Ganymede, also called Jupiter III, is the largest of Jupiter’s satellites and of all the satellites in the solar system. According to astronomer and historian Xi Zezong, Ganymede was first discovered by ancient astronomer Gan De, and 2,000 years later was "re-discovered" Galileo Galilei and Simon Marius.
Ganymede has a diameter of about 5,270 km (3,275 miles), which makes it larger than the planet Mercury. It orbits Jupiter at a distance of 1,070,000 km (665,000 miles). Ganymede’s relatively low density of 1.93 grams per cubic cm indicates that its composition is roughly half rock and half water ice by mass. Spacecraft investigations of its gravity field reveal that the interior consists of a dense, iron-rich core with a radius of 1,500 km (930 miles) surrounded by a rocky lower mantle, which is wrapped with a layer of ice some 700 km (430 miles) thick. The iron core produces a magnetic field that is 1 percent as strong as Earth’s. Above the ice layer is likely a subsurface ocean possibly 100 km (60 miles) deep. The top layer of the satellite is an icy crust about 150 km (90 miles) thick.
Ganymede was observed at close range in 1979 by the Voyager 1 and 2 spacecraft and by the Galileo orbiter beginning in the mid-1990s. Previously, in addition to water ice, spectroscopic observations of Ganymede from Earth had detected molecular oxygen and ozone trapped in the ice. Spectra obtained by Galileo’s instruments showed evidence for hydrated minerals resembling clays; solid carbon dioxide; traces of hydrogen peroxide probably produced from the ice by photochemical reactions; sulfur compounds, some of which may have come from Jupiter’s volcanically active satellite Io; and organic material that may have been deposited by impacting comets. The polar regions are lightly frosted with fresh ice and are crowned by flickering auroras produced by subatomic particles following the satellite’s magnetic field lines. (Ganymede is the only solar system satellite with a magnetic field.)
The surface comprises two principal types of terrain, one dark and one bright. The dark terrain is present in broad, roughly polygonal regions that are separated by bands of bright terrain. Both terrains have impact craters. The density of craters is higher in the dark terrain, indicating that it is the older of the two types. Craters of a given diameter on Ganymede are generally much shallower than comparably sized craters on rocky bodies like the Moon or Mercury, suggesting that they have become partially filled in through cold viscous flow of the icy crust.
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- Kivelson, M.G.; Warnecke, J.; Bennett, L. et.al. (1998). "Ganymede’s magnetosphere: magnetometer overview" (pdf). J.of Geophys. Res. 103 (E9): 19,963-19,972. doi:10.1029/98JE00227. http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf.
- Kivelson, M.G.; Khurana, K.K.; Coroniti, F.V. et.al. (2002). "The Permanent and Inductive Magnetic Moments of Ganymede" (pdf). Icarus 157: 507-522. Template:Citation error. http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS1572507.pdf.
- Zezong, Xi, "The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo", Chinese Physics 2 (3) (1982): 664–67.
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