In two previous articles, we considered the Galilean satellites and the fact that tidal flexing, due to their resonant orbits, provides heat for
volcanism on Io and could result in the presence of liquid water beneath Europaís icy surface. The Galileo spacecraft, which is currently finishing
its mission in the Jovian system, took a number of images of Europaís surface between 1995 and 2001 that can be used to study the geology of this
small moon and look for evidence of liquid water beneath the surface.
The Voyager spacecraft took the first close-up images of Europa in the late 1970's. Before Voyager, little was known about Europa's surface except
that it was very bright, and measurements taken from Earth with spectrometers on telescopes suggested that there could be water. Images of Europa
taken by Voyager revealed a surface covered with crack-like features, and very few impact craters. The lack of craters was surprising, since all
bodies in the solar system are continually hit by debris. This process results in a pockmarked surface like the Moon's unless geologic activity takes
place to remove craters from the surface. The relative lack of craters on Europa means that the surface is young, perhaps as young as a few tens of
Images of Europa's surface taken by the Galileo spacecraft have shown surface features that could be consistent with the presence of liquid water
beneath Europa's surface, but do not prove it [see image]. Europa's surface is primarily covered by a vast set of interconnecting cracks and ridges.
Also present are areas of disrupted "chaotic terrain", where the surface appears to have been broken up into coherent iceberg-like blocks that seem
to have "rafted" into new positions. Such areas can be reconstructed by fitting the preexisting features on the blocks back together like pieces of
a jigsaw puzzle. Other features of interest on Europa's surface include regions that could possibly be low viscosity surface flows, and impact
craters that are anomalously shallow.
A number of models have been proposed for the formation of the variety of features visible in Galileo images of Europa's surface. There is currently
no consensus among these often-contradictory views of Europa's geophysics. In general, models of Europaís subsurface fall into two categories, one in
which a thin ice layer (at most a few kilometers thick) is present on top of a layer of liquid water, and another in which the surface ice layer is
much thicker, perhaps tens of kilometers or more, with liquid water (if it exists) at a much lower depth. Models of the formation of various geologic
features on Europa seem to follow either the thin-ice or the thick-ice view.
For example, the shapes of Europa's impact craters suggests that they formed within a solid target, but their shallow depths suggest that the surface
rebounded somewhat after their formation. Models of this rebound suggest that most craters on Europa formed in a 5-15 km thick brittle surface layer,
overlying a lower-viscosity subsurface layer. This subsurface material, however, could either be liquid water or warm, low-viscosity ice. A separate
model of central peak formation in Europan craters suggests that the ice crust layer must be at least 3-4 km thick, and a model of crater topography
suggests a much thicker ice layer, at least 19-25 km.
Cryovolcanic surface flows, made of a mixture of water, ice, and maybe other volatile materials such as ammonia, would be intriguing, but there are
very few regions where the shape of surface features is suggestive of flows. There is also a substantial buoyancy problem in their formation, as it is
difficult to get liquid water to the surface of Europa since it is denser than ice. One possible such region is visible in the figure, and is about 3
km across. This region could have been formed when some type of fluid-like material covered over pre-existing ridges and other features.
Models of ridge formation range from cryovolcanism to tidal squeezing to linear diapirs to compression and plastic deformation. These models range in
requirements from a very thin crust overlying liquid water (the tidal squeezing model) to completely solid-state models with a thin brittle crust on
top of a lower-viscosity, warm ice layer (diapirism or compression). One interesting feature called cycloidal ridges seem to correspond in orientation
and location to cracking of the surface in response to the changing daily tidal stresses. This model would require the existence of a global ocean
near the surface to obtain sufficient tidal stresses to crack the ice. Clearly, current models in the literature are contradictory and have very
different implications for Europa's subsurface structure.
Chaotic terrain was first seen as a "smoking gun" for the presence of liquid water beneath Europa's surface, but formation models that involve only
solid materials are also possible. At the liquid end of the spectrum, regions of chaotic terrain are seen as areas of localized heat flow where the
ice layer melted all the way to the surface. In this model, the blocks are buoyant remnants of the preexisting icy crust that move about in a slushy
matrix, both translating and tilting. Eventually the matrix freezes solid, ending the blocks' motion and preserving their final positions. This model
requires localized heating of the crust, but it may be difficult to concentrate the heating in both space and time. The solid-state formation model
suggests that ice rises to the surface in a diapir, eventually disrupting the brittle surface. These convective upwellings make it difficult to tilt
the blocks as observed, however. A third, intermediate model suggests that runaway melting within rising diapers produces chaos. Thus, models of chaos
formation do seem to favor the existence of either water or an ice-water slurry at shallow depths near the surface, but such patches could be
localized and not require the existence of a global liquid ocean layer.
The geology of Europa provides some tantalizing clues that liquid water may have been, or currently be, present beneath Europaís icy surface. However,
images of the surface can not prove the existence of liquid water. Fortunately, some other types of geophysical evidence do provide more definitive
detections of water, and these will be examined in a future article.