Europa (moon)
Europa is one of Jupiter's four Galilean moons and is slightly smaller than the Moon. Its bright icy surface is crossed by long fractures and bands. Multiple lines of evidence indicate that a global saltwater ocean lies beneath this shell. The presence of liquid water at depth makes Europa one of the principal targets in the study of habitability beyond Earth.[7][8]
Size and physical characteristics
Europa has a mean diameter of approximately 3,122 km, about 350 km less than that of the Moon. It is the smallest of the four Galilean moons. Its mass is 4.80 × 1022 kg and its mean density 3.01 g/cm³. Rocky and metallic material accounts for most of the mass, while water and ice form the outer layer.[1][3]
Surface gravity is approximately 1.31 m/s², or 13% of Earth's. An object would weigh about seven and a half times less on Europa than on Earth. Despite its icy appearance, Europa is far denser than water and is not a solid sphere of ice.[1]
Orbit and rotation
Europa orbits at a mean distance of about 671,000 km from the centre of Jupiter, between Io and Ganymede. One revolution takes approximately 3.55 Earth days, or 85 hours. Europa rotates once in the same interval and therefore keeps the same hemisphere facing Jupiter.[4][5][3]
Io, Europa and Ganymede are linked by the Laplace resonance: for every orbit of Ganymede, Europa completes approximately two and Io four. Their repeated gravitational interactions maintain the slight eccentricity of the moons' orbits. Jupiter's changing tidal force deforms Europa, and energy dissipated by this flexing heats its interior. Tidal heating helps keep water liquid despite the great distance from the Sun.[9][7]
Surface and icy terrain
Europa's surface consists predominantly of water ice. Images show long dark lines, double ridges and broad bands cutting across older terrain. Some areas resemble a broken mosaic in which individual blocks of ice have shifted and rotated relative to one another. These regions are known as chaos terrain.[3][7]
Impact craters are comparatively scarce, indicating that the surface is geologically renewed as older impact features are covered or disrupted by the movement and reorganisation of ice. The opposing edges of some bands fit together like sections of ice crust that have pulled apart. The precise formation mechanisms of the ridges and chaos terrain remain under investigation.[7]
Subsurface ocean
Some of the most important evidence for an ocean came from the Galileo spacecraft. Measurements showed that Europa responds to Jupiter's changing magnetic field by producing an induced field of its own. This behaviour is best explained by a global layer of electrically conductive material beneath the surface—a saltwater ocean.[7]
Models indicate that this ocean may contain more than twice as much water as all of Earth's oceans combined. Its exact depth, salinity and temperature distribution are not yet known. Because the ocean is hidden beneath ice, its properties are reconstructed from magnetic, gravitational and other indirect measurements.[8][7]
Thickness of the ice shell
In January 2026, the Juno team published an estimate based on microwave measurements collected during the spacecraft's flyby on 29 September 2022. In the observed region, the cold, rigid part of a pure-water ice shell was approximately 29 km thick.[10]
This figure applies to the cold outer layer. If warmer convecting ice lies beneath it, the total shell must be thicker. Dissolved salts would reduce the estimated thickness. The result therefore does not establish a single uniform value for all of Europa.[10]
Atmosphere and magnetospheric interaction
Europa possesses an extremely thin oxygen-bearing atmosphere. Charged particles in Jupiter's magnetosphere strike the surface and split water-ice molecules into hydrogen and oxygen. This oxygen is produced by radiation chemistry, not photosynthesis.[2]
Data from Juno's JADE instrument place the rate of oxygen production at approximately 12 kg per second, or about one thousand tonnes per day. Some oxygen can take part in chemical reactions at the surface. Its possible transport through the ice into the ocean is important to estimates of available chemical energy, but the rate of such transport remains unknown.[2]
Surface composition
Observations by the James Webb Space Telescope published in September 2023 detected carbon dioxide, with a particularly strong concentration in the chaos terrain of Tara Regio. The distribution of CO2 indicates that the carbon probably came from Europa's interior, possibly its ocean, rather than being delivered chiefly by meteorites.[11]
Carbon dioxide is not stable on Europa's surface over long geological intervals. Its presence in relatively young terrain links the observed surface chemistry with the renewal of the ice shell. These measurements help constrain which substances may be present in the subsurface water.[11]
Potential habitability
Scientific interest in Europa centres on the possible combination of liquid water, essential chemical elements and sources of energy. If the ocean is in contact with a rocky seafloor, reactions between water and rock may create chemical conditions relevant to living systems. The ocean's composition and the intensity of this exchange remain unknown.[8]
No evidence of life on Europa has been found. Europa Clipper is designed to determine whether environments on the moon could support life. Its investigation covers the ice shell, ocean, composition and geological activity.[8]
Discovery and exploration
Galileo Galilei discovered Europa in January 1610 together with Jupiter's other large moons. These observations demonstrated that celestial bodies could orbit a planet other than Earth. Europa can be seen through a small telescope as one of the bright points close to Jupiter.[6]
In 1979, Voyager 1 and Voyager 2 returned images of the icy surface. The next major stage was the Galileo mission, which operated in the Jovian system from 1995 to 2003. It made 12 close flybys of Europa and measured its terrain, gravity and magnetic environment.[6][7]
On 29 September 2022, Juno passed approximately 352 km above Europa's surface. The spacecraft obtained new images and measurements of particles and microwave emission, later used to study oxygen production and the ice shell.[12][2][10]
Europa Clipper and Juice
NASA's Europa Clipper launched on 14 October 2024 and is scheduled to reach Jupiter in April 2030. It will orbit the planet and conduct 49 close flybys of Europa. Its instruments will investigate the ice shell, magnetic properties, composition and internal structure of the moon.[8]
The European Jupiter Icy Moons Explorer—Juice launched on 14 April 2023 and is expected to reach the Jovian system in July 2031. Its programme includes observations of Europa, Ganymede and Callisto. The two missions will provide complementary measurements of Jupiter's icy moons and their internal oceans.[13]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.0 2.1 2.2 2.3 NASA's Juno Mission Measures Oxygen Production at Europa // NASA. 4 March 2024.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Europa Facts // NASA Science.
- ↑ 4.0 4.1 4.2 4.3 4.4 Planetary Satellite Mean Elements // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 5.0 5.1 Galilean Satellite Fact Sheet // NASA Space Science Data Coordinated Archive.
- ↑ 6.0 6.1 6.2 Europa Exploration // NASA Science.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Why Europa: Evidence for an Ocean // NASA Science.
- ↑ 8.0 8.1 8.2 8.3 8.4 Europa Clipper // NASA Science.
- ↑ NASA's Juno Mission Uncovers Heart of Jovian Moon's Volcanic Rage // NASA. 12 December 2024.
- ↑ 10.0 10.1 10.2 NASA's Juno Measures Thickness of Europa's Ice Shell // NASA. 27 January 2026.
- ↑ 11.0 11.1 NASA's Webb Finds Carbon Source on Surface of Jupiter's Moon Europa // NASA Science. 21 September 2023.
- ↑ NASA's Juno Shares First Image From Flyby of Jupiter's Moon Europa // NASA. 29 September 2022.
- ↑ Juice // European Space Agency.