S

Fig. 6.36. The Sippar Sulcus area on Ganymede contains curvilinear and arcuate scarps or cliffs. These features appear to be depressions that might be sources for water ice volcanism thought to form the bright grooved terrain on Ganymede. This structure is the best candidate seen for an icy volcanic lava flow on Ganymede. The morphology of this structure suggests the possibility of volcanic eruptions creating a channel and flow, and cutting down into the surface. (Credit: NASA/JPL/Brown University)

Fig. 6.36. The Sippar Sulcus area on Ganymede contains curvilinear and arcuate scarps or cliffs. These features appear to be depressions that might be sources for water ice volcanism thought to form the bright grooved terrain on Ganymede. This structure is the best candidate seen for an icy volcanic lava flow on Ganymede. The morphology of this structure suggests the possibility of volcanic eruptions creating a channel and flow, and cutting down into the surface. (Credit: NASA/JPL/Brown University)

Fig. 6.37. This cut-out model represents the probable internal structure of Ganymede. Ganymede's surface is rich in water ice. The Galileo spacecraft confirmed that the moon is highly differentiated, with a rock and iron core overlain by a deep layer of warm soft ice, capped by a thin cold rigid ice crust. Data suggests that a dense metallic iron core exists at the center of the rock core. (Credit: NASA/JPL/Caltech)

Fig. 6.37. This cut-out model represents the probable internal structure of Ganymede. Ganymede's surface is rich in water ice. The Galileo spacecraft confirmed that the moon is highly differentiated, with a rock and iron core overlain by a deep layer of warm soft ice, capped by a thin cold rigid ice crust. Data suggests that a dense metallic iron core exists at the center of the rock core. (Credit: NASA/JPL/Caltech)

[437]. According to John Anderson of JPL, 'the Galileo data showed clearly that Ganymede was differentiated into a core and a mantle, that there was a 800 km thick layer of warm ice beneath a warped and faulted ice crust, an equally thick mantle of rock, and an iron core.' It was not clear whether the metallic core was pure iron or a mixture of iron and iron sulfide [438].

There is indirect geological and geophysical evidence that Ganymede may posses a subsurface salty liquid water ocean [439]. Ganymede possesses an intrinsic magnetic field and the most likely source is dynamo action in a liquid Fe-FeS core. It has therefore been concluded that Ganymede's interior should have an iron-rich core surrounded by a silicate rock mantle and by an outer shell of ice. The ice shell is suggested to be ~800 km thick and the core may have a radius of between 400 and 1,300 km. Interpretations of magnetic data from Galileo passes of Ganymede have suggested the presence of a conducting layer at a depth between 170 and 460 km in which a magnetic field is being induced. This suggests that Ganymede, E

just like Europa and Callisto, may have a subsurface ocean. However, it must be £

noted that the magnetic data can also be fitted with models that do not require c ¡X

a subsurface ocean [440]. The melting temperature of ice will be significantly o ^

reduced by small amounts of salts and/or incorporated volatiles such as methane *> ®

and ammonia. If these elements are present, they could contribute to the liquidity ,0

of the ice, further enhancing the chance of Ganymede currently having a liquid ^ ®

6.1.4 Callisto

Being the Galilean moon furthest from Jupiter, Callisto completes an orbit about its parent in 16.689 days [442]. It has a surface temperature of 142-157 K [443], a diameter of 4,820.6 ± 3.0 km with no detectable deviation from sphericity, and a density of (1,834.4 ± 3.4) kg m-3 [444]. It is the second largest Galilean moon. Callisto has the lowest surface brightness of the four Galilean moons, although it is brighter than Earth's Moon (Fig. 6.38).

Spectral data clearly show that the surface of Callisto is covered by water ice. Its surface is evenly distributed dark, heavily cratered terrain, lacking volcanic or tectonic landforms. Callisto's low albedo also indicates the presence of non-icy components on its surface [445]. Of interest, small craters less than 3 km in diameter occur less frequently on Callisto than on Ganymede (Fig. 6.39). This suggests the presence of an erosional process at scales smaller than 1 km such as sublimation of surface ices, CO2 degassing, or the existence of more volatile ammonia ice [446]. The surface of Callisto is the oldest among the Galilean satellites, and its largest craters are hundreds of kilometers in diameter. Its main features were probably formed during heavy bombardment [447].

Magnetic field data from the Galileo spacecraft suggests that Callisto has a magnetic field that may be induced by interaction with Jupiter's magnetic field. An internal conducting layer, such as a subsurface ocean of at least 10 km thickness with the salinity of terrestrial saltwater, could explain this. If it exists, the subsurface ocean is probably at great depth since there is no outward sign of any internal activity affecting the surface [448]. The observed magnetic field perturbations are approximately those expected for moons responding as perfectly conducting spheres. Such a response requires a globally distributed highly conducting medium located close to the surface of the moon. This result has been interpreted as support for the

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