438 Encyclopedia of the Solar System
FIGURE 7 An example of a triple band, consisting of a central
ridge about 5 km wide that is flanked on each side by diffuse,
dark material (arrows). This material can be patchy and
discontinuous, and may be related to cryovolcanic eruptions
during formation of the band, although the exact mechanism is
poorly understood. (NASA/JPL.)
4.1.5 PULL-APART BANDS
Polygonal dark and gray bands on Europa’s surface have
margins that can be closed together almost perfectly, recon-
structing structures that were apparently laterally displaced
when the bands formed along fractures. Many bands are
bounded along their margins by an individual ridge, sug-
gesting that a double ridge was split along its axis during
band formation (Fig. 8). These structures have been termed
“pull-apart” bands, and are a clear indication of movement
of a brittle surface layer atop a more viscous, yet mobile,
subsurface. Where the bands pulled apart, dark, probably
low-viscositysubsurface material moved up to fill the gap.
Limited topographic data across bands suggest that many
stand somewhat higher than the surrounding terrain, con-
sistent with formation by upwelling buoyant ice, rather than
liquid water. Bands have been shown to have brightened
over time, possibly because of frost deposition or radiation
damage, leading to a wide range of brightnesses ranging
from relatively dark, through gray, to as bright as the bright-
est background plains on the surface.
Almost all bands exhibit bilateral symmetry, with V-
shaped central troughs and hummocky textures, and some
have zones of ridges and troughs parallel to the central
axis, which may include faults (Fig. 9). Morphological com-
parisons between bands and terrestrial midocean ridges
suggest that band formation may have been analogous to
FIGURE 8 Points along a large gray band (arrows, top) can be
reconstructed if the band is removed (arrows, bottom), with the
preexisting terrain matching up perfectly along the margins. The
band appears to have exploited two existing double ridges during
its formation. Reconstructions like this show that a completely
new surface has been created by band formation, suggesting that
they represent a considerable amount of extension of Europa’s
surface. (After Prockter et al., 2002.)seafloor spreading centers on the Earth, where plates are
pulled apart and new volcanic material erupts along the
spreading axis. Features on both planets exhibit central
troughs and subparallel ridges where volcanic (in Europa’s
case, cryovolcanic) material has apparently erupted inter-
mittently through the spreading process. Newly formed