436 Encyclopedia of the Solar System
tend to be near theangle of repose(∼ 30 ◦BA, and at
some, preexisting topography can be traced up the flank,
suggesting that they may have formed by upwarping of the
surface. Most ridges tend to be relatively linear, or only gen-
tly curved, and mass wasting is prevalent along ridge flanks.
The cycloidal ridges discussed later in this section have
notably arcuate shapes, but are otherwise morphologically
identical to other double ridges on Europa. Another form
of ridge is the “complex” ridge, which may have from three
or more subparallel ridge crests (Fig. 4). Some complex
ridges appear to be sets of several double ridges, running
parallel to, or in some cases intertwined with, each other,
while others seem to be composed of bundles of ridge crests
separated by intervening troughs.
Ridge formation on Europa is not yet fully understood,
and several models have been suggested. Europa’s cracks
are probably modified by other processes to form ridges
with distinct and uniform crests. In one model, ridges form
through the buildup of cryovolcanic material erupted from
fissures, as is the case with many terrestrial eruptions such
as those that form Hawaiian volcanoes. A major drawback
with this model is that it is hard to explain the remarkable
uniformity of ridge crests, and the distinct V-shaped trough
along their axes. An alternative model suggests that dou-
ble ridges form in response to cracking and subsequent rise
of warm or compositionally buoyant ice. Possibly aided by
tidal heating, the buoyant ice intrudes and lifts the surface
to form ridges. Although this model does explain some ob-
servations, such as why some ridge flanks have preexisting
terrain running up them, it does not explain how multiple
ridges might form within complex ridges.
Another model proposes that the ridges form in a man-
ner similar to pressure ridges in arctic sea ice. In this model,
cracks created by diurnal tidal stresses allow water to seep
up from the ocean below, filling the crack and partially freez-
ing into a slurry. It is envisioned that diurnally varying tidal
stresses would then push the crack margins back together,
and this partially frozen ice is easily smashed up, forming a
jumbled pile of ice that squeezes out of the crack. Although
the process that forms pressure ridges is understood well on
the Earth’s sea ice, where the ice is thin and ocean currents
cause movement of the ice, it is unknown whether Europa’s
ice is sufficiently thin and mobile as to pull apart atop the liq-
uid layer. Even if Europa’s ice is thin, it is not clear that this
model can explain the morphology of Europa’s ridges, in-
cluding distinct V-shaped troughs along double ridges, their
uniform parallel ridge crests, and the apparently upwarped
features along some ridge flanks.
Alternative models have suggested that the cracks in-
stead penetrate upward from the ocean into the ice shell,
and that liquid injected into cracks from beneath then up-
warps the surface. This model could explain the general
morphology of ridges, but it has difficulty explaining the
uniformity of the crests and the morphologies of complex
ridges.
The model that seems to best fit the observations of ridge
morphology is one in which a fracture forms along the sur-
face and then undergoesshear stressesand strike-slip mo-
tion as a result of Europa’s diurnal tides. This strike-slip
motion along the crack produces frictional heating as the
walls of the crack rub past each other, warming the subsur-
face ice (Fig. 5). This shear heating may trigger warm iceFIGURE 5 Double ridges are ubiquitous on Europa, but their
origin is not well understood. In the shear heating model,
strike-slip, or shear, motion along a fracture results from diurnal
tidal stresses. Friction between the fracture walls warms the ice,
softening or partially melting it. Warm ice close to the fracture
rises buoyantly, upwarping the ridge crests. Downward drainage
of melt may aid formation of the axial depression. (Image:
NASA/JPL. Diagram: Topography courtesy B. Giese, DLR.)