Europa is one the 67 moons of Jupiter that orbit the planet. It was discovered by Galileo Galilei in 1610.

Diameter: 3122 km
Mass: 4.80 x 1022 kg
Composition: Ice and rock
Orbit: 670,900 km from Jupiter

Europa is the sixth moon of Jupiter and is one of its four largest. This group of the four largest moons, collectively known as the Galilean satellites, also includes Ganymede, Io and Castillo. Europa is the smallest of these four. On discovering the moon, Galileo originally saw Europa and Io as a single entity, but further viewings revealed them to be separate light sources.

Galileo used Roman numerals to define the first four moons he found, with Europa referred to as Jupiter II. As more satellites were discovered, this system became complicated and was changed to a naming system previously proposed by Simon Marius. Marius claimed to have discovered the four satellites around the same time as Galileo. Europa, as with all Galilean satellites, is named after a Greek noblewoman. Europa was the daughter of the King of Tyre and became a lover of Zeus after being taken to Crete. Later, she became the Queen of Crete and had many children.

The composition of Europa is an iron core with a rocky mantle. Its surface is covered in smooth ice, with what is believed to be salty water lying under its surface. Europa’s distance from the sun has meant that the water has frozen over, giving it the smoothest surface of any object in the Solar System.

Covering the icy surface are dark markings which are also described as cracks. With an ocean believed to lie beneath the icy surface, it is widely surmised that the cracks have been caused by tidal movements below the shell of ice.

Europa orbits Jupiter in three and a half days. It is phase locked, meaning that the same side of the moon always faces Jupiter. The moon moves around Jupiter in an oval-shaped orbit rather than a circular one. As a result, the closer the satellite gets to Jupiter, the higher the tide. This movement below the surface is thought to create the marks and cracks that are evident on the surface.

In the 1970s, Pioneer 10 and Pioneer 11 went on missions to Jupiter and took photographs of the moons. These were rather dark and blurry. The Voyager spacecraft, later that decade, were able to take more pictures of Europa’s surface which clearly showed an icy surface. It was the Galileo mission which was sent to orbit Jupiter that had a chance to study all four of the Galilean satellites in depth. Many flybys of Europa took place between 1995 and 2003. The last time images of Europa were taken was in 2005, when New Horizons was en route to Pluto.

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