Europa (moon)

Europa is the fourth-largest moon of Jupiter and the sixth closest to the planet. Its diameter is about 3,100 kilometres, a little smaller than Earth's Moon. It completes an orbit of Jupiter in roughly three and a half Earth days and is tidally locked, so the same hemisphere generally faces the planet.

The moon’s surface is largely water ice. Long, intersecting bands, ridges and cracks mark the terrain, while large impact craters are comparatively scarce. Images from spacecraft suggest that the surface is geologically young and has been reshaped, though the processes and timing are not fully settled. Jupiter’s gravitational pull varies over Europa’s slightly elliptical orbit; the resulting tidal flexing is thought to contribute to internal heating and surface fractures.

Several independent observations support the idea that a global ocean of salty liquid water lies beneath Europa’s ice. A key clue came from NASA’s Galileo spacecraft: measurements of Jupiter’s magnetic field near the moon indicated an electrically conductive layer inside it. Salty water is the leading explanation for that signal. Surface geology and tidal-response models provide additional evidence.

The ocean has not been directly observed, and estimates of the ice-shell thickness and ocean depth depend on models. NASA describes an ice shell roughly 15–25 kilometres thick above an ocean estimated to be about 60–150 kilometres deep; these are scientific estimates, not direct measurements. The amount of water could exceed the volume of Earth’s global ocean, but details of the interior remain uncertain.

Europa is a major target in astrobiology because life as known on Earth requires liquid water, chemical building blocks and an energy source. A subsurface ocean, possible exchange between the ocean and surface, and energy from tidal heating make the moon scientifically compelling. These are reasons to investigate its potential habitability, not evidence that organisms are present.

NASA’s Europa Clipper mission is designed to assess whether places beneath the ice have conditions that could support life. It is not a life-detection mission. Europa’s surface is exposed to intense radiation from Jupiter, so researchers focus especially on the protected environment that may exist below the crust.

Galileo Galilei observed Europa in 1610 while studying Jupiter’s moons; Simon Marius also reported observing the Jovian moons around that time and is commonly credited as a co-discoverer. The Voyager 1 and 2 spacecraft made close observations during their 1979 Jupiter encounters, revealing Europa’s bright, intricately marked surface. NASA’s Galileo spacecraft later studied Jupiter and its moons from orbit, making repeated flybys of Europa between 1995 and 2003. Its measurements provided some of the strongest evidence for a subsurface ocean.

NASA’s Europa Clipper launched on 14 October 2024. It is planned to orbit Jupiter and make repeated close passes of Europa to investigate the moon’s ice, ocean, geology and potential habitability. The European Space Agency’s Jupiter Icy Moons Explorer (Juice), launched in April 2023, will also observe Europa as part of a wider study of Jupiter and its large icy moons, with Ganymede as its main orbital target.