Europa (moon)

Europa (moon)

Europa is one of the four largest moons orbiting the planet Jupiter. It was discovered by two famous astronomers, Simon Marius and Galileo Galilei. Though small compared to Earth's Moon, Europa has caught scientists' attention because it might hide something special: a warm ocean under 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 think 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 an important 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. 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 called "Jupiter II" because it was the second moon found around Jupiter. Later, more moons were found 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 about 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, so one side always faces Jupiter.

The small wobble in Europa's orbit, caused by the pull of other moons, gently stretches and squeezes the moon. This movement may help keep a warm, liquid ocean beneath its icy surface by creating heat inside Europa. Scientists think Europa might have tilted on its axis in the past, which could explain some of the cracks on its surface. These cracks show the strong tides caused by Jupiter's gravity on Europa's ocean.

Bulk properties

Europa is a little smaller than the Earth's moon. It is about 3,100 kilometres (1,900 mi) across, making it the sixth-largest moon in the Solar System. It is made mostly of silicate rock, like terrestrial planets.

Scientists think Europa has a thick layer of water – some frozen on top and some as a liquid ocean below the ice. Data from the Galileo spacecraft shows Europa has a weak magnetic field, which suggests there is a salty ocean under the surface. Europa might also have a metallic iron core.

Most scientists agree that Europa has a liquid ocean under its icy surface. Heat from movements caused by Jupiter keeps this ocean warm. Europa's surface is very cold, about 110 K (−160 °C; −260 °F) at the equator and even colder at the poles. This keeps the ice as hard as rock.

The Galileo spacecraft also found Europa has a weak magnetic moment. This needs a layer of conductive material inside, most likely a big ocean of salty water.

Since the Voyager spacecraft flew by Europa in 1979, scientists have tried to learn what makes the reddish-brown colour on its surface. They think it might be salts like magnesium sulfate left by water coming up from below. Another idea is sulfuric acid hydrate.

The ocean inside Europa may contain sodium chloride. This was found in images from the HST. The ocean also has carbon, seen on the surface as carbon dioxide in Tara Regio.

In 2012, the Hubble Space Telescope took a picture that looked like a burst of water vapour near Europa's south pole. It might be as tall as 200 km (120 mi), but later studies think these bursts are usually smaller. These bursts may happen when Europa is farthest from Jupiter, due to tidal force.

Europa gets heat from movements caused by Jupiter. In 2016, studies showed these movements can create more heat in Europa's ice than previously thought. Most of this heat comes from how the ice changes shape, not from friction between ice pieces. The more the ice changes shape, the more heat it makes.

Surface environment

Europa is one of the smoothest objects in our solar system. It does not have large mountains or craters like many moons. Instead, it has dark streaks that cross its surface. These streaks are called lineae. They show where the moon’s icy crust has cracked and moved.

Europa also has unusual rounded features called lenticulae. They look like freckles. Some 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 has very little oxygen. The moon has a very thin atmosphere made mostly of oxygen. This oxygen is created when sunlight and particles from Jupiter break down water vapor on the surface. The oxygen does not stay long but is made again and again.

Exploration

Scientists are interested in Europa because they think it might have water. In the 1950s, they learned 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, suggesting that Europa might have 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

There is no proof that life exists on Europa, but it is one of the best places in our Solar System where life might live. Scientists think life could exist in a warm ocean hidden under Europa's icy surface, similar to life near deep-ocean vents on Earth. Even without volcanic activity, Europa might still create the right conditions for life because of processes that make oxygen and hydrogen.

Europa's ocean could help life in many ways, such as around warm spots on the ocean floor or floating in the water. But if Europa's water is too cold or too salty, it might be hard for most life to survive. Scientists are also checking if Europa's surface has important materials that could help life. While Europa may not be very active now, in the past it might have had conditions that could support life.