Europa (astronomy) - astronomy.

Europa (astronomy) - astronomy. Europa (astronomy), large moon of the planet Jupiter that scientists suspect may have an ocean under its icy surface. It orbits Jupiter at an average distance of 671,000 km (416,900 mi), completing an orbit once every 3.55 Earth days. Europa rotates once in exactly the same amount of time that it takes to complete one orbit around Jupiter and therefore keeps a single face toward the planet at all times. The moon's slightly elliptical (oval-shaped) orbit parallels Jupiter's equator. Europa is slightly smaller than Earth's moon, with a radius of 1,565 km (972 mi). Like Earth's moon, it ranks among the ten largest moons in the solar system. Europa is made up mostly of silicate rock, rock containing compounds of silicon and oxygen. The surface of Europa is covered by water ice. About 5 km (about 3 mi) beneath this ice, there may be a 50-km-deep (30-mi-deep) ocean of water or a 100-km-deep (60-mi-deep) ocean of slushy ice. Gravitational forces keep Europa's interior warm. The moon's orbit is not a completely smooth ellipse because Europa is pulled slightly by the gravity of the nearby moons Ganymede and Io. As a result, Europa rocks as it orbits Jupiter and is squeezed and stretched slightly by the competing pulls of Jupiter, Ganymede, and Io. This flexing causes friction inside the moon, producing heat that keeps the subsurface ice slushy or liquid. Like Earth, Europa has a geologically young surface, less than 100 million years old. Most craters have been erased, perhaps by water from beneath the ice. Europa's only distinctive features are orange lines that cover the surface, concentrated near the equator. They appear to be cracks in sheet ice that are full to overflowing with orange material from inside the moon, forming ridges. Pictures taken by the Galileo orbiter indicate that geysers erupt along the cracks, supporting the theory that Europa has an ocean of water under its ice. Further observations taken by Galileo in 2000 lent strength to this theory. The spacecraft's magnetometer recorded regular changes in the direction of Europa's magnetic field that correspond to fluctuations in Jupiter's magnetic field at Europa's position. These changes suggest the presence of an underlying material that conducts electricity, such as a salty ocean. Europa has an oxygen atmosphere much too thin to breathe. Scientists theorize that the oxygen is produced by sunlight or energetic particles trapped in Jupiter's magnetic field that break water down into its component hydrogen and oxygen. The hydrogen is too lightweight to be retained by Europa's gravity and escapes into space, leaving the oxygen behind. Europa was discovered independently in 1610 by Italian astronomer Galileo and German astronomer Simon Marius. Europa and three other moons of Jupiter found in 1610 are called the Galilean moons. Marius named Europa and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Europa was a princess abducted by Zeus. Zeus and Europa were the parents of Minos, ruler of Crete. The orange lines on Europa's surface are named for characters in the Greek myth about Europa. Most of what is known about the structure and surface of Europa has come from information provided by spacecraft studying Jupiter. Voyager 1 and 2 passed Europa in 1979, mapping only a fraction of the moon's surface. The Galileo orbiter made its first flyby of Europa in December 1996 and returned several times before the end of its mission in 2003. Scientists at the National Aeronautics and Space Administration (NASA) hope to design a mission to search for life under Europa's ice crust. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.