When Europa’s orbit takes it close to Jupiter, the planet’s gravity raises tides on this ocean, causing the icy crust above it to bulge and crack open. It is the power of these tides that warms the ocean enough to keep it in a liquid state. Just possibly, Europa's ocean could be home to living things.

Orbit

Of the four Galilean moons Europa is the second nearest to Jupiter. It orbits Jupiter at an average distance of about 670,900 kilometres (416,700 miles), completing a single orbit in just over three and a half days. Like its fellow Galileans, Europa is tidally locked to Jupiter, with one hemisphere constantly facing its parent planet.

Due to its orbital resonances with Io and Ganymede, Europa's orbit is slightly elliptical rather than circular. When Europa comes a little nearer to Jupiter, the giant planet's gravitational pull increases, causing Europa to bulge. As Europa moves further away, Jupiter's gravitational force decreases, causing Europa to relax back into a spherical shape. This is known as tidal flexing. The same process is at work on Io, where tidal flexing creates intense volcanic activity. On Europa, tidal flexing causes crustal movements (see below).
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Evidence for an ocean

The Galileo space probe, which orbited Jupiter from 1995 to 2003, discovered that Europa has a weak magnetic field. Its existence indicates there is a layer inside the moon that conducts electricity. The most likely candidate for this is a large ocean of liquid saltwater beneath its solid icy crust.

In 2012 the Hubble Space Telescope detected water vapour plumes erupting from Europa to a height of about 200 kilometres (120 miles) above the surface. Scientists think these burst through the cracks in the moon’s thin crust.

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 of very hot, molten rock. The movement of Europa’s outer layer strongly indicates that it, too, is not solidly attached to its internal core, but floats on a liquid underlayer—not of molten rock but of a warm water.
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Europa’s geology may therefore resemble that on Earth, where the tectonic plates are being eased apart at the mid-oceanic ridge by magma pushing up from beneath, powered by geothermal energy. On Europa, these crustal movements are caused by the gravitational pull of its giant parent planet, Jupiter (see above). Where Europa's crust cracks open, water from below forces its way to the surface, spreads out and freezes. These cracks are known as lineae.

Life on Europa?

There are three key ingredients that make life possible in any world: liquid water, energy and chemistry. Europa probably has liquid water in the vast ocean beneath its crust. Energy comes in the form of heat produced by tidal flexing caused by Jupiter's immense gravity. The presence of the necessary chemical elements is the least certain. Key elements forming the building blocks of life, such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur, may have been incorporated into Europa as the moon formed.
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Europa’s internal ocean, estimated at about 100 kilometres (60 miles) deep, is thought by many scientists to be, along with Enceladus, a moon of Saturn, the most likely location of extraterrestrial life in the Solar System—if it exists anywhere at all. Other candidates include Mars, Titan, Pluto, Europa's fellow Galileans Ganymede and Callisto, all of which are suspected to have underground oceans, and the atmosphere of Venus.

On Europa, organisms might live clustered around mineral-rich hydrothermal vents in the ocean floor just like those in Earth’s oceans, or inside the rocks that form the Europan ocean bed. Alternatively, they might float or swim freely in the ocean waters, or cling to the lower surface of the icy crust, like algae and bacteria do to the Arctic ice on Earth. The level of oxygen in the oceans might even be enough to support not just microbes, but larger, more advanced life-forms.