More Facts about Europa: Facts About Jupiter's Icy Moon and Its Ocean

It is possible Galileo actually observed Europa a day earlier, on Jan. 7, 1610. However, because he was using a low-powered telescope, he couldn't differentiate Europa from Io, another of Jupiter's moons. It wasn't until later that Galileo realized they were two separate bodies.

The discovery not only had astronomical, but also religious implications. At the time, the Catholic Church supported the idea that everything orbited the Earth, an idea supported in ancient times by Aristotle and Ptolemy. Galileo's observations of Jupiter's moons — as well as noticing that Venus went through "phases" similar to our own moon — gave compelling evidence that not everything revolved around the Earth.As telescopic observations improved, however, a new view of the universe emerged. The moons and the planets were not unchanging and perfect; for example, mountains seen on the moon showed that geological processes happened elsewhere. Also, all planets revolved around the sun. Over time, moons around other planets were discovered — and additional moons found around Jupiter.Marius, the other "discoverer," first proposed that the four moons be given their current names, from Greek mythology. But it wasn't until the 19th century that the moons were officially given the so-called Galilean names we know them by today. All of Jupiter's moons are named for the god's lovers (or victims, depending on your point of view). In Greek mythology, Europa was abducted by Zeus (the counterpart of the Roman god Jupiter), who had taken the form of a spotless white bull to seduce her.She decorated the "bull" with flowers and rode on its back to Crete. Once in Crete, Zeus transformed back to his original form and seduced her. Europa was the queen of Crete and bore Zeus many children.

Discovery
Galileo Galilei discovered Europa on Jan. 8, 1610. It is possible that German astronomer Simon Marius (1573-1624) also discovered the moon at the same time. However, he did not publish his observations, so it is Galileo who is most often credited with the discovery. For this reason, Europa and Jupiter's other three largest moons are often called the Galilean moons. Galileo, however, called the moons the Medicean planets in honor of the Medici family. [Photos: Europa, Jupiter's Mysterious Icy Moon]


 * JUpiter Icy Moons Explorer (JUICE) (proposed for 2020s). Will look for molecules, such as organic molecules, that are associated with life-giving processes. (Organics are common in the solar system, but the molecules themselves do not always indicate life.)


 * Europa Clipper (proposed for 2020s). Will fly by Europa dozens of times. One of its major goals is to seek out evidence of the apparent plumes that Hubble researchers spotted several times.


 * Galileo (orbited Jupiter between 1995-2003). Its most famous discovery at Europa was finding strong evidence of an ocean beneath the icy crust at the moon's surface.


 * Pioneer 10 (1973 flyby of Jupiter system). This passed too far away from Europa to get a detailed picture, but the mission did note some variations in albedo (brightness) on the moon's surface.
 * Pioneer 11 (1974 flyby of Jupiter system). The spacecraft made a flyby of Europa at nearly 375,000 miles (600,000 km) away, only allowing it to see some variation on the surface.
 * Voyager 1 (1979 flyby of Jupiter system). Made a distant flyby of Europa, and also yielded insights about how the gravity of one moon in Jupiter's system influences the gravity of others. For example, Io's volcanism was traced in part to interaction of Io with the moons, as well as with massive Jupiter.Voyager 2 (1979 flyby of Jupiter system). One of its major discoveries was confirming brown stripes across the surface of Europa, suggesting cracks in the icy surface.