Europa is a smooth white-tan ice world criss-crossed by reddish-brown cracks, hiding a global ocean of salty liquid water beneath its frozen shell. That ocean - kept liquid by tidal heating - makes Europa one of the most promising places to look for life beyond Earth.
Its ocean may hold twice as much liquid water as all of Earth’s oceans combined.
The ice shell is thought to be roughly 15-25 km thick, over an ocean tens of kilometres deep.
Its surface is one of the smoothest and youngest in the Solar System - a few tens of millions of years old.
An ocean you cannot see
How do you find an ocean under kilometres of ice? The Galileo spacecraft noticed that Jupiter’s rotating magnetic field induces a magnetic response in Europa - exactly what a shell of electrically conducting salty water would do. Combined with the moon’s young, cracked, nearly craterless surface, the evidence points to a global liquid ocean beneath the ice.
Physical characteristics
Diameter
3,122 km
Mass
4.800e+22 kg
Surface gravity
1.31 m/s²
Density
3.01 g/cm³
Tidally locked
Yes
Mean temperature
-160 °C
Geometric albedo
0.67
Atmosphere and composition
Atmosphere
A tenuous oxygen exosphere, produced as radiation splits water ice on the surface.
Composition
A rocky interior with an iron core, wrapped in a ~100 km layer of water - an ice shell over a salty liquid ocean.
Interior structure
Metallic coremodelled
An iron centre, size only loosely constrained.
Rocky mantlemodelled
Seafloor rock in contact with the ocean - interesting chemistry for life.
Salt-water oceanmodelled
Likely holds twice the water of all Earth’s oceans combined.
Ice shellmodelled20 km thick
15-25 km of cracked, shifting ice.
How we know: Galileo detected an induced magnetic field that almost certainly requires a conducting salt-water layer; Europa Clipper is on its way to confirm the ocean.
Discovery
Discovered by
Galileo Galilei
Year
1610
How
one of the first astronomical telescopes
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
GalileoNASA · Orbiter · 1989–2003 · completed
The first spacecraft to orbit an outer planet, Galileo circled Jupiter from 1995 to 2003 and dropped a probe into its atmosphere. In 2003 it was deliberately flown into Jupiter so it could never crash into - and contaminate - Europa’s possible ocean.
JunoNASA · Orbiter · 2011–2025 · completed
Juno looped over Jupiter’s poles from 2016 to 2025, peering beneath the clouds to map the planet’s interior, gravity and magnetic field. Its extended mission added daring close flybys of Ganymede, Europa and volcanic Io.
Europa ClipperNASA · Orbiter · 2024 · en route
The largest spacecraft NASA has ever built for a planetary mission is on its way to Jupiter, arriving in 2030. From Jupiter orbit it will make dozens of close Europa flybys, using ice-penetrating radar to probe the ocean hiding beneath the moon’s frozen shell.