2.3.3 Fracturing and motion of the ice shell
If the rigid surface layer of Europa's ice is thin (or, at least, has been thin for some of the time), and overlies either water or some kind of weak and mushy ice as indicated by large craters such as Pwyll, then we might expect to find some evidence for fracturing and motion of the rigid ice shell. This is precisely what the pattern of dark bands such as those on Figure 16 (see Section 2.3.1) appears to be showing us. An area from Figure 16 is enlarged in Figure 18, with an interpretation of how plates bounded by fractures in the rigid ice shell could have moved relative to one another.
The arrows on Figure 18b suggest that the plates labelled A-D have all moved westwards relative to the ice at the right-hand (eastern) edge of the map. In addition, plate B has rotated about 5° anticlockwise relative to plate A (opening up the intervening wedge-shaped band that extends south from y); plate C has moved west relative to plate B and plate D has moved west relative to plate C.
It is tempting to make an analogy with plate tectonics on Earth, and to regard the stepped dark bands forming the north and south boundaries of plate C as lengths of spreading axis (or mid-ocean ridge) offset by transform faults. However, even if the interpretation in Figure 18b is correct, there are several important differences between plate tectonics on Europa and the Earth. First, Europa's jumble of overlapping dark bands (Figure 16) suggests that old spreading axes are abandoned and replaced by new ones after only a few tens of kilometres of spreading. However, on Earth most spreading axes last for tens to hundreds of millions of years, during which time they add hundreds or even thousands of kilometres of new lithosphere to the edges of the adjacent plates. On Earth, creation of new lithosphere at spreading axes is balanced globally by destruction of lithosphere at subduction zones. (On Earth, a subduction zone is where one lithospheric plate descends at an angle below another.)
There is no analogue to terrestrial subduction zones on Europa, but it is obvious that if new areas of surface ice are being added to make the dark bands then other areas must be being destroyed at an equal rate. The processes operating on Europa to achieve such a balance remained a mystery until Galileo's more detailed images became available. You will examine this evidence soon, but first it is worth exploring the extra information that Galileo images can give about the dark bands themselves. Figure 19a is one such image. It shows that the pale areas between the dark bands that seemed relatively featureless at the resolution of the Voyager images can be seen at higher resolution to be criss-crossed by low ridges. At this level of detail, Europa's surface has been aptly described as looking like a ball of string. Furthermore, the 'ball of string' ridges also occur within the dark bands (running parallel with their edges). When we move up to even higher resolution, as in Figure 19b, the 'ball of string' ridges are even more obvious (and some can be seen to have central grooves running along them), whereas the distinction between dark bands and pale terrain has become hard to see.
Click here for a bigger version of Figure 19b
It is uncertain exactly how the ridges on Europa have been built. Each is probably the result of some form of cryovolcanic eruption along a crack or fissure. If this is the case, the material erupted must have been in the form of mushy ice, or perhaps a fountain-like spray of fragmented ice, analogous to a volcanic fissure eruption on Earth (Figure 20) and involving the escape of gaseous volatiles during eruption. Fortunately, the details of ridge-building are not important in order to understand the general surface history and its implications for ice thickness, which appear to be as follows:
Each 'ball of string' ridge is symptomatic of a small amount of surface extension.
The ridges occur in sets of up to about a dozen parallel ridges, and each set can usually be seen to be cut across by a younger set. There are at least four such sets within the portion of the dark band shown in Figure 19b. Although not quite parallel to each other, each set runs lengthways relative to the dark band, and would in total be responsible for the kind of spreading across a dark band indicated in Figure 18b.
In the older pale terrain outside the dark band the ridge sets are oriented more variably, showing a long and complex history of surface creation.
The dark bands are the youngest parts of the 'ball of string' surface, and evidently become paler as they age. (There are many ways in which this could happen. Some involve growth or fragmentation of ice crystals over time, others depend on chemical changes caused by long-term exposure to radiation.)
There are two things to add to finish the story of surface creation in the area covered by Figure 19. First, the bright bands cut across the 'ball of string' texture and so are clearly younger than it. These bands may be a slightly different kind of cryovolcanic feature - their feathery edges, seen at the highest resolution (Figure 19b), could represent debris shed downslope from a central high. Second, there are some very narrow grooves (barely visible in Figure 19b) that also cut both dark and pale 'ball of string' texture, one of which widens towards the east where it becomes an otherwise unremarkable contributor to the texture. Many features such as these are probably cracks where extension occurred without an accompanying eruption. Others are evidently the surface expressions of faults with sideways (instead of extensional) movement (as you will see shortly).
The dark bands and the intervening tracts of pale terrain were constructed by a long and complicated series of events, each of which was associated with spreading on a local scale.