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Preventing the Forward Contamination of Europa
The task group recommends a number of studies that would improve knowledge of Europa and that would better define the issues related to minimization of forward contamination. These include studies on the following topics:
Ecology of clean room and spacecraft-assembly areas, with emphasis on extremophiles such as radiation-resistant microbes;
Detailed comparisons of bioload assay methods;
Desiccation- and radiation-resistant microbes that may contaminate spacecraft during assembly;
Autotroph detection techniques; and
Europa’s surface environment and its hydrologic and tectonic cycles.
The task group was unable to reach complete agreement on the central issue of the planetary protection standards that must be met by future missions to Europa. The majority if its members believe that Europa ’s potential importance to studies of chemical evolution and the origin of life is great but that detailed understanding of the europan environment and the survival of terrestrial organisms in extreme conditions is so limited that the current planetary protection methodology is not readily applicable to Europa missions. Uncertainties demand conservatism, and, thus, the very first mission to Europa must meet the highest reasonable level of safeguard.
In practice, this means that the bioload of each Europa-bound spacecraft must be reduced to a sufficiently low level at launch that delivery of a viable organism to a subsurface ocean is precluded at a high level of probability. This approach allows mission planners to take advantage of the bioload reduction likely to occur en route, particularly while in Jupiter’s radiation environment. One consequence of this view is that Europa must be protected from contamination for an open-ended period, until it can be demonstrated that no ocean exists or that no organisms are present. Thus, we need to be concerned that over a time scale on the order of 10 million to 100 million years (an approximate age for the surface of Europa), any contaminating material is likely to be carried into the deep ice crust or into the underlying ocean.
Thus, the task group’s majority concluded that spacecraft sent to Europa must have their bioload at launch reduced to such a level that, taking into account the natural additional reduction that occurs after launch, the probability of contaminating a europan ocean with a viable terrestrial organism at any time in the future should be less than 10-4 per mission. How this standard might be implemented by a combination of Viking-level cleaning and sterilization, accompanied by bioload reduction in the europan radiation environment, is illustrated by a probabilistic calculation offered by the task group (Appendix A).
In addition to the majority view, this report presents two independent minority viewpoints that argue for less stringent planetary protection requirements.*
The minority viewpoints supporting less-stringent planetary protection procedures than those advocated by the majority are based on two independent arguments. One subset of the task group argued that the planetary protection provisions for Europa should be broadly consistent with the current policies, practices, and protocols. The other subset argued that studies of the organisms found in extreme terrestrial environments suggest that no known terrestrial organism has a significant probability of surviving and multiplying in a europan ocean. The practical consequences of both of these views is that Europa missions should be subject to essentially the same planetary protection requirements that are currently applied to Mars missions. That is, spacecraft (including orbiters) without biological experiments should be subject to at least Viking-level cleaning, but sterilization is not necessary.