Ganymede (astronomy) - astronomy.

Ganymede (astronomy) - astronomy. Ganymede (astronomy), satellite of the planet Jupiter and the largest moon in the solar system. With a radius of 2,634 km (1,637 mi), Ganymede is even larger than the planets Pluto and Mercury. The moon is only half as massive as Mercury, however, because about half of Ganymede's composition is frozen water, which has a lower density than Mercury's rock and iron. Ganymede orbits Jupiter at an average distance of 1.07 million km (665,000 mi), completing an orbit once every 7.155 Earth days. The moon is tidally locked, meaning that it rotates only once per orbit and keeps the same face toward Jupiter at all times. Ganymede's nearly circular orbit parallels Jupiter's equator. Ganymede's solid ice crust is thought to be about 75 km (about 45 mi) thick. Some scientists believe that the moon's mantle, the layer that underlies the crust, may contain liquid water or ice slush. The moon's surface is a combination of old, heavily cratered dark areas and younger light areas. Unlike craters on rocky worlds, such as Earth's moon, Ganymede's craters have flat floors and sagging walls because of slowly flowing ice that is smoothing the moon's surface. The largest cratered area is Galileo Regio. This young terrain is striped by parallel grooves and ridges called sulci. Scientists theorize that the sulci date from a time when the gravity of the large, neighboring moon Callisto pulled Ganymede into a slightly elliptical (oval-shaped) orbit. The combination of Jupiter's gravity and Callisto's gravity squeezed and stretched Ganymede slightly. Rocks inside the moon rubbed together, producing heat and softening the crust. This softening caused large sections of ice to slide and bump into each other, rumpling the crust into sulci. Ganymede's orbit is now more circular, so its crust is colder and more solid. Ganymede probably has an oxygen atmosphere much too thin to breathe. The oxygen may be produced as sunlight or charged particles trapped in Jupiter's magnetic field break water into oxygen and hydrogen. In 1995 the Hubble Space Telescope found evidence of ozone, a molecule made up of oxygen, around Ganymede. Much of scientists' knowledge of Ganymede comes from spacecraft that have flown past the moon. In 1979 Voyager 1 and 2 passed close enough to Ganymede to produce detailed images of the moon. The Galileo spacecraft made its first flyby of Ganymede in June 1996 and came within 260 km (162 mi) of the moon in September 1996. Images from Galileo show a string of 13 craters that astronomers believe was created by the impact of a breaking comet. Images from Galileo also suggest that volcanoes on Ganymede spewed hot water in the past, which melted down into the moon's icy surface, creating channels and valleys. Astronomers hypothesize that the source of this hot water may have been a global liquid ocean located under the moon's icy surface. Galileo also detected a magnetic field generated by Ganymede that is stronger than any other moon's magnetic field yet discovered. Ganymede was discovered independently by Italian astronomer Galileo and German astronomer Simon Marius. Ganymede and the other three large moons of Jupiter discovered in 1610--Io, Europa, and Callisto--are collectively known as the Galileans. Marius named Ganymede and the other Galileans for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Ganymede was a Trojan boy whom Zeus made cupbearer of the gods. Ganymede's sulci are named for places on Earth that are associated with myths. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.