Europa (moon)

Europa is one of Jupiter's four largest moons, known as the Galilean satellites.

It is a planetary-mass moon that is slightly smaller and less massive than Earth's Moon. As an icy moon, it is the closest of the three icy Galilean moons to Jupiter.

Scientists consider Europa a high priority for study because it may be habitable. Its unique features suggest it could support extraterrestrial life. The moon is the smoothest known solid object in the Solar System. It lacks large-scale features like mountains or heavy cratering.

The moon's surface is shaped by a process called tidal heating.

Europa follows a nearly circular orbit with an eccentricity of only 0.009. However, gravitational disturbances from other moons maintain this slight eccentricity. As Europa moves closer to Jupiter, the planet's gravity pulls harder on the moon. This causes Europa to elongate toward the planet. As it moves away, the gravitational force decreases and the moon relaxes. This constant stretching and squeezing is called tidal flexing. This mechanical energy generates internal heat that keeps the moon's interior warm.

This internal heat likely maintains a massive subsurface ocean.

Most scientists believe a layer of liquid water exists beneath a thick icy crust. The outer crust is estimated to be approximately 15 to 25 kilometers thick. Beneath this lies a ductile layer of warm ice. Below that, the liquid ocean may be about 100 kilometers deep. This ocean could contain a volume of water between two and three times that of Earth's oceans. Some models suggest the ocean interacts with the silicate rock mantle below. This interaction might bring essential chemicals to the water.

Europa's surface is marked by a complex network of cracks and streaks. These features are called lineae and appear as light tan stripes.

The surface is geologically young due to the movement of ice. Some researchers suggest a process similar to plate tectonics occurs on Europa. This movement could drive chemicals from the surface into the ocean. There is also evidence of "chaos terrain," where the ice appears to have melted or broken apart.

Additionally, the Hubble Space Telescope and the Galileo probe detected water vapor plumes. These plumes act like cryogeysers erupting from the subsurface. Studying these plumes could allow researchers to search for life without landing.

The history of Europa's discovery dates back to early 1610. Galileo Galilei observed the moon using a 20x-magnification refracting telescope. Simon Marius also discovered the moon independently around the same time. Marius suggested naming the moon after Europa from Greek mythology. This character was the daughter of the Phoenician king of Tyre. For a long time, astronomers simply called it the second satellite of Jupiter. The name was not widely used again until the mid-20th century.

Space exploration has provided most of our modern data. The Galileo mission orbited Jupiter from 1995 to 2003 and remains a primary source of information.

In September 2022, the Juno spacecraft flew within 320 km of the surface. New missions are currently underway to learn more. The European Space Agency's Juice mission launched in April 2023. NASA's Europa Clipper mission launched on 14 October 2024. These missions will help scientists understand the moon's habitability in much greater detail.

Europa's physical makeup is similar to terrestrial planets. It is primarily composed of silicate rock and may have an iron-nickel core.

It also possesses a very thin atmosphere made mostly of oxygen. Recent studies in March 2024 suggest this atmosphere may have less oxygen than previously thought. The moon is also tidally locked to Jupiter. This means one side always faces the planet. The gravitational relationship between Europa, Io, and Ganymede is known as a Laplace resonance. This orbital dance is what helps pump energy into Europa's system through tidal forces.