In 1610, when Galileo Galilei turned his telescope toward Jupiter, he saw four tiny points of light accompanying the planet. These are the Galilean satellites, the four largest of the 67 known moons of Jupiter. The smallest of the Galileans is Europa.
Under its hard crust of ice, Europa is thought to host an ocean of liquid water, up to 106 miles (170 kilometers) deep. Tides caused by nearby Jupiter could be warming Europa's ocean, providing energy that could be driving an ecosystem of alien life forms.
Europa receives about 5.4 sieverts of radiation per day, a fatal dose. Someone at sea level on Earth receives 0.0014 sievert per day.
Europa is 1,940 miles (3,122 km) in diameter. Its surface temperature is minus 260 degrees F (minus 160 degrees C). Its gravity is 0.134 of Earth’s. The moon has a very thin atmosphere composed of molecular oxygen.
Europa has more water than the entire Earth. Europa's water would form a sphere 1,090 miles (1,754 km) in diameter. Earth’s water-sphere would be 860 miles (1,384 km) across. Europa's ocean is 10 times deeper than the seas of Earth.
Like many moons in the solar system, Europa is tidally locked to its parent: one side permanently faces Jupiter. As Europa orbits, its trailing hemisphere develops deep cracks probably caused by tidal stresses. These features are called lineae (Latin for "lines").
Although we have no current evidence of existing life in the sea of Europa, some scientists estimate that the water contains enough oxygen to support 3 million tons of fishlike creatures. Other scientists don’t expect life there to be bigger than microbes.
Europa has been explored in many science fiction stories and films, including "Space Odyssey: Voyage To The Planets" and "Europa Report." In reality, manned missions to Jupiter and its moons are unlikely due to the enormous distance and the high radiation environment around Jupiter.
The next robotic probe of Europa will be the European Space Agency's Jupiter Icy Moon Explorer (JUICE), planned for launch in 2022. Further in the future, a nuclear-powered submersible could descend below the frozen crust to explore Europa's ocean.