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Infecting Other Worlds

B. Randall Tufts, Richard Greenberg

Tainting an Alien Ocean

Recognizing that Europa might be vulnerable to forward contamination, NASA recently commissioned a study by the National Research Council to evaluate standards of planetary protection for upcoming missions. At the heart of the resulting report, issued just last year, is a specific recommendation: “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.” The rationale given for this number was citation of the 1964 COSPAR resolution for Mars, a source that is neither appropriate nor relevant.

In fact, the proposed Europa standard is flawed for a completely different—and rather disturbing—reason. According to several members of the committee that prepared the report (including a statement by its chair), the recommended 10–4 value did not really come from the COSPAR resolution they cited; rather it was simply a compromise among the subjective judgments of the members of the group. The reference to COSPAR was added afterward to lend an appearance of objectivity.

Before NASA proceeds too much further with planning for its Europa campaign, the scientific community needs to reopen the discussion of proper guidelines for preventing forward contamination. The starting point might be Sagan and Coleman’s preserve-it-until-we-are-done-with-it, the prime directive, the natural contamination standard or some other principle that gains a consensus. Once such a principle is in place, quantitative standards for mission design, construction and operations can be developed using the kinds of scientific information assembled in the National Research Council report.

Another important reason for continuing discussion is that knowledge of Europa has increased tremendously—even since that report was issued—thanks to interpretation of data from the Galileo mission. There is now a considerable body of evidence that the icy crust has cracks and openings that may connect with the interior ocean. So the Europan biosphere, if it exists, may well extend to within centimeters of the surface. Any assessment of the probability of forward contamination should include the latest understanding of these conditions.

The new work makes Europa an even more inviting target for exploration, because its ice-covered ocean appears more likely to be able to support life, and organisms might be available for sampling more easily. Of course, Europa would also be even more vulnerable to forward contamination than most planetary astronomers had thought possible just a year or two ago. Recognizing the potential vulnerability, NASA should not continue with exploration of the Jovian system—indeed, the agency should probably not go too far even in planning—unless proper deliberations and calculations are included to ensure that humanity will do no inadvertent harm to any neighbors in space.

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