these various forms can be present at concentrations that are far from chemical equilibrium. As an example, H2S in contact with Earth's O2- rich atmosphere is unstable and should react to form sulfate. If the reaction is slow, as this oxidation reaction is at low temperatures, then coexisting H2S and O2 represent food to a sulfide-oxidizing microorganism. At high temperatures, terrestrial geochemical constraints necessitate that most energy-yielding reactions are reduction reactions, like the formation of CH4 from bicarbonate and H2 in submarine hydrothermal systems.22 In any event, the thermodynamic calculations require analytical data on iron, sulfur, carbon, nitrogen, and other elements that can exist in variable redox states.

Quantitative inventories of available energy in terrestrial environments can be complemented with laboratory studies of the energetic demands of microorganisms. It will then be possible to answer questions about the amount of biomass that can be supported by a geochemical process in a surface or subsurface environment.23 At present, microbial growth experiments on extremophiles focus on optimum temperatures, and ranges of salinity, pH, and temperature, at which an organism can live. Missing from this approach is an assessment of the amount of energy provided by chemical reactions that is required to grow and maintain a given cell concentration. For example, although we know that Methanococcus jannaschii is an autotrophic methanogen living optimally at 85° C in submarine hydrothermal systems, we do not know how many cells of this organism can be supported on the disequilibrium between hydrogen and bicarbonate present in the natural system.

In addition, little is known about the energy requirements of the metabolic pathways used by chemolitho-autotrophs and other extremophiles at the actual temperatures and pressures where they live. Does a hyperthermophile require more or less energy to make peptide bonds than is required by a low-temperature organism? What is the energy yield of ATP hydrolysis at the high temperatures and pressures encountered deep in hydrothermal systems? How do the redox potentials of important biochemical processes change with temperature and pressure? Although hints about the answers to these questions are currently available, more definitive results could help identify likely metabolic strategies used by novel organisms in unfamiliar environments.

A unifying theme of energy availability and energy demand, if quantified, would enable transport of microbial growth data from the laboratory to the study of natural environments. It would then be possible to test whether a given geochemical process can yield enough energy to support a given metabolic strategy, and, if it can, how many organisms can be supported. These studies would permit quantitative estimates of the potential for life on Europa or in terrestrial environments that might be close analogs to conditions on Europa.

LABORATORY STUDIES

Space Weathering of the Surface Materials

There is considerable observational evidence that it is necessary to understand space weathering in order to interpret reflectance properties and ages of surface features on Europa. The surface materials are weathered by plasma ions and electrons from the jovian plasma torus, solar ultraviolet photons, and micrometeorite bombardment, all occurring in the presence of a tenuous oxygen atmosphere that is itself a product of ion bombardment. These are processes that also occur on the surfaces of other objects that have tenuous or no atmospheres. Whereas laboratory studies directed toward lunar materials are extensive, however, the materials of interest for Europa (ices, salts, and organics) have been little studied. The principal need is data on irradiation effects — plasma and ultraviolet weathering of these materials. The types of data required pertain to the effects of weathering on the production of gas-phase molecules (sputtering and decomposition), changes in reflectance, and chemical alterations induced in the surface. These effects are clearly interrelated. Because of the detection of S (as SO2) and Na, the chemistry of an H2O-S-Na system subjected to a radiation environment needs to be understood.

In the last 10 years there has been a considerable effort to understand the ion bombardment of water ice. This process results in redistribution of H2O molecules across the surface of Europa and also in the production of O2 that contributes to Europa's atmosphere and H2 that escapes into space. These data, along with those for the sputtering and chemistry of other frozen volatiles of potential importance (e.g., SO2 and CO2), have been summarized in Solar System Ices.24 However, there is still a dearth of data on the spectral changes linked to irradiation of these ices, including the absorbance properties produced by implantation of S and Na ions from the jovian torus



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