Encyclopedia of the Solar System 2nd ed

Europa 439

terrain at terrestrial midocean ridges undergoes normal
faulting as it cools and moves away from the ridge crest.
On Europa, normal faults parallel to the central axis may
have similarly formed as new band material cooled suffi-
ciently for faulting to take place (Fig. 9).

FIGURE 9 Model for band formation suggests they are
analogous to midocean ridges on Earth. (Top) Distinct
morphological zones are mirrored on either side of the central
axis of this band. Closest to a central trough, the terrain is
hummocky and relatively fine-textured. Further away from the
axis, the terrain breaks into normal faults. These types of terrains
are also found at spreading centers on midocean ridges, leading
to suggestions that Europa’s bands form in a similar way
(cracking followed by extension), allowing new, warmer ice to
well up to fill the gap. As this material cools and moves away
from the central axis, it thickens enough that it can form normal
faults. This process is analogous to the way new seafloor forms on
Earth. (After Prockter et al., 2002.)

The major difference between terrestrial plate tectonics

and Europan band formation is the lack of subduction zones

on Europa. Thus, because band formation has clearly re-

sulted in a large amount of extension (many tens of percent

in some areas), there must be some mechanism for bal-

ancing this extension elsewhere on Europa’s surface. Some

fraction of Europa’s extension is related to net global expan-

sion, as would be the case if the ice shell were thickening

with time.

4.1.6 FOLDS

Analysis of high-resolution images of Europa has identi-

fied regional-scale contractual folds in a handful of regions

on Europa. The most apparent are identified in the band

Astypalaea Linea, where they appear as subtle hills and

valleys; several have warped the band at a wavelength of

∼25 km, with fine-scale fractures along the crests of the

hills and small compressional ridges within the valleys (Fig.

10). It is unlikely that such folds can represent the primary

mechanism by which the icy satellite’s considerable surface

FIGURE 10 The best example of folds on Europa, within the

gray band Astapalea Linea. (Top) Close examination of the band

reveals fine-scale ridges and fractures. (Bottom left) If a low-pass

filter is applied to the image, the 25 km wavelength folds can be

distinguished. (Bottom right) Map showing sets of ridges (blue

arrows) within the fold valleys and fractures (green arrows) that

mark the fold crests. (Prockter and Pappalardo, 2000.)