Jupiter: Moons: Ganymede
Ganymede is the largest satellite in our solar system. It is larger than Mercury and Pluto, and three-quarters the size of Mars. If Ganymede orbited the Sun instead of orbiting Jupiter, it would easily be classified as a planet.
Since Ganymede has a low density, it was originally estimated that the satellite is half water ice with a rocky core extending to half of the satellite's radius. However, the Galileo spacecraft found a magnetic field around Ganymede, which strongly indicates that the satellite has metallic core about 250 to 800 miles in. The mantle is composed of ice and silicates and a crust which is probably a thick layer of water ice. Ganymede's magnetic field is embedded inside Jupiter's massive magnetosphere.
Astronomers using the Hubble Space Telescope found evidence of thin oxygen atmosphere on Ganymede in 1996. The atmosphere is far too thin to support life as we know it.
In 2004, scientists discovered irregular lumps beneath the icy surface of Ganymede. The irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years. The findings have caused scientists to rethink what the interior of Ganymede might contain. The reported bulges reside in the interior, and there are no visible surface features associated with them. This tells scientists that the ice is probably strong enough, at least near the surface, to support these possible rock masses from sinking to the bottom of the ice for billions of years. But this anomaly could also be caused by piles of rock at the bottom of the ice.
Ganymede has three main layers. A sphere of metallic iron at the center (the core), a spherical shell of rock (mantle) surrounding the core, and a spherical shell of mostly ice surrounding the rock shell and the core. The ice shell on the outside is very thick, maybe 800 kilometers (497 miles) thick. The surface is the very top of the ice shell. Though it is mostly ice, the ice shell might contain some rock mixed in. Scientists believe there must be a fair amount of rock in the ice near the surface. Variations in this amount of rock may be the source of these possible rock formations.
Spacecraft images of Ganymede show the moon has a complex geological history. Ganymede's surface is a mixture of two types of terrain. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain which forms intricate patterns across Ganymede. The term sulcus, meaning a groove or burrow, is often used to describe the grooved features. This grooved terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 meters (2,000 feet) have been observed and the grooves run for thousands of kilometers across Ganymede's surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough and the dark, cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike Earth's Moon). The largest area on Ganymede is called Galileo Regio.
The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to slow and gradual adjustment to the soft icy surface. These large phantom craters are called palimpsests, a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing. Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede's craters - rays tend to be bright from craters in the grooved terrain and dark from the dark cratered terrain.
Discovery
Ganymede was discovered by Galileo Galilei on 7 January 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the Sun, instead of our solar system revolving around Earth.
How Ganymede Got its Name
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III and IV. Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800's that the names of the Galilean moons, Io, Europa, Ganymede and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
In mythology, Ganymede ("GAN uh meed") was a beautiful young boy who was carried to Olympus by Jupiter disguised as an eagle. Ganymede became the cupbearer of the Olympian gods.
Satellites in the Jovian system are named for Zeus/Jupiter's lovers and descendants. Names of outer satellites with a prograde orbit generally end with the letter a (although an o ending has been reserved for some unusual cases), and names of satellites with a retrograde orbit end with an e.
