The Search for Life on Europa
The first Europa lander should investigate a site where liquid water from the ocean has recently reached the surface. However, it is difficult on the basis of current knowledge to determine where these sites may be (or even if any exist). The Europa Orbiter mission will be crucial in helping to decide where to land. Galileo spacecraft-based models for Europa's geology are evolving rapidly, and there is no guarantee that they will converge to the correct model. When first described (37), chaos regions seemed to provide candidate locations where the ocean may have reached the surface through catastrophic melt-through events. Now, however, models of viscous creep in Europa's ice argue against this explanation (74). Whether large cracks represent sites where ocean water reaches Europa's surface on a diurnal basis remains controversial, but if so they might be of special interest in a search for life (41). It is unclear how to interpret europan "ponds," which seem to indicate the eruption of liquid water from a subsurface source (35). However, if we had to choose a site for the first europan lander based on Galileo data alone, and assuming the ability to target a region only kilometers across, we might well recommend landing in such a place. Consistent with the recommendations of a recent National Academy of Sciences committee (9), the exploration of Europa should be seen as analogous to that of Mars, demanding a systematic program.
Chemical context should be established before or simultaneous with any biology experiments. Appropriate measurements would include abundances of the major cations and anions present, the salinity, the pH, an analysis of the volatiles (e.g., CO2, O2, CH4, etc.) present in the water, and a search for organic molecules. In fact, the latter probably represents the highest-priority "biology" experiment to be conducted. Additional experiments might include high-sensitivity searches for specific indicative organic molecules (such as amino acid enantiomers), a determination of key stable isotope ratios (such as 12C/13C) or fluorescent microscopy.
Any search for life on Europa should either scan a large amount of material in a manner that chooses particular sites for subsequent high-sensitivity investigation, and/or take advantage of the opportunity to concentrate sample by melting and filtering (or perhaps evaporating) ice.
Current estimates (61) of charged-particle flux and gardening suggest that substantially radiation-processed material may extend down to 1 m on Europa for 107-yr-old terrain. Ideally, sample acquisition would take place below the processing depth. This emphasizes the importance of targeting the youngest terrain (where the gardening depth will be less), and of improving our models for impact gardening on Europa.
Planetary Protection It is unclear whether any terrestrial microorganism could withstand a spacecraft journey to Europa plus subsequent transportation to and survival in Europa's ocean. But the fact that we can already speculate about possible europan ecologies using terrestrial analogies suggests that the recommendations of a recent National Research Council study (75) should be taken seriously until our knowledge improves: Spacecraft to Europa should have their bioload at launch reduced to a level consistent with a very low probability of contaminating a europan ocean with viable terrestrial microorganisms.