Jupiter and its moons

1.11 Callisto

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

Callisto, the outermost galilean satellite, has a dark heavily cratered surface (Figure 9.18) with an albedo of only 0.2. It is the only one of its family to resemble what was expected before the role of tidal heating became appreciated. Callisto is not currently in orbital resonance with any of its neighbours, and shows no clear sign of having experienced tidal heating in the past.

Figure 9.18: Galileo view of a large part of Callisto showing its dark, heavily cratered surface[.] The younger impact craters are the most prominent, because their ejecta blankets have not yet become radiation-darkened[.] Towards the left is a multiringed impact basin named Valhalla[.] The 600-km-diameter pale zone in Valhalla's centre marks the site of the original crater, but this was too large a structure for the lithosphere to support and it has long since collapsed[.] This is surrounded by rings of concentric fractures, the outermost having a diameter of 4000 km[.]
NASA

Despite this, Callisto is far from boring. Its weakly differentiated structure has already been remarked upon, and appears to consist of an ice-rock mixture throughout except for an ice-rich crust. There could possibly be a rocky core occupying up to 25 per cent of Callisto's radius, but an iron core would seem to be ruled out by the Galileo gravity data. Therefore it is surprising that Galileo found that a magnetic field is induced within Callisto by its passage through Jupiter's magnetosphere.

Given the lack of an iron core, the only reasonable way to explain this magnetic field is to appeal to a salty (and therefore electrically conducting) ocean at least 10 km thick and no more than 100 km below the surface. A liquid layer at such a shallow depth seems incompatible with the undeformed, ancient and heavily cratered nature of Callisto's surface, so here is yet another mystery awaiting resolution.

However, some features on Callisto that used to be a mystery are now understood thanks to comet Shoemaker Levy 9. These are linear chains of overlapping craters (Figure 9.19). It is now agreed that each chain is the scar of impacts by fragments of a different comet, each of which hit Callisto on its outward path immediately after having been tidally disrupted during a close passage by Jupiter. There are about a dozen of these, mostly on Callisto's leading hemisphere which is the side most exposed to the risk of such impacts.

Figure 9.19: A chain of 25 10-km-diameter craters on Callisto believed to result from the serial impacts of fragments of a tidally disrupted comet[.] Bottom: Voyager image showing the whole chain in context[.] Top: Galileo view, showing an oblique close-up view of parts of three overlapping craters in this chain[.]
NASA

High resolution views like the Galileo image in Figure 9.19 reveal a surprising paucity of craters less than 1 km across. Given the number of larger craters that are present, it is inconceivable that smaller craters have not formed in even greater abundance. Therefore something must be acting to remove them. The Figure 9.19 Galileo image contains clues as to what might be going on. Hilltops and some slopes are markedly brighter than the dark surface from which they crop out. This can be explained if the bright surfaces are relatively clean ice whereas the dark surfaces are covered by a regolith that is enriched in rock debris.

This may be a result of the continual bombardment of the surface by charged particles and micrometeorites. We have seen that this contributes to the breakdown of water molecules, but it can also simply vaporize the ice. On Callisto, those water vapour molecules that do not escape to space or become split into hydrogen and oxygen will recondense on any icy surface that they bump into. This condensation process is at its most efficient on hilltops because these are exposed to the sky all round and so are slightly colder than the surrounding area. Once the brightness difference is established, the temperature contrast gets reinforced because the brighter surfaces will absorb less solar warmth than the darker ones. Therefore, over time frost migrates towards the bright hilltops and other exposed areas, and the residual silicate dust becomes progressively concentrated in the places from where the ice is being selectively removed. Perhaps this process is capable of eroding the rims of small craters faster than the average rate at which such craters are forming.

Another erosional process that occurs on Callisto is landslips, as can be seen in Figure 9.20, where the inner part of a crater rim has collapsed.

Figure 9.20: Galileo image showing a 12-km-wide crater on Callisto with a landslip extending a third of the way across its floor from its eastern rim[.] Note the degraded morphologies of many of the smaller craters, and the relative brightness of the hilltops compared to the dustier low ground[.]
NASA