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Fig. 6.38. An image of Callisto by the Voyager 2 spacecraft. The surface of Callisto is the most heavily cratered of the Galilean moons. The bright areas are ejecta thrown out from relatively young impact craters. (Credit: NASA/JPL/Caltech)

presence of a salty subsurface ocean. A subsurface ocean with the salinity typical of Earth's oceans and a few kilometers thickness can easily produce the observed induction response. Perhaps the most spectacular result of these observations is that the magnetic evidence indicates the existence of an ocean on Callisto at this present epoch, not just during the recent past [449]! Modeling also supports the hypothesis that Callisto may have an internal liquid water ocean [450].

The surface of Callisto is very old and shows no evidence of extensive resurfacing. Unlike Ganymede, Callisto does not show areas of grooved terrain, probably due to its undifferentiated state and the lack of significant tidal stress from Jupiter [451]. The surface of Callisto is heavily cratered, more so than Ganymede, with craters and more craters everywhere we look (Fig. 6.40). Some scientists have characterized its surface as boring, especially compared to the other Galilean moons, and its craters show very little evidence of any kind of significant erosion that would be caused by tectonic activity or cryovolcansim.

Unlike the other Galilean moons, there is no convincing evidence of volcanism of any kind. This lack of erosion can possibly be attributed to the fact that Calliso is at a greater distance from Jupiter than the other three moons, and therefore does not suffer from tidal heating, nor from the orbital resonances that the other moons impose on each other [452]. The monotony of its surface is broken by a fair number of large, multiple-ringed, impact basins. This indicates that Callisto suffered from asteroid related bombardment [453] (Fig. 6.41). The largest and most outstanding feature on its surface is the Valhalla multiple-ringed impact basin. Valhalla is comprised of a smooth central plain ~600 km in diameter and a series of rings extending to a radius of ~2,000 km [454]!

Valhalla Crater Callisto

Fig. 6.39. Craters on Callisto as imaged by the Galileo spacecraft. Notice the dark, mobile blanket of dust-like material that seems to cover everything on Callisto. Some crater walls show movement of this material. While the moon has a significant number of large craters, it seems to lack a related number of small craters. While the dust-like material would fill in some small craters on the slopes of larger ones, it is not clear what process would erase the others. (Credit: NASA/JPL/Caltech)

Fig. 6.39. Craters on Callisto as imaged by the Galileo spacecraft. Notice the dark, mobile blanket of dust-like material that seems to cover everything on Callisto. Some crater walls show movement of this material. While the moon has a significant number of large craters, it seems to lack a related number of small craters. While the dust-like material would fill in some small craters on the slopes of larger ones, it is not clear what process would erase the others. (Credit: NASA/JPL/Caltech)

In many places, the surface of Callisto gives the appearance of having been dusted in a dark powder. Whatever this dusty material is, it has the effect of softening the features of many of the craters on Callisto's surface [455]. Although not extensively resurfaced, there is some evidence of degradation of many of Callisto's craters. Landslides, slumping, and crater rim failure is seen in many areas [456, 457] (Fig. 6.42). There are some small 'smooth' areas suggestive of cryovolcanic resurfacing; however, there is no definitive evidence of this. There is quite a lot of evidence of other processes that have degraded Callisto's surface, such as sublimation-driven landform modification and mass wasting or slumping [458].

Like Ganymede, 'palimpsests' are evident on the surface of Callisto. Palimpsests are large, circular, low relief impact scars, and consist of four surface units: central plains, unoriented massif facies, concentric massif facies, and outer deposits. Palimpsest deposits represent fluidized impact ejecta rather than cryovolcanic deposits or ancient crater interiors [459]. Palimpsests give the impression of large craters that are almost not there.

Fig. 6.40. Another portion of Callisto's surface imaged by the Galileo spacecraft. Note the varied landscape and the many features that are almost completely filled in by the dust-like material on the moon's surface. (Credit: NASA/JPL/Caltech)

Callisto possesses an exosphere. The ultraviolet spectrometer on board Galileo detected hydrogen atoms escaping from Callisto, and scientists determined that oxygen was therefore being disassociated from water molecules in the crust as a result of bombardment from solar ultraviolet radiation! The infrared spectrometer also found evidence of carbon dioxide frost and even gaseous carbon dioxide. According to Robert Carlson, Callisto's atmosphere is so tenuous that the carbon dioxide molecules drift around without bumping into one another. Callisto is not able to retain this atmosphere because ultraviolet radiation from the Sun breaks the molecules down into ions and electrons that are then swept away by Jupiter's magnetic field. It may be that carbon dioxide is periodically vented from the surface [460]. Otherwise, Callisto has no significant atmosphere.

There is indirect geological and geophysical evidence that Callisto may posses a subsurface salty liquid water ocean [461]. Callisto possesses one of the oldest known surfaces in the Solar system. Being the furthest from Jupiter of the Galilean moons, Callisto is not tortured by the tidal forces that can contribute to constant upheaval and resurfacing, and therefore avoided an internal heating process. The moment of inertia determined by the Galileo spacecraft suggests that Callisto has only a partially differentiated interior (Fig. 6.43). That is, it does not posses a distinct

Fig. 6.41. A Voyager spacecraft image of the Valhalla multiple-ringed structure on Callisto. This feature consists of a light floored central basin some 300 km in diameter surrounded by at least eight discontinuous rhythmically spaced ridges. The rings are indicative of the moon's low density and probable low internal strength. (Credit: NASA/JPL/Caltech)

Fig. 6.41. A Voyager spacecraft image of the Valhalla multiple-ringed structure on Callisto. This feature consists of a light floored central basin some 300 km in diameter surrounded by at least eight discontinuous rhythmically spaced ridges. The rings are indicative of the moon's low density and probable low internal strength. (Credit: NASA/JPL/Caltech)

separate core and separate mantle, but ice and rock incompletely separated. Callisto is believed to have avoided differentiation through large-scale melting but may be incompletely differentiated through the convective gradual unmixing of two solid components, ice and metal-rich rock. It is not clear whether this unmixing process is still ongoing or has been arrested. Magnetic field data returned by Galileo suggests that Callisto has a conducting layer at a depth of not more than a few 100 km in which a magnetic field is induced. The simplest explanation for this observation calls for a subsurface ocean or layers of partially molten ice. Spectroscopic data suggests that ice is the major component on Callisto's surface. Its density suggests that Callisto contains approximately equal shares of rock/iron and ice [462, 463]. The source of heat that would keep a subsurface ocean from freezing completely on Callisto is probably radiogenic. Also, any salt in the ice or slush will act as a natural antifreeze. A salt mixture no more concentrated than the oceans on Earth would be enough to explain the magnetic field data collected by Galileo [464].

Kruskov and Kronrod suggested a six-layer, partially differentiated interior structure for Callisto, consisting of the following shells: (1) an outer water-ice envelope; (2) an intermediate water-ice mantle, which is subdivided into three reservoirs and composed of a mixture of high-pressure ices and rock material (dry silicates and/or hydrous silicates + Fe-FeS alloy); and (3) a central iron rock

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