<Tr> <Th> Surface pressure </Th> <Td> 0.1 μPa (10 bar) </Td> </Tr> <P> Europa / jʊəˈroʊpə / (listen) yoor - OH - pə (Jupiter II) is the smallest of the four Galilean moons orbiting Jupiter, and the sixth - closest to the planet . It is also the sixth - largest moon in the Solar System . Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the legendary mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter). </P> <P> Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water - ice crust and probably an iron--nickel core . It has a tenuous atmosphere composed primarily of oxygen . Its surface is striated by cracks and streaks, whereas craters are relatively rare . In addition to Earth - bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s . </P> <P> Europa has the smoothest surface of any known solid object in the Solar System . The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably harbor extraterrestrial life . The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below . Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor . This may be important in determining if Europa could be habitable . In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers . In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter between 1995 to 2003 . Such plume activity could help researchers in a search for life from the subsurface European ocean without having to land on the moon . </P>

What is the atmosphere of europa made of