Europa

Europa is the smallest of Jupiter's four Galilean moons and, at about 3,120 km across, slightly smaller than Earth's Moon. Yet this modest ball of rock and ice has become one of the most compelling destinations in the entire solar system, because beneath its bright, frozen shell almost certainly lies a global ocean of liquid salt water — an ocean thought to hold roughly twice the volume of all of Earth's seas combined.

That single fact reframes everything about the little moon. Europa is not just another cratered iceball orbiting a giant planet; it is a candidate for life beyond Earth, and the target of two flagship spacecraft now on their way to Jupiter to find out.

Discovery and the Galilean family

Europa was spotted in January 1610, when Galileo Galilei turned his new telescope on Jupiter and saw four points of light shifting position night after night — the first moons ever found orbiting another planet, and proof that not everything in the heavens circled the Earth. The German astronomer Simon Marius observed them at nearly the same moment and supplied the names we still use, drawn from the mythological loves of Zeus. Europa, a Phoenician princess carried off by the god, gives her name to the second-closest of the quartet.

The four Galilean moons — Io, Europa, Ganymede, and Callisto — form a miniature system in their own right. Europa orbits Jupiter once every 3.55 days at a distance of about 671,000 km, locked in a precise gravitational rhythm known as the Laplace resonance: for every orbit Ganymede completes, Europa makes two and Io makes four. That clockwork is the engine behind almost everything that makes Europa interesting.

An ocean under the ice

Europa's surface is water ice, but the water does not stop there. Because its resonant orbit is slightly elliptical, Jupiter's immense gravity flexes and kneads the moon on every pass, and that tidal friction generates heat deep inside. Enough heat, models suggest, to keep a layer of the interior liquid. Beneath an ice shell estimated at 15 to 25 km thick lies an ocean thought to be 60 to 150 km deep — so deep that, despite Europa's small size, it may contain more than twice the water of every ocean on Earth.

The strongest evidence is magnetic. As Europa moves through Jupiter's powerful magnetic field, NASA's Galileo orbiter detected an induced field emanating from the moon itself — exactly the signature expected from a shell of electrically conductive salt water circulating below the ice. Hubble observations later added tantalizing hints of intermittent water-vapour plumes venting some 200 km above the south polar region, as if the ocean occasionally finds a path to the surface.

A cracked and youthful surface

Seen up close, Europa is one of the smoothest large bodies in the solar system — and one of the youngest. Impact craters are strikingly rare, implying the surface is resurfaced and only about 40 to 90 million years old. Crisscrossing the whole globe instead are lineae: long, reddish-brown fractures thousands of kilometres in length, where the shifting ice shell has split and refrozen. Some arc in repeating cycloidal curves, tracing the daily rise and fall of tidal stress as the moon circles Jupiter.

Elsewhere the ice dissolves into chaos terrain — jumbled fields of tilted, broken plates frozen back into a lumpy matrix, as seen in the well-studied Conamara Chaos. Such regions look as though warm, briny water welled up from below, softened the shell, and let rafts of ice drift before locking solid again. The rusty tint staining the cracks is thought to be salts and sulphur compounds carried up from the ocean — a possible window onto the chemistry hidden beneath.

Could anything live there?

Europa checks the boxes astrobiologists care about most: liquid water, a rocky sea floor that could host hydrothermal vents, and a steady energy source in the form of tidal heating. On Earth, similar vents support entire ecosystems in total darkness. If Europa's ocean has stayed liquid for billions of years and touches a chemically active rock mantle, it is one of the likeliest places in the solar system to harbour microbial life.

The surface itself is hostile — airless, bathed in lethal doses of Jupiter's radiation, and colder than roughly minus 160 °C at the equator, plunging far lower toward the poles. But the ocean is shielded by kilometres of ice, and it is that buried environment, not the frozen crust, that draws the interest.

The missions on their way

NASA's Europa Clipper launched on 14 October 2024 and will reach the Jupiter system in 2030. Rather than orbit Europa directly — which the radiation would quickly destroy — it will loop around Jupiter and make dozens of close flybys, using radar to sound the ice, cameras to map the surface, and instruments to sniff any plumes for signs of the ocean's makeup. Its goal is not to detect life outright but to judge whether Europa is habitable.

Alongside it, the European Space Agency's JUICE spacecraft, launched in 2023, will study Europa, Ganymede, and Callisto through the early 2030s, making two flybys of Europa before settling into orbit around Ganymede. Together the two missions promise the closest look yet at this ocean world since Galileo's mission ended in 2003.

Finding Europa yourself

You cannot see Europa with the naked eye, but almost any small telescope — or even good binoculars held steady — will reveal it. Point them at Jupiter and you will find up to four tiny star-like dots strung out in a line to either side: the Galilean moons, exactly as Galileo first saw them. Europa is the faintest and closest-in of the four bright ones, and its position shifts noticeably from night to night, so that sketching the moons over several evenings lets you watch it circle Jupiter in just a few Earth days.