MACROSCOPE
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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