Jupiter has four main moons, which were discovered by Galileo, as well as many smaller ones (47 Jovian moons are currently known). Europa, seen here photographed by Voyager 1, is the second closest of Jupiter's main moons. It has a very smooth surface with few craters and no volcanoes, but features distinct long markings, probably cracks or presure ridges in the water ice which covers the moon's surface. Scientists believe that it is posible that liquid water, heated by tidal energy, exists beneath Europa's icy surface, making it a posible home for extraterrestrial life. A tenuous oxygen atmosphere also exists. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Both pased Jupiter in 1979.
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