Europa moon

Europa is one of the four Galilean moons of Jupiter and is the smallest and least massive among them. Slightly smaller and less massive than Earth’s Moon, it is a planetary-mass satellite that can be observed from Earth with common binoculars. Europa is an icy world with a geologically young surface, shaped largely by tidal heating caused by its close orbit around Jupiter. Its internal structure is believed to include an iron–nickel core surrounded by silicate rock and an extensive outer shell of water ice, beneath which lies a global subsurface ocean. The Jovian moon possesses a very thin atmosphere composed mainly of oxygen.
Europa’s bright, whitish–beige surface is characterised by smooth icy plains, a web of dark cracks and streaks, and very few impact craters, giving it the smoothest surface of any known solid body in the Solar System. Its surface and interior have been studied through telescopic observations and numerous spacecraft flybys, beginning in the 1970s and continuing with recent close approaches, including a 320-kilometre flyby by the Juno spacecraft in 2022.

Discovery and naming

Europa was discovered on 8 January 1610 by Galileo Galilei, after first being observed the previous night when it appeared so close to Io that the two could not be resolved separately. Simon Marius also independently observed the moon around the same time. Marius proposed the name “Europa” after the figure from Greek mythology, a Phoenician princess courted by Zeus and mother of King Minos of Crete. Although this name was suggested in the early seventeenth century, it did not come into common usage until the twentieth century; earlier literature generally referred to the moon by the designation “Jupiter II”, based on Galileo’s numbering system.
Europa is now classified as Jupiter’s sixth satellite by modern ordering, following later discoveries of inner moons by the late nineteenth and twentieth centuries. The adjectival form “Europan” is the standard terminology when describing features related to the moon.

Orbit and rotation

Europa orbits Jupiter every 3.55 days at an average distance of roughly 670,900 kilometres. Its orbital eccentricity is very small, leading to a nearly circular orbit, with its inclination to Jupiter’s equatorial plane at approximately 0.47°. Like the other large Jovian satellites, Europa is tidally locked to Jupiter, always showing the same hemisphere to the planet. The prime meridian on Europa therefore passes through the point on its surface where Jupiter appears overhead.
Research suggests that Europa may not be perfectly locked, as small deviations known as nonsynchronous rotation have been proposed. These would imply that the icy crust is somewhat decoupled from the rocky interior by a liquid ocean layer. The orbital eccentricity of Europa is maintained by its three-body resonance with Io and Ganymede, a configuration that generates continuous tidal flexing. This tidal heating is a principal energy source driving Europan geological processes and is a key factor in maintaining the subsurface ocean in a liquid state.
Some studies propose that Europa’s axis may have been tilted in the past, leading to variations in stress on the icy crust. Its vast network of cracks, fractures, and lineae may preserve a record of past changes in spin orientation, tidal forces, and internal heat distribution.

Physical characteristics

Europa is slightly smaller than the Moon, with a diameter just over 3,100 kilometres. Its density indicates a composition dominated by silicate rock with a metallic core, comparable to terrestrial planets. Overlying the rocky mantle is a global layer of water estimated to be around 100 kilometres thick, composed of both solid ice and liquid water. This water layer is believed to host a deep subsurface ocean that may be in contact with a rocky sea floor.
Europa’s surface is extremely smooth, lacking significant topography compared with most other solid bodies. The characteristic dark streaks cutting across the globe, known as lineae, likely form as the icy crust fractures and shifts due to tidal flexing. In addition, blocks of surface ice known as chaos terrain appear to have broken, rotated, and refrozen, suggesting active geological processes.
The moon’s tenuous atmosphere consists primarily of molecular oxygen, which is generated by the interaction of surface ice with solar radiation and charged particles from Jupiter’s magnetosphere.

Subsurface ocean and potential habitability

A leading interpretation of Europa’s geology holds that a subsurface ocean lies beneath the icy crust. Evidence includes magnetic field measurements, the smoothness of the surface, and the presence of features resembling plate-tectonic behaviour within the ice. Heat from tidal flexing keeps this ocean liquid and may drive convection within the ice shell.
Sea salt detected on some surface formations suggests that water from the subsurface has interacted with rock on the seafloor before resurfacing through cracks. This interaction could provide chemical energy sources essential for habitability. Models of ice movement propose that surface materials may be cycled downward, enabling energy and chemical exchange between the surface and the ocean.
Cryovolcanic activity is also strongly suspected. Observations by the Hubble Space Telescope detected intermittent plumes of water vapour, and reanalysis of Galileo spacecraft data further supported the presence of water plumes erupting from the surface. These potential plumes offer an accessible way to study the ocean without landing, as passing spacecraft could sample material ejected from below.
Recent studies in 2024 have suggested that the surface oxygen concentration may be lower than previously inferred, potentially affecting estimates of oxygen transfer to the ocean.

Exploration and scientific investigation

Europa has been examined extensively by spacecraft beginning with early flybys in the 1970s. The Galileo mission, launched in 1989, provided the most detailed survey of Europa during its eight-year orbit of Jupiter from 1995 to 2003. More recent observations have been conducted by the Juno spacecraft.
Future missions are expected to greatly expand knowledge of Europa:

• Jupiter Icy Moons Explorer (JUICE), launched by the European Space Agency in April 2023, will perform two flybys of Europa before entering orbit around Ganymede.
• Europa Clipper, launched by NASA in October 2024, will carry out numerous close flybys to investigate surface composition, subsurface structure, and possible plume activity.