The Icy Satellites

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 The Icy Satellites Saturn's SatellitesGanymede

Ganymede is Jupiter's third Galilean satellite and the largest in the solar system. It's surface is similar is some ways to an other Galilean satellite, Europa. Ganymede was geologically active in the past. It was photographed by the Voyager probes, Galileo and more recently by New Horizons.
Distance to Jupiter1 070 000 km
Period of revolution7.15 in earth days
Diameter5268,2 km
Mass (Earth=1)0.0247757
Density (water =1)1.936
CompositionIce water, rocks and some metals
Temperature on the surface-117 °C
The surface of Ganymede shows two terrains of different ages (image 2): one part which is quite recent and not unlike the rifts on Europa and the other, darker and older (many millions of years) where lots of craters can be seen. These darker parts are parallel to the the border between the two types of terrain. The more recent is smoother, but on a smaller sale, fractures do appear (image 3). These fractures are consequences of the tectonics on Ganymede.

The greater craters are visible as shiny star shaped spots (image 1). These are ice water, risen from below to the surface because of impacts.
Internal structure and composition:
Following the measurements of the Gravitational field around Ganymede made by the Galileo probe, this satellite was seen to be the most differentiated in the solar system. The measurements also showed the existence of a magnetic field induced by the satellite.

Though Ganymede is almost the same size as Mercury, its mass is half that of the planet. The explanation lies in its composition. This satellite is half rock and half ice water with a little metal. These materials are disposed in a relatively separated fashion so that the ice forms the external crust, about 800 km thick, covering a coat of silicates and finally a core which could have a radius of up to half that of the satellite itself. The idea of an internal liquid ocean is not to be rejected either.

We still do not know for sure the origins of the magnetic field intrinsic to Ganymede. On the one hand, the present rate of cooling of the core should not allow for convection currents(and therefore a dynamo effect) to be established. On the other hand, a changing field coming from the silicate coat needs an initial magnetisation by a field created by the core, as long as the core's cooling rate was enough in the past.

Figures 1 to 3 (credit: JPL/NASA)
1) image from New Horizons
2) The different terrains on Ganymede
3) more detail on the young terrain

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