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Definition: Europa from Philip's Encyclopedia

Smallest of Jupiter's Galilean satellites, with a diameter of 3138km (1950mi). Mainly rock, Europa's smooth water-ice crust is criss-crossed by a network of light and dark linear markings. A form of ice tectonics might be operating on the planet, since there are very few craters.

Summary Article: Europa
From Collins Dictionary of Astronomy

The smallest of the four Galilean satellites of Jupiter and the only one inferior in size and mass to the Moon. It has a diameter of 3138 km and its density is 2.97 g cm–3. Its albedo is 0.67. Studied by the two Pioneer spacecraft 10 and 11, by Voyager 2, and in particular depth by the Galileo spacecraft, Europa has been found to be a smooth-surfaced world with no mountains and very few craters – none more than about 20 km in diameter. Its relatively young icy crust (at least 100 km thick) overlies what is thought to be a metallic core and is crisscrossed by a series of streaks and cracks. Most of the cracks appear to be filled with dark-colored material, some with a light-colored substance. The gravitational influence on Europa not only of Jupiter but also of the satellites Io and Ganymede serve to squeeze the satellite, resulting in tidal flexing, which is believed to heat Europa's interior. Tidal flexing may have melted some of the ice crust to form a substantial ocean of liquid water between it and the core. Water from this ocean may seep through cracks produced in the crust and may refreeze at the surface, thereby renewing it or forming icy ridges. It has been suggested that this ocean, if it exists, may provide a possible suitable environment for extraterrestrial microorganisms. The Hubble Space Telescope has revealed that Europa has a vanishingly thin oxygen atmosphere, due not to biological action but to the interaction between charged particles from the Sun and water molecules in the satellite's icy crust. The charged particles break up some of the water molecules, liberating hydrogen, which dissipates into space, and oxygen. See also Jupiter's satellites; Table 2, backmatter.

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