chemical extremes on Europa has no counterpart on Earth, so that no terrestrial organism could have adapted simultaneously to all of them.

Although less than 1 percent of all living species have been characterized to date, both the physiological ecology and the behavior of microbial communities, as well as the environments to which terrestrial microorganisms can adapt, are reasonably well studied. The minority maintained that the known facts are sufficient to form scientifically valid conclusions about the survival and proliferation of terrestrial organisms on Europa.

It argued further that even if organisms that had simultaneously adapted to all the extreme environmental parameters on Europa did exist on Earth, the probability that a spacecraft would be contaminated with significant numbers of these organisms is infinitesimally small. This minority subset would nonetheless be willing to subject future Europa missions of all types to the Viking-level cleaning procedures, so as to significantly reduce their initial bioload.

A majority of the members of the task group did not accept these views. They recommended a more conservative approach and set the probability of proliferation at the relatively small, but finite, value of 10-6(see F7 in the Appendix A). They argued that prudence is necessary given the variety of life seen in extreme environments on Earth, our ignorance of the extremes of life’s adaptability, and our lack of knowledge of the europan ocean. As we learn more, F7, like the other probability factors discussed in Appendix A, may be updated.


The majority viewpoint is that a common standard should be set according to which, for every mission to Europa, the probability of contaminating a europan ocean with a viable terrestrial organism at any time in the future should be less than 10-4per mission. NASA would establish the assays and calculations for confirming this figure. The two independent minority viewpoints would both allow future missions to Europa to be governed by the (possibly updated) standards for planetary protection of Mars.


1 See, for example, COSPAR, “Resolution 26 COSPAR Position with regard to the Florence Report of its Consultative Group on Potentially Harmful Effects of Space Experiments,” Article 5, COSPAR Information Bulletin No. 20, 1964, page 26.

2 Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies, National Academy Press, Washington, D.C., 1999.

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