A magnetometer and a plasma detector on a spacecraft orbiting Europa would provide global data on the charged-particle populations and magnetic field over long periods of time and be able to distinguish a permanent intrinsic field from time-varying plasma and induction fields. New, lightweight plasma detectors, such as the Plasma Experiment for Planetary Exploration (PEPE) instrument now operating on Deep Space 1, are able to make good measurements while placing fewer demands on spacecraft mass or power resources than past instruments. Additional data on the induction field would also help to characterize the highly electrically conducting near-surface layer, perhaps a liquid-water ocean, in terms of depth, thickness, and electrical conductivity.

If it turns out that Europa does not have an intrinsic magnetic field, it will be possible to conclude only that there is no convection or dynamo activity in its core. Whether its core is liquid or solid will still be uncertain.

More accurate and complete determinations of Europa's gravity, global shape, and topography will enable refinement of interior structural models; tests of hydrostaticity; and inferences about ice thickness and variations thereof, topography on the water-rock boundary, and mechanical properties of the ice.

DETERMINATION OF THE GEOCHEMICAL ENVIRONMENT OF THE SURFACE AND POSSIBLE OCEAN

The prospect of skipping a systematic geochemical assessment of Europa and discovering some form of life is exciting but has a low probability of success. It is more likely that many geochemical properties of the europan environment will have to be characterized before the probability of the origin and development of life there can be sensibly assessed. Among the properties of interest are the presence and concentrations of chemicals that might serve as nutrients or as poisons, the energy sources available that might support life, the present and past redox states, organic materials that might be residues from living organisms or prebiotic processes, or the characteristic times for physical processes, both in sequence and duration. In this section COMPLEX takes a ''top-down" approach to the study of europan geochemistry, beginning with the atmosphere.

The Atmosphere and Ionosphere

Europa has a thin neutral atmosphere and an ionosphere derived from it. The current state of knowledge of Europa's neutral atmosphere and its embedded ionosphere is still rudimentary, however. The vertical structure and the horizontal patterns (latitudinal and longitudinal morphologies) are yet to be measured. The day-to-day variability of both the neutral atmosphere and the ionosphere and their responses to the stresses caused by electrodynamic interactions with the magnetosphere in which they reside are essentially unknown. Atmospheric species cannot yet be accurately sorted into endogenous and exogenous components. Most of the atmosphere consists of chemical species produced by sputtering of the materials that are found in the surface ice, but the compositions of those materials are surely changed as a consequence of the chemical reactions induced during their ejection into the atmosphere. Certainly, without the strong interaction of the bombarding plasma, the atmosphere would be much more meager than it is.

The atmosphere of Europa informs us of the presence of molecular oxygen (O2) and molecular hydrogen (H2) and perhaps of their rates of formation. The O2 and H2 are produced by plasma irradiation of water ice, and it is likely that some of these gases do not escape from the ice but remain buried in it (as is observed, for example, on Ganymede). In principle, if the ice convects more rapidly than those species in the ice are destroyed, they might be transported downward to the water-ice interface. There, the O2 could serve either as an energy source or as a poison for possible organisms.

Also, because ion sputtering is an important process contributing to the atmosphere, atoms and molecules of all types that are present in the ice are released into the atmosphere. For example, ground-based spectroscopy in 1995 revealed the presence of sodium (Na) in the atmosphere. Additional detailed sampling of the atmosphere can provide information about these minor and even trace constituents of the ice. Key element ratios may be obtainable from analysis of atmospheric species. Thus, if the atmosphere is sampled in detail, so that its trace constituents are measured, the composition of the surface materials can be determined.



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