tions for the composition and physical properties of the lower-most layers of ice, as well as the possibility that living systems could emerge in the dark using chemical energy supplied by the disequilibrium between hydrothermal fluids and molten ice.
Although essential, characterization of geophysical, petrological, and geochemical properties is not sufficient to reveal the difference between a hydrothermal system and a hydrothermal ecosystem. The possibility that a hydrothermal system can support life depends on how well it can meet the demands that life places on it. Is the supply of carbon sufficient for biosynthesis? Are nutrients available at useful concentrations? Is sufficient energy available in usable forms? As mentioned above, the energetic requirements of hyperthermophilic organisms are largely unknown. Given sufficient analytical data, it is possible to evaluate the geochemical energy available from various inorganic reactions used by autotrophs in terrestrial hydrothermal ecosystems. As an example, it is now possible to determine the amount of energy that can be obtained by a methanogen in a seafloor hydrothermal system at the precise temperature and pressure at which it lives. However, we do not know why this is enough energy, because the energetics of many biochemical reactions and metabolic processes have not been studied at elevated temperatures and pressures.
Terrestrial ice sheets deform under the force of gravity in a fashion determined by boundary conditions at the sides and bottom. On short time scales, ice behaves elastically and, for large stresses, ice undergoes brittle fracture. For low stresses, operating over long times, ice undergoes creep deformation.
The terrestrial sea-ice pack moves under the influences of ocean currents and winds that frequently exert forces strong enough to fracture individual floes. The effective rheology of the polar ice pack is governed primarily by consequent macroscopic processes such as ridging, rafting, and lead formation rather than the intrinsic rheology of the sea ice itself.
The surface of Europa seems to be composed of features suggestive of icebergs locked in a sea-ice matrix, block rotation and displacements, brittle fracture, and, perhaps, creep deformation as evidenced by flow-stripe-like structures on the surface of some blocks. Consequently, and perhaps at different times, the surface of Europa may have behaved somewhat like terrestrial ice sheets and sea ice cover. More enigmatic are the banded ridges running for hundreds of kilometers across the surface. These features, more reminiscent of terrestrial tectonic processes, have no apparent analog in the terrestrial ice environment and may be related to the tidal stresses exerted on Europa by Jupiter.
Observations such as these lead to several questions that could be explored by modeling of ice dynamics. These include the following:
What long-term processes might lead to creep deformation?
Where and how did the ice sheet fracture, and did fracturing result in an upwelling of liquid water?
What forces caused rotation of the ice blocks? and
Is convection possible within the ice sheet?
Earth-based telescopic observations have played an important role in advancing current understanding of Europa (see Chapters 2 and 3). The key advantages of future Earth-based (as opposed to spacecraft) data collection are the ability to look for long-term (yearly or decadal) variability, the capability of using very-high-spectral-resolution spectroscopy, and the advantages afforded by telescopes with large apertures.
At ultraviolet wavelengths, the Hubble Space Telescope (HST) will continue to provide high-resolution spectral capability, along with moderate spatial resolution. These are the most important for studies of the atmospheric composition and the morphology of the gas distribution. Detection of magnesium, for example, could rule out Io as a source of Europa's tenuous atmospheric constituents. Additional HST observations of the oxygen atmosphere could provide limits on its temporal variability.