An ocean hidden under ice
Europa
Moon of Jupiter · discovered 1610 by Galileo Galilei
Europa is the smallest of Jupiter's four Galilean moons, a little smaller than Earth's Moon, and one of the most promising places to look for life beyond Earth. Beneath a bright shell of water ice, scientists are confident there is a global ocean of salty liquid water, kept warm by Jupiter's tides.
That hidden ocean may hold about twice as much water as all of Earth's oceans combined. If it touches a rocky seafloor, it could have the chemistry life needs, which is why NASA and ESA are both sending spacecraft to take a closer look.
Key facts
- Diameter
- 3,122 km90% of Earth's Moon
- Mass
- 4.80 × 10²² kg
- Surface gravity
- 1.314 m/s²0.134 g
- Density
- 3,013 kg/m³
- Distance from Jupiter
- 671,034 km≈ 9.4 Jupiter radii
- Orbital period
- 3.55 daysSame as its rotation (tidally locked)
- Discovered
- 1610Galileo Galilei
- Surface
- Water ice crossed by ridges, cracks and chaos terrain
- Atmosphere
- Extremely thin exosphere dominated by oxygen
- Surface temperature
- About −160 °C at the equator, −220 °C at the poles
- Notable feature
- Conamara Chaos, a jumble of shifted ice rafts
- Named after
- Europa, a Phoenician princess courted by Zeus
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Europa has one of the smoothest surfaces in the Solar System, with few large craters and very little relief. Crater counts suggest the surface is only about 40 to 90 million years old on average, meaning it has been renewed recently in geological terms.
The ice is criss-crossed by long dark bands called lineae, many of them double ridges running for hundreds or even thousands of kilometres. Elsewhere lies chaos terrain, such as Conamara Chaos, where blocks of crust have broken apart, drifted and refrozen, possibly over pockets of water or slushy ice. Pwyll, one of the few large young craters, is surrounded by bright rays.
Reddish-brown material stains the cracks and chaos regions. Scientists think it includes salts and sulfur compounds, some perhaps brought up from below and altered by Jupiter's radiation. Hubble observations have hinted at sodium chloride, ordinary table salt, and the James Webb Space Telescope has detected carbon dioxide in Tara Regio that appears to originate from inside the moon.
Interior and composition
Europa has an iron-rich core, a rocky mantle and an outer layer of water roughly 100 kilometres deep. The upper part is a frozen ice shell, estimated at anywhere from a few kilometres to more than 25 kilometres thick, with liquid ocean beneath. The strongest evidence came from Galileo's magnetometer, which detected a magnetic field induced by Jupiter in a salty, electrically conducting layer.
Because the ocean likely sits on rock, chemical reactions at the seafloor could supply energy and nutrients, as hydrothermal vents do on Earth. Some telescope observations have suggested plumes of water vapour escaping through the ice, though these detections remain tentative.
Orbit and rotation
Europa orbits Jupiter every 3.55 days and is tidally locked. It shares the 1:2:4 Laplace resonance with Io and Ganymede, which keeps its orbit slightly eccentric. The changing pull of Jupiter flexes the moon, generating heat that keeps the ocean liquid and stresses the ice shell, helping to crack it.
Some studies suggest the ice shell may rotate slightly faster than the interior, decoupled by the ocean beneath, which could explain the orientation of certain fracture patterns.
Origin
Europa formed from the disc of gas and dust that surrounded the young Jupiter. It sits between rocky Io and the ice-rich outer moons, and it ended up mostly rock with a substantial water layer. Over time, the heavier materials sank to form the core while water rose to the surface and partly froze.
Exploration
Discovered by Galileo Galilei in January 1610, Europa has become a flagship target for planetary science. Two dedicated missions are now on their way to the Jupiter system.
Voyager 1 and 2 (1979)
The Voyager flybys revealed Europa's remarkably smooth, fractured surface and prompted the first serious speculation about liquid water below.
Galileo (1995–2003)
Galileo's close flybys produced detailed images of ridges and chaos terrain, and its magnetometer readings provided the key evidence for a subsurface ocean.
Juno (2022)
Juno passed about 350 kilometres above Europa in September 2022, capturing high-resolution images of its icy surface.
Europa Clipper (launched 2024)
NASA's Europa Clipper launched in October 2024 and is due to arrive at Jupiter in 2030. It will make dozens of close flybys to measure the ice shell, ocean and habitability.
JUICE (launched 2023)
ESA's Jupiter Icy Moons Explorer will make two Europa flybys in the early 2030s before settling into orbit around Ganymede in 2034.
The moons of Jupiter
Surprising facts about Europa
- Europa's hidden ocean may contain about twice as much water as all of Earth's oceans put together.
- The surface stands in such intense radiation from Jupiter that an unprotected person would receive a lethal dose in about a day.
- Europa's ice is constantly bombarded by charged particles, which split water molecules and produce oxygen, some of which might eventually reach the ocean.
- Europa is one of the brightest moons in the Solar System, reflecting around 60 per cent or more of incoming sunlight.
- Europa Clipper is the largest spacecraft NASA has ever built for a planetary mission, with solar arrays spanning more than 30 metres.
Keep exploring
- Ganymede, moon of Jupiter: the giant moon with a magnetic heart
- Enceladus, moon of Saturn: ice fountains from a hidden sea
- Io, moon of Jupiter: the solar system's volcanic powerhouse
- Jupiter: the planet guide
- Watch Europa orbit Jupiter in the interactive 3D solar system
- All the major moons of the Solar System