Jupiter and its moons

3.4 Ganymede and Callisto

The images of Ganymede and Callisto in Section 1.11 and Section 1.12 and below demonstrate that ancient icy surfaces can display just as large a range of features as any rocky world. Given the similarity in properties between ice and rock listed in Section 1.10, this should not be surprising. For example, most impact craters on Callisto and the younger impact craters on Ganymede (Figure 13) look pretty much the same as impact craters on Mercury, the Moon or Mars. Similarly, Callisto's Valhalla impact basin (Figure 9.18) resembles multiringed impact basins of terrestrial planets such as Mercury's Caloris basin.

Figure 13: Left: relatively young craters on Ganymede, the largest having a well-formed central peak. This is an enlarged extract of part of Figure 9.17. Right: a higher resolution view of a similar crater on Ganymede, 32 km in diameter.

Question 8

From which direction is the illumination coming in each of the images in Figure 13?

Answer

The shadows on the crater floors cast by the walls and central peaks demonstrate that the illumination is from the right in both cases.

The only important distinction between cratering on the terrestrial planets and on Jupiter's satellites concerns the rate of cratering. There is clear evidence from the relative abundances of craters of different sizes that the population of impactors responsible for craters on Ganymede and Callisto differs from the population that caused the cratering of the terrestrial planets. The reason is that the latter is probably dominated by asteroids, whereas the former may include a greater proportion of comets plus locally generated debris arising from collisions within the particular satellite system.

Whatever the explanation, it means that we cannot use the lunar cratering time-scale to determine the absolute ages of the surfaces of the galilean satellites. Even more frustrating is that the impactor populations at Saturn, Uranus and Neptune all seem to have been different, so that we cannot use craters to compare ages between satellite systems in the outer Solar System. All that it is usually safe to assume is that when comparing regions on the same body, a more densely cratered surface must be older than a less densely cratered surface.

The surfaces of Ganymede and Callisto are much older than any surface that has survived on Europa, and this is where you first meet the concept of icy surfaces becoming darker as they age. This is a principle that seems to apply pretty well among the satellites of the outer planets. Age-darkening is a combination of two processes. One is the gradual concentration of residual rocky or sooty (carbonaceous) particles as the ice is removed by molecules being broken apart by radiation (water ice, for example, gets split into hydrogen and oxygen) or vaporised in small meteorite impact events. The other is loss of oxygen or hydrogen from carbon-bearing ice molecules (such as carbon dioxide, CO2, or methane, CH4) to leave sooty carbon or tarry molecules known as tholins.

Age is not the only factor controlling the albedo of ice. Its physical state is important too. If you make a powder out of anything solid, the powder will probably be paler than what you started with. This is why young craters and their ejecta are usually such bright features, although these too darken with age. Age-darkening is too slow to have affected Europa's young surface, and here the contrast between pale and dark areas must be largely due to the texture of the ice. For example, in Figure 9.13 the dark 'cracks' are clearly younger than the bright plains that they cut. Rather than being related to age, this contrast could reflect differences in the size of the ice crystals or the roughness of the surface.

Try to bear in mind when you compare images of different icy satellites that this will not usually tell you about their albedos, because the brightness of each image will usually have been adjusted to show each body to the best advantage. For example, in Figure 9.16 Callisto has been brightened relative to Ganymede so that features in each one show up well. With an average albedo of 0.7, Europa is more than three times as reflective as Callisto, whose albedo is only 0.2. Figure 14 below shows the true relative brightnesses of the three icy galilean satellites.

Figure 14: Europa, Ganymede and Callisto shown at their correct relative sizes and their correct relative brightnesses. In this rendering, the image of Europa is rather too bleached out to show its features well, whereas Callisto is very dark except for its more recent impact craters.

You should not come away from this course with the impression that the galilean satellites are well understood. For example, the great abundance of craters on Callisto suggests an ancient surface, and the absence of fractures or cryovolcanic features suggests that its outer shell (which would equate with the lithosphere of a terrestrial planet) must be very thick. However, Callisto's magnetic field suggests a salty ocean no more than 100 km below the surface. It is hard to see how both can be right. I am inclined to suspect that the magnetic data collected by Galileo have been misinterpreted, and that Callisto has no ocean, or at least not at such a shallow depth - but I could be wrong.