Europa

Identity and context

Europa is a moon of Jupiter and one of its four Galilean moons, along with Io, Ganymede, and Callisto. It is the planet’s fourth-largest moon and the sixth closest to it. Its name comes from Europa, a figure in Greek mythology abducted by Zeus, the equivalent of the Roman Jupiter. In NAIF nomenclature, Europa has code 502 and the Roman numeral JII.

With an equatorial diameter of about 3,100 kilometres, it is around 90% the diameter of Earth’s Moon. Its icy surface reflects about 5.5 times as much sunlight as the Moon, so Europa would appear brighter if it occupied the Moon’s place in the sky. Jupiter is about 780 million kilometres from the Sun, and sunlight takes approximately 45 minutes to reach Europa; there, sunlight is about 25 times fainter than on Earth.

Surface and structure

Spectroscopic observations made in 1972 showed that the surface is composed mainly of water ice. Images from the Voyager spacecraft revealed very bright, relatively smooth terrain crossed by bands, cracks, and ridges. In some dark bands, the edges fit together like pieces of a jigsaw puzzle. This suggests that the fractures separated and that dark, icy material may have flowed into the openings. The images also showed few large craters, a sign that geological processes may have renewed the surface; explanations considered include flows of ice or the gradual settling of the crust.

Important and robust evidence for an ocean beneath the ice comes, among other things, from Galileo’s magnetic measurements. Jupiter disturbed the magnetic field around Europa in a way that points to an induced field within the moon itself, probably generated by a conductive fluid. Given Europa’s icy composition, scientists consider a global saltwater ocean the most likely candidate. The ice shell is estimated to be between 15 and 25 kilometres thick, and the ocean could be between 60 and 150 kilometres deep. That ocean could contain approximately twice as much water as all of Earth’s oceans.

In 2019, a team announced the direct detection of water vapour above Europa, using infrared observations from the Keck Observatory. Other observations with telescopes and a reinterpretation of Galileo data have suggested that water plumes may rise to about 160 kilometres above the surface. Their existence and possible connection to the ocean have not been conclusively established. If the ejected material came from the ocean, a spacecraft could pass through a plume and analyse it without reaching the surface. It has also been proposed that ocean samples could become incorporated into the base of the crust and, as less-dense ice rises, become accessible to remote study.

Orbit and activity

Europa completes an orbit around Jupiter in about 3.5 days and is tidally locked: it always keeps the same hemisphere facing the planet. Its orbit is slightly elliptical, so its distance from Jupiter changes. The moon is in an orbital resonance with Io and Ganymede: for every orbit Ganymede completes, Europa completes two and Io completes four. Repeated gravitational attractions among the three moons maintain a small eccentricity in their orbits.

As the distance varies, the strength of Jupiter’s gravitational pull also changes between Europa’s near and far sides. The resulting tides deform and relax the moon. This flexing probably contributes to surface fractures and could produce internal heating. If an ocean exists, tidal heat could promote volcanic or hydrothermal activity on the ocean floor. Jupiter bombards the surface with radiation, which is harmful to life exposed there, although this radiation could indirectly help generate substances that could be used in an ocean protected beneath the ice.

Habitability and exploration

The possibility of habitability is a scientific hypothesis, not a detection of life. For life as it is known on Earth, liquid water, certain chemical elements, and an energy source are considered necessary. Studies cited by NASA indicate that Europa may have water and elements such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Sunlight does not reach beneath the ice, so any hypothetical organisms would probably depend on chemical reactions rather than photosynthesis. Nor does the surface radiation imply that the underground ocean has the same conditions as the surface.

The first close observations came from Pioneer 10 and 11; Voyager 1 and 2 obtained more detailed images during their Jupiter flybys in 1979. Galileo, which entered orbit around Jupiter in 1995, made 12 close flybys of Europa and provided much of what is known about it. Juno has also made close flybys, and the Hubble Space Telescope is among the instruments used for observations from Earth. NASA indicates that six robotic spacecraft have explored Europa.

Europa Clipper was launched on 14 October 2024. Its main scientific objective is to determine whether there are places beneath Europa’s surface that could support life. The spacecraft will make numerous flybys and study the moon in detail; its purpose is to assess potentially habitable conditions, not to confirm on its own that life exists.