David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.
Copyright © David Rothery
Ganymede is the largest planetary satellite in the Solar System, being bigger (though less massive) than the planet Mercury. It is shown in comparison with its outer neighbour Callisto in Figure 9.16. Although these two satellites are similar in size, with bulk densities implying a roughly 60:40 rock:ice mixture in each, Galileo indicated that Ganymede is a fully differentiated body, with an iron inner core (filling 22 per cent of its radius), a silicate outer core (filling 55 per cent of its radius) and an icy mantle, but that Callisto is only weakly differentiated. The difference in evolution between these two bodies is probably because Ganymede was formerly subject to much more intense tidal heating than it receives today, whereas Callisto has never experienced much heating, tidal or otherwise. Ganymede has a magnetic field with about 1 per cent the strength of the Earth, which could be generated in the core or in a salty ocean deep within the ice layer.
With an albedo of about 0.45, Ganymede's surface is darker than Europa's. Spectroscopic studies show that it is dominantly water-ice, with scattered patches of carbon dioxide ice, and that the darkening is caused by silicate minerals (probably in the form of clay particles) and tholins. The darkening is at least partly attributable to the much greater age of Ganymede's surface, allowing more time for the action of solar radiation to produce tholins and for silicate grains to become concentrated in the regolith by the preferential loss of ice during impacts.
There are also faint traces of oxygen and ozone, apparently trapped within the ice as suggested for Europa, and Galileo found an extremely tenuous (and continually leaking) atmosphere of hydrogen that is presumably the counterpart to the oxygen produced by the breakdown of water molecules.
It is obvious even on images of the resolution of Figure 9.16 that there are two distinct terrain types on Ganymede, one darker than the other. At higher resolution (e.g. Figure 9.17) it is apparent that the pale terrain must be younger than the dark terrain, because belts of pale terrain can be seen to cut across pre-existing tracts of dark terrain. However, the density of impact craters on both terrain units shows that each must be very old, perhaps as much as 3 billion years.
Considered in more detail, it becomes clear that multiple episodes of pale terrain generation are required to explain the cross-cutting relationships between belts of pale terrain. Images like Figure 9.17 and others at a higher resolution, show the complex grooved nature of the pale terrain and hint that each belt of pale terrain might require just as complex a series of events to explain it as parts of the bright plains on Europa.
This is not to say that the same processes were involved. One important difference between the two bodies is that on Europa there is abundant evidence of the two halves of a split tract of terrain having been moved apart to accommodate the new surface that has formed in between, whereas on Ganymede signs of lateral movement are scarce. Ganymede's pale terrain appears to occupy sites where the older surface has been dropped down by fault movements, allowing cryovolcanic fluids to spill out. It remains a matter of debate whether the grooves within each belt of pale terrain represent constructional cryovolcanic features or reflect subsequent deformation, perhaps related to underlying faults.
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