Discover

'Ganymede' ''(gan'-ə-meed,'' ; Greek ''Γανυμήδης)'' is Jupiter's largest natural satellite, and the largest natural satellite in the Solar System; it is larger in diameter than Mercury but has only about half its mass. Ganymede is one of four planetary satellites of Jupiter that could be seen with the naked eye under favorable conditions, if not for the brighter light of nearby Jupiter. Other than possibly the Galilean Satellites, the only planetary satellite visible with the naked eye is our own Moon.
Ganymede's discovery is generally credited to Galileo Galilei who documented its existence in 1610.^[2] The name Ganymede was suggested soon after by Simon Marius, for the cup-bearer of the Greek gods, beloved of Zeus (see Ganymede).^[3] This name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation (a system introduced by Galileo) as 'Jupiter III' or as the "third satellite of Jupiter". Ganymede is the only Galilean moon of Jupiter named after a male figure.

Contents

Physical characteristics

Internal structure

Surface features

Atmosphere

Magnetosphere

Possibility of Life

Ganymede in fiction

See also

References

External links

Physical characteristics

Internal structure

Ganymede is composed of silicate rock and water ice, with an ice crust floating over a warmer ice mantle that may contain a layer of liquid water. Indications from the ''Galileo'' orbiter data suggest that Ganymede is differentiated into a three-layer structure: a small partially molten iron or iron/sulfur core surrounded by a rocky silicate mantle with an icy shell on top. This metallic core suggests a greater degree of heating at some time in Ganymede's past than had previously been proposed. In fact, Ganymede may be similar in its internal structure to Io.

Surface features

The Ganymedean surface is a mix of two types of terrain: very old, highly cratered dark regions and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges. Their origin is clearly of a tectonic nature, probably formed by the extension, stretching, and faulting of the icy crust. Analogous tectonic features form the ridges and valleys in the Basin and Range province of the southwestern United States. Features reminiscent of old lava flows have also been observed. Similar ridge and groove terrain is seen on Enceladus, Miranda and Ariel. The dark regions are similar to the surface of Callisto.
Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the bright grooved terrain (but how much younger is uncertain). Craters both overlay and are cross-cut by the groove systems indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.^[4] Unlike on the Moon, however, Ganymedean craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests.
The largest feature on Ganymede is a dark plain named Galileo Regio, as well as a series of concentric grooves, or furrows, that are remnants of an ancient impact crater long since obscured by subsequent geological activity.

Atmosphere

In 1972, a team of Indian, British and American astronomers working at Indonesia's Bosscha Observatory in Lembang detected a thin atmosphere around Ganymede during an occultation when Jupiter (and Ganymede) passed in front of a star.^[5] Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has since been found by the Hubble Space Telescope.^[6] Note that this is not necessarily evidence of life: it is thought that the oxygen is produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation and then the hydrogen is lost due to its low atomic mass.

Magnetosphere

The ''Galileo'' orbiter's first flyby of Ganymede discovered that Ganymede has its own magnetic field, embedded inside Jupiter's huge field. Ganymede is the only moon known to have a magnetosphere. Ganymede's intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior, likely originating in its metallic core. Ganymede also has an induced magnetic field component, indicating that the satellite contains a subsurface layer that acts as a conductor. It is thought that this conductive material is a layer of liquid water containing salt, located at about 150 km depth and sandwiched between layers of different density forms of ice.

Possibility of Life

It has been suggested that Ganymede may be amenable to life. However, scientists say that the chances of life existing on Europa are much higher than on Ganymede, because Europa's ocean is closer to the surface than Ganymede's.^[8]
While temperatures are extremely low, and there is very little atmosphere, it does have its own magnetic field that might protect life from harmful radiation, and it lies in a relatively quiet radio zone around Jupiter unlike Europa.

Ganymede in fiction

Main articles: Jupiter's moons in fiction

See also

★ List of craters on Ganymede

★ Lunar and Planetary Institute

★ Colonization of Ganymede

References

1. Using the mean radius
2. Galilei, G.; ''Sidereus Nuncius'' (March 13, 1610)
3. Marius, S.; (1614); ''Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici'', where he attributes the suggestion to Johannes Kepler
4. Ganymede
5. R. W. Carlson et al., ''An Atmosphere on Ganymede from Its Occultation of SAO 186800 on 7 June 1972'',Science 5 October 1973:
Vol. 182. no. 4107, pp. 53 - 55
6. Ganymede page
7. http://spaceflightnow.com/news/n0012/29ganyflyby/
8. Bridges, A.; ''Ocean Lurks Deep in Ganymede, Galileo Finds'', Space.com (18 December 2000)

External links

★ Ganymede Profile by NASA's Solar System Exploration

★ Ganymede page on ''The Nine8 Planets''

★ Jupiter's Moon Ganymede on ''Views of the Solar System''

★ Ganymede Crater Database Lunar and Planetary Institute
''... | Europa | 'Ganymede' | Callisto | ...''