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

So far, more than 60 moons of various sizes have been discovered around the planet Jupiter. The four largest moons of Jupiter are Io, Ganymede, Callisto, and Europa. The surface of Europa is frozen, covered with a layer of ice, leading scientists to believe there is a very active ocean beneath the surface of this intriguing moon.

Facts about Europa

Age: Europa is estimated to be about 4.5 billion years old, about the same age of Jupiter.

Distance from the sun: On average, Europa's distance from the sun is about 485 million miles (or 780 million kilometers).

Distance from Jupiter: Europa is Jupiter's sixth satellite. Its orbital distance from Jupiter is 414,000 miles (670,900 km). It takes Europa three and a half days to orbit Jupiter. The same side of the Europa faces Jupiter at all times.

Size: Europa is 1,900 miles (3,100 km) in diameter, making it smaller than the Earth's moon, but larger than Pluto. It is the 15th largest body in the solar system, and the smallest of the Galilean moons.

Temperature: Europa's surface temperature at the equator never rises above minus 260 degrees Fahrenheit (minus 160 degrees Celsius). At the poles of the moon, the temperature never rises above minus 370 degrees Fahrenheit (minus 220 degrees Celsius).

The Discovery of Europa

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, 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]

Galileo Transcript
A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. Galileo made these sketches after discovering four moons orbiting Jupiter in January 1610. The moons, later named Io, Europa, Callisto and Ganymede, were the first discovered beyond Earth.
Credit: NASA

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 of the Galilean moons orbiting Jupiter helped scientists realize that the planets in our solar system, including Earth, revolved around the sun and not the Earth.

Europa's name

In Greek mythology, Europa was abducted by Zeus, 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—who is a counterpart of the Roman god Jupiter—then transformed back to his original form and was seduced by Zeus. Europa as the queen of Crete and bore Zeus many children.

For centuries, the four largest moons of Jupiter didn't have actual names. Instead, Galileo gave each moon a number, from one through four. Of the four large moons, Europa was believed to be the second closest to Jupiter, which is why Galileo also called it Jupiter II.

Voyager's Europa
This color image of the Jovian moon Europa was acquired by Voyager 2 during its close encounter on July 9, 1996.
Credit: NASA/JPL

In 1892, Amalthea was discovered. Amalthea was closer to Jupiter than any of the Galilean moons, so it was believed Europa was actually the third closest moon to Jupiter, making the name Jupiter II obsolete. Now, with the discovery of even more satellites closer to the planet, Europa is considered Jupiter's sixth satellite.

It was Simon Marius who first proposed that the four moons be given their current names. But it wasn't until the 19th century that the moons were officially given the so-called Galilean names we know them by today.

Exploration of Europa

The Galileo mission, launched by NASA in 1989, is responsible for much of the information we have on Jupiter and the bodies surrounding it. It took more than six years for the unmanned Galileo spacecraft to reach its destination. The craft stayed in orbit of Jupiter from December 8, 1995 until Sept. 21, 2003. [Best Jupiter Missions of All Time]

Characteristics of Europa

A prominent feature of Europa is its high degree of reflectivity. Europa's icy crust gives it an albedo—light reflectivity—of 0.64, one of the highest of all of the moons in the entire solar system.

Scientists estimate that Europa’s surface is about 20 million to 180 million years old, which makes it fairly young.

Observing pictures taken by the Galileo spacecraft, scientists believe Europa is made of silicate rock, and has an iron core and rocky mantle, much like the Earth does. Unlike the interior of Earth, however, the rocky interior of Europa is surrounded by a layer of ice that is approximately 62 miles (or 100 km) thick.

Experts also believe there is an ocean deep beneath the surface of the moon, and that it is possible this ocean contains some form of life. The possibility that there is extraterrestrial life on the Europa has sparked the imaginations of many, and is one of the reasons interest in Europa remains high. In fact, recent studies have given new life to the theory that Europa can support life.

The surface of the moon is covered by a salt water ocean. And, because the moon is so far from the sun, this ocean is frozen across the surface of the moon.

Europa - Galileo
These images show the trailing hemisphere of Jupiter's moon Europa taken by the Galileo spacecraft at a distance of about 677,000 km. The left image shows Europa in approximately true color and the right image shows Europa in enhanced color to bring out details. The bright feature towards the lower right of the disk is the 45 km diameter crater Pwyll.
Credit: NSSDC Photo Gallery

The surface of Europa is covered with cracks. Many believe these cracks are the result of tidal forces on the ocean beneath the surface. It's possible that, when Europa's orbit takes it close to Jupiter, the tide of the sea beneath the ice rises higher than normal. If this is so, the constant raising and lowering of the sea caused many of the cracks observed on the surface of the moon.

It is also believed that the ocean beneath the surface sometimes erupts through the surface (much like lava erupts from a volcano) and then freezes. Icebergs observed on the surface of the moon may support this theory.

Europa's Atmosphere

Europa has a tenuous oxygen atmosphere, likely the result of charged particles from the sun hitting water molecules on the moon's surface, breaking the molecules into oxygen and hydrogen atoms. While the hydrogen escapes the moon's surface, oxygen is left behind.

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