life elsewhere, it is tempting to try to initiate a spacecraft mission that will immediately search for europan life or return samples of surface ice to Earth for such analyses. However, the history of space exploration suggests that a phased approach, in which the results of one mission provide the scientific foundation for the next incremental advance, is more productive in the long term.

We need only look to the history of the search for life on Mars, however, to see the difficulty of crafting such an approach. While the Viking missions seemed very well conceived in 1970, they look naive today in the light of current understanding of the martian environment, and of the diversity of life on Earth and its ability to survive in extreme environments. Viking did not sample the most appropriate environments in its search for extant life on Mars. The results from the Viking biology experiments, though, have provided a remarkable foundation for an understanding of martian geochemistry that is playing a key role in knowing how and where to look for life on Mars today.

In a similar vein, the absence of identifiable surface environments that might support life or contain evidence of life on Europa and our complete lack of understanding of the chemical environment of the icy surface layer, the liquid water layer that may or may not underlie it, and the rocky interior of Europa suggest that a detailed exploration of the satellite will provide the best opportunity to answer these exciting questions.

Thus, COMPLEX recommends that Europa be explored within the framework of a well-conceived and planned strategy designed to create a scientific base of information that is sufficient to provide a global context for interpreting data pertaining to the possible presence of life on Europa. A comprehensive understanding of the geology, geochemistry, and geophysics of Europa, and of the nature of its atmosphere, is not strictly necessary in order to determine if liquid water is present. Knowledge of these is necessary, however, to assess the potential for life, and to determine whether life is present.

COMPLEX concludes that, should it turn out that liquid water is not present on Europa and has not been present in geologically recent times, the strong evidence for comparatively recent or ongoing geologic activity still makes it an appropriate target for exploration. However, the priority accorded Europa in the solar system exploration program and the sequence of exploration activities would have to be reassessed at that time.

The search for extinct or extant life on Mars, and the geophysical and geochemical analyses that are a fundamental part of the search, will provide substantial new insights into the environments in which life might exist and the precursor and resulting molecules that might obtain. Similarly, the search for life in extreme environments on Earth is providing key new insights into the potential for life elsewhere in the universe. In both cases, the new results need to be integrated into the ongoing Europa program to ensure a solid basis for investigation and analysis.

Thus, COMPLEX recommends that the search for evidence of present or past life on Europa, or for evidence of chemical evolution that has the potential to lead to life, should be coordinated with other aspects of the search for possible abodes of life in the solar system.

ELEMENTS OF A COMPREHENSIVE EXPLORATION PROGRAM

A comprehensive exploration of Europa that can address the major scientific goals will require a combination of spacecraft missions, ground-based telescopic observations, technology development, and supporting research and analysis. The scientific priorities for exploring Europa should proceed from the global to the local scale in searching for liquid water, determining the composition of the surface and near-surface ice, and exploring any pockets or oceans of liquid that might be discovered. The set of spacecraft missions to Europa that follows from this, then, likely should proceed from a polar orbiter, to landed experiments, to subsurface devices that can penetrate to depths necessary to reach liquid water. COMPLEX recognizes that implementation of such an ambitious sequence of spacecraft, with each being able to take advantage of results from the earlier missions, may require decades.

COMPLEX recommends that a staged series of missions be utilized to explore Europa, with the scientific focus of the first mission being to determine whether liquid water exists at the present epoch or has existed



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