Europa is one of the so-called Galilean moons of Jupiter, along with Io, Ganymede and Callisto. Astronomer Galileo Galilei gets the credit for discovering these moons, among the largest in the solar system. Europa is the smallest of the four but it is one of the more intriguing satellites.
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. The icy surface also makes the moon one of the most reflective in the solar system.
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]
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.
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 Europa was seduced by Zeus. Europa was 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.
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.
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 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 F (minus 220 C).
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 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 think 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 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 saltwater ocean. And, because the moon is so far from the sun, this ocean is frozen across the surface of the moon.
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 thought 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.
Obtaining samples of the ocean may not require drilling through the icy crust. In 2013, the Hubble Space Telescope identified geysers of water vapor spewing from the moon's south pole. The plumes subsequently disappeared, leading scientists to wonder if the features were a cyclical event.
In 2014, scientists found that Europa may host a form of plate tectonics. Previously, Earth was the only known body in the solar system with a dynamic crust, which is considered helpful in the evolution of life on the planet.
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.
Europa: Where life may evolve?
The presence of water beneath the moon's frozen crust makes scientists rank it as one of the best spots in the solar system with the potential for life to evolve.
"The story of life on Earth may have begun in our oceans, and that's because — of course — if we've learned anything about life on Earth, it's that where you find the liquid water, you generally find life," Kevin Hand, an astrobiologist at the NASA Jet Propulsion Laboratory, said in a video about Europa.
"Hidden beneath Europa's icy surface is perhaps the most promising place in our solar system beyond Earth to look for present-day environments that are suitable for life," NASA officials wrote in a statement. "The Galileo mission found strong evidence that a subsurface ocean of salty water is in contact with a rocky seafloor. The cycling of material between the ocean and ice shell could potentially provide sources of chemical energy that could sustain simple life forms."
The icy depths of the moons are thought to contain vents to the mantle much as oceans on Earth do. These vents could provide the necessary thermal environment to help life evolve.
If life exists on the moon, it may have gotten a kick from deposits from comets. Early in the life of the solar system, the icy bodies may have delivered organic material to the moon.
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]
In 2013, the U.S. National Research Council's Planetary Science Decadal Review issued its 10-year recommendation for NASA's planetary exploration program. Exploration of Europa was ranked as the highest-priority mission. Since then, NASA has been working toward a mission to Jupiter's icy moon.
Additional reporting by Nola Taylor Redd, Space.com Contributor