Jupiter and its moons

3.3 Europa

Although Io may be more spectacular, Europa is perhaps more intriguing. To get the most out of this section you needed to look again carefully at images in Chapter 9 of Teach Yourself Planets (such as Figure 9.14, Figure 9.15 and Plate 9 in Section 1.10). This is because even though Europa's landscape is so alien, the images contain clues as to the processes that have shaped it. Figure 12 shows part of the larger region surrounding Figure 9.14 and Plate 9 (in Section 1.10).

Figure 12: A 200 km wide area of Europa, illuminated from the right. 'Ball-of-string' surface patterns predominate, but this surface pattern has been disrupted in several places, notably in Conamara Chaos on the left, which is shown in a more detailed image in Plate 9 in Section 1.10. The colours are exaggerated. The reddest areas are probably the saltiest, purer ice appears blue, and white patches are powdery ejecta from a distant impact crater.

Question 7

Look carefully at the area shown in Figure 12 and see if you can use your logic to put the following events into the correct time sequence within this area, starting with the oldest first:

(a) disruption of surface by breakup into chaos

(b) generation of ball-of-string surface (refer to Figure 9.14 for a good example of ball-of-string surface)

(c) formation of ridge systems larger in scale than the typical ball-of-string texture

(d) patchy distribution of white ejecta.

This is a challenging question. If you are really stuck then study the answer carefully, but please have a go first - working this out does not depend on any skills specific to planetary science, instead it is a matter of logic and common sense.

Answer

The correct sequence is (b), (c), (a), (d), although there may be local exceptions. Generally, ball-of-string texture is the oldest. Several generations of larger ridges can be seen cutting across (overprinting) the finer scale ball-of-string texture, so these larger ridges must be younger. Ball-of-string texture and some of the larger ridges are both disrupted into chaos, notably in the large area of Conamara chaos in the left (see Plate 9) but also in smaller chaos regions that cut across some of the large ridge systems in the lower right, so chaos generation is younger than ridge generation. Distribution of the ejecta patches is probably the last event, because the splashes of white can be seen even on the re-frozen matrix between the chaos rafts (most clearly seen in the lower left of Plate 9).

Section 1.10 introduces the properties of Solar System ice that enable it to behave so much like rock, and the strange term 'cryovolcanism' that you will meet in connection with several other icy satellites. Europa is atypical because the ice (being so thin and warm at its base) can sometimes behave rather like frozen sea-ice on Earth, but on the other icy satellites of the outer planets the likenesses to silicate rock in a terrestrial planet are much stronger.

The Europa section concludes with speculation about the possibility of life nourished by chemical energy beside hot springs on the floor of Europa's ocean. This is the kind of setting where it is now believed that life is most likely to have begun on the Earth. However, you should bear in mind that there is no proof that Europa's ocean exists, although at present that does seem to be the simplest way to explain the drifted rafts in some of the chaos regions.