entering into the calculation or the possibility that some parameters may be correlated. Indeed, the task group explicitly assumes that all factors are independent.

NASA may decide that more detailed calculations and considerations are necessary or that the calculations for a particular mission show that the probability threshold of 10-4 is exceeded at some high level of confidence, given the error bars estimated for the various factors. Future studies such as those recommended in Chapter 7 will naturally change the numerical values in the required calculation.

MAJORITY AND MINORITY VIEWPOINTS

The task group was unable to reach complete consensus on a number of issues relevant to determining the appropriate planetary protection requirements for Europa. Two independent minority viewpoints were expressed by two subsets of the task group. Recognizing that reasonable people may disagree on the interpretation of complex scientific issues, the task group presents here the majority viewpoint and both minority viewpoints so that they may be discussed and retained for the historical record.

Applicability of the Current Planetary Protection Strategies to Europa

The first point of disagreement concerned the applicability to Europa of the current approach to planetary protection as recommended in NRC reports, as adopted by NASA, and as ratified by the international scientific community, embodied by COSPAR. According to this approach, the planetary protection measures applicable to a particular spacecraft depend on the type of mission envisioned and the degree to which its destination is of interest to studies of the processes of chemical evolution and/or the origin of life (see Chapter 1). Application of this methodology requires some detailed knowledge about the object to which the spacecraft is being sent.

One minority subgroup expressed the view that the current strategy of protection via categorization is broad enough to be applicable to Europa. Indeed, this approach has already been applied to recommendations for the prevention of back contamination when europan sample are returned to Earth.2 The implication of this minority view is that the first missions to Europa should be subject to a somewhat augmented version of the protocols currently applied to Mars missions. Thus, orbiters and simple landers would be subject to Viking-level cleaning, while landers with life-detection experiments and/or deep penetrators would be subject to a stricter Viking-level sterilization procedure. Suggested augmentations to the existing cleaning protocols for Mars missions would include assaying for radiation-resistant microbes in addition to spores and the use of molecular-based, cell-detection methods in addition to conventional culturing techniques.

The majority viewpoint is more conservative and argues that Europa must be treated as a special case. The basis for this viewpoint is the current relative ignorance of Europa’s possible biology, its possible subsurface ocean—which could allow life to be globally connected —and its possible geologic activity, which may recycle surface material into the ocean on a time scale comparable to the age of the surface, and may also provide a source of chemical energy in the form of organic debris and inorganic substrates entrained from the surface. The majority viewpoint is also based on the possibility that an impacting spacecraft could implant debris sufficiently deep within the ice that it would be protected from radiation.

Survival of Terrestrial Microbes on Europa

The second point of disagreement within the task group concerned the likelihood of the survival and proliferation of a terrestrial organism that might reach a europan ocean (see F7in the sample calculation contained in Appendix A).

This minority subset of the task group argued that while it is just conceivable that a terrestrial organism might survive in an oceanic environment on Europa, experience from studies of extreme terrestrial environments suggests that such an organism’s ability to grow and multiply—the real danger to future scientific studies and, potentially, to the survival of indigenous organisms, if they exist—is indistinguishable from zero.

This second subset asserted that no known terrestrial organisms could survive the successive assaults of cold, aridity, and radiation likely to be experienced in transit from Earth to Europa and then finally proliferate in a saline, oceanic environment under extreme hydrostatic pressure. They believe that the combination of physical and



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