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to well up beneath the crack, forming ridges, and perhaps
inducing partial melting beneath the ridge axis at the same
time. This model predicts that a ridge a few hundred me-
ters high could be built by upwelling warm ice in only a
decade or so, and because both sides of the crack are sub-
ject to the heating, the ridges would be expected to be of
uniform width and height, as is observed. If shear heating
were sufficient to induce partial melting below the ridge,
it would tend to drain downward, perhaps forming the V-
shaped axial trough above. This model would soften the ice
along the ridge, perhaps enabling contractional deforma-
tion to occur in response to compressional stress. A model in
which contraction occurs across ridges may be viable based
on reconstruction of preexisting features and kinematic
arguments. If ridges do hide contraction along initially ex-
tensional structures, as in the shear heating model, then
ridges could help to balance the abundant extension on
Europa that is represented by its pull-apart bands, as dis-
cussed later. We may ultimately find that ridge formation is
a combination of several models, but they currently remain
an enigma.
4.1.3 CYCLOIDAL RIDGES
While most double ridges are linear in overall planform,
cycloidal ridges are shaped like a chain of distinct arcs
(Fig. 6). Cycloidal ridges and some other structures on Eu-
ropa’s surface are likely explained by the action of diurnal
stresses. If a fracture propagates slowly enough—at about
walking speed—the rotation oftensile stressesover a Eu-
ropan day occurs on a timescale such that the propagating
fracture can be affected by these changing stresses, trac-
ing out an arc instead of a straight path. As Europa moves
in its orbit, the tensile diurnal stresses will drop below the
critical value needed for fracture propagation, until the next
orbit, when tensile diurnal stresses again increase above the
critical value for fracture propagation, generating the next
cycloid arc. This model requires that the diurnal stresses
needed to crack the ice and create cycloidal fractures be
relatively small, just a few tens of kilopascals. Ridges would
evolve from cycloidal fractures in a manner similar to the
formation of other ridges, and some pull-apart bands with
scalloped margins may have pulled apart along cycloidal
ridges. Tides imparted by Jupiter’s gravitational pull would
be insufficient to crack the surface into cycloidal patterns
if Europa had no ocean, so the presence of the cycloid
features is strong argument for the existence of an under-
lying ocean at the time the fractures formed.4.1.4 TRIPLE BANDS
One specific type of lineament consists of a bright central
ridge, flanked by patchy, diffuse, low-albedo margins, hence
the term “triple band” (Fig. 7). These are most commonly
larger ridges. It has been suggested that the dark flanks were
created by the eruption of icy cryovolcanic material (similar
to some explosive volcanoes on Earth), which either seeped
out along the ridge flanks or rained darkpyroclasticmate-
rial onto the surface alongside the ridge. Another possibility
is thatintrusionsof ice that is warmer than its surroundings
might result in localsublimationof icy surface materials
leaving a layer of morerefractorydark deposits. The un-
usual brightness of the central ridge relative to the flanks is
as yet unexplained: It may be coated by frost or depleted in
dark materials.FIGURE 6 (Top) Cycloidal ridges on
Europa. (Bottom) Model for cycloidal ridge
formation, in which each arc forms during
one orbital cycle. A crack initiates when
stresses reach a critical value and then
propagates slowly enough that the changing
diurnal stress field affects its orientation,
causing it to curve. When the stress drops
below a critical level, the crack ceases
propagation until the stresses are once again
sufficiently large to reinitiate cracking. When
the crack reinitiates, Europa has moved
along in its orbit, and the stresses are now in
a different orientation, leading to a sharp
cusp as the next arc begins to propagate.
(Top: NASA/JPL. Bottom: After Hoppa et
al., 1999.)