Europa (moon)

Europa is one of the four largest moons orbiting the planet Jupiter. It was discovered independently by two famous astronomers, Simon Marius and Galileo Galilei. Though small compared to Earth's Moon, Europa has captured scientists' imaginations because it might hide a secret: a warm ocean beneath its icy surface.

This icy moon has a very smooth and young-looking surface, marked by cracks and streaks instead of mountains or large craters. Scientists believe a salty water ocean lies below this icy shell, kept warm by the gravitational pull of Jupiter. This ocean could possibly support life, making Europa a key target in the search for life beyond Earth.

In recent years, space probes have flown close to Europa, sending back exciting images and data. The Galileo spacecraft provided much of what we know today, and newer missions like NASA's Europa Clipper, launched in October 2024, aim to study the moon even closer. These missions help scientists understand if Europa's ocean could be a home for living things.

Discovery and naming

Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610. Galileo first saw Europa and another moon, Io, close together through a telescope but could not tell them apart at first. The next night, he saw them as two separate objects.

The moon is named after a character from Greek mythology, Europa, who was a lover of Zeus, the ruler of the gods. The naming idea came from another astronomer, Simon Marius. For a long time, Europa was just called "Jupiter II" because it was the second moon found orbiting Jupiter. Later, more moons were discovered closer to Jupiter, so Europa is now known as Jupiter's sixth moon, but it is still sometimes called Jupiter II.

Orbit and rotation

Europa orbits Jupiter in about 3.55 days, traveling a distance of roughly 670,900 km from the planet. Its path is almost circular and stays close to Jupiter's equatorial plane. Like the other large moons of Jupiter, Europa is tidally locked, meaning one side of it always faces Jupiter.

The small wobble in Europa's orbit, caused by the pull of other moons, creates gentle stretching and squeezing of the moon. This movement may help keep a warm, liquid ocean beneath its icy surface by generating heat inside Europa. Scientists think Europa might have tilted on its axis in the past, which would explain some of the cracks seen on its surface today. These cracks record the powerful tides caused by Jupiter's gravity acting on Europa's ocean.

Bulk properties

Europa is slightly smaller than the Earth's moon. At just over 3,100 kilometres (1,900 mi) in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System. It is the least massive of the Galilean satellites. Its bulk density suggests that it is similar in composition to terrestrial planets, being primarily composed of silicate rock.

It is estimated that Europa has an outer layer of water around 100 km (62 mi) thick – a part frozen as its crust and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. This layer is likely to be a salty liquid-water ocean. Europa probably contains a metallic iron core.

The scientific consensus is that a layer of liquid water exists beneath Europa's surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. Europa's surface temperature averages about 110 K (−160 °C; −260 °F) at the equator and only 50 K (−220 °C; −370 °F) at the poles, keeping Europa's icy crust as hard as granite. The first hints of a subsurface ocean came from theoretical considerations of tidal heating.

The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 nT) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's. The existence of the induced moment requires a layer of a highly electrically conductive material in Europa's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater.

Since the Voyager spacecraft flew past Europa in 1979, scientists have worked to understand the composition of the reddish-brown material that coats fractures and other features on Europa's surface. Spectrographic evidence suggests that the darker, reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically.

The presence of sodium chloride in the internal ocean has been suggested by a 450nm absorption feature, characteristic of irradiated NaCl crystals. It has been spotted in HST observations of the chaos regions and is presumed to be areas of recent subsurface upwelling. The subterranean ocean of Europa contains carbon and was observed on the surface ice as a concentration of carbon dioxide within Tara Regio, a geologically recently resurfaced terrain.

The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole. The image suggests the plume may be 200 km (120 mi) high, or more than 20 times the height of Mt. Everest., though recent observations and modeling suggest that typical Europan plumes may be much smaller. It has been suggested that if plumes exist, they are episodic and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions.

Europa receives thermal energy from tidal heating, which occurs through the tidal friction and tidal flexing processes caused by tidal acceleration. Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed. Their results indicate that most of the heat generated by the ice actually comes from the ice's crystalline structure (lattice) as a result of deformation, and not friction between the ice grains. The greater the deformation of the ice sheet, the more heat is generated.

Surface environment

Europa is one of the smoothest objects in our solar system. Unlike many moons, it lacks large mountains and craters. Instead, it has dark streaks that crisscross its surface. These streaks are called lineae, and they show where the moon’s icy crust has cracked and shifted.

Europa also has unusual rounded features called lenticulae, which look like freckles. Some of these are domes, while others are pits or dark spots. Scientists think these formed when warmer ice pushed up through the colder surface ice.

Europa’s surface gets very little oxygen, and the amount may be lower than we once thought. The moon has a very thin atmosphere made mostly of oxygen, created when sunlight and particles from Jupiter break down water vapor on the surface. This oxygen does not stay long but keeps getting made again.

Exploration

Scientists have long been interested in Europa because they think it might have water. In the 1950s, observations showed that Europa is made mostly of ice. The first spacecraft to pass by Europa were Pioneer 10 and Pioneer 11 in the 1970s. Later, the Voyager probes sent back better pictures, hinting that Europa might hide a warm ocean under its icy surface.

The Galileo space probe studied Europa closely from 1995 to 2003, giving us our best looks yet. More recently, the Juno spacecraft flew by Europa in 2022. In 2024, NASA launched the Europa Clipper mission to orbit Jupiter and fly by Europa many times, learning more about its surface and possible ocean below.

Future missions

People wonder if Europa could have life, so scientists plan more trips there. Future missions might look for signs of life in Europa’s hidden oceans. These missions must be built to survive the strong radiation around Jupiter.

• Europa Lander is a new idea for a spacecraft that would land on Europa’s surface.
• Buoyant Rover for Under-Ice Exploration (BRUIE) is a robot designed to swim in Europa’s oceans to search for life.

Habitability

So far, there is no evidence that life exists on Europa, but the moon is one of the most likely places in our Solar System where life might be able to live. Scientists think life could exist in a warm ocean hidden under Europa's icy surface, similar to life found around deep-ocean vents on Earth. Even without volcanic activity, Europa might still create the right conditions for life because of processes that produce oxygen and hydrogen.

Europa's ocean could allow life to exist in many ways, such as around warm spots on the ocean floor, or even floating in the ocean. However, if Europa's water is too cold or too salty, it might be too difficult for most life to survive. Scientists are also studying whether Europa's surface contains important materials that could support life. While Europa may not be very active today, in the past it might have had conditions that could support life.