present in the atmosphere or on the surface of Titan that cannot be simulated readily in the laboratory or analyzed satisfactorily in situ. For the high-altitude organic haze, the answer appears to be no, but this conclusion might not hold for organic phases that have undergone longer-term chemistry on the surface. The risk associated with sample returns from Titan should be small, since organic compounds found on the surface exist under cryogenic conditions (95 K), and organics in the upper atmosphere are processed by free-radical chemistry that breaks down biopolymers. In any event, the extreme distance of Titan from Earth will limit organic analysis on the latter to in situ experiments for the foreseeable future.
There are practical and societal reasons for ensuring planetary protection for all interplanetary missions (see the Session 2 paper by Rummel). Although the probability that an extraterrestrial life form could be pathogenic to humans, or even viable at all in the terrestrial environment, is very low, it cannot be shown to be zero. During the past few years it has become well established that microorganisms inhabit environments thought at one time to be too extreme to harbor life. Some of these environments are associated with geothermal or hydrothermal systems and thus exhibit high temperatures, high concentrations of heavy metals and volatiles, and acidic pHs. Other inhabited environments include desert rocks and deep subsurface basaltic aquifers. These findings have expanded the number of extreme environments on Earth and other solar system bodies that might harbor organisms.
One of the recommendations of a previous National Research Council (NRC) report for handling Mars samples returned to Earth is that they should be contained and treated as though potentially hazardous until proven otherwise.3 This recommendation poses problems in both sample containment and the kinds of analyses to be used for life detection. What criteria will be used to declare whether a returned sample is hazardous or not? A recent NRC report attempts to define these criteria and outlines necessary containment procedures.4
Important issues related to forward contamination include compromising the search for life and the possibility that Earth life could grow elsewhere, a possibility of particular concern for missions to Europa. 5 The essential issue in forward contamination is ensuring that the landing vehicle is free of contamination by Earth microorganisms. For back contamination, the important issues are identifying biosignatures for extraterrestrial life and developing methods for detecting these biosignatures at low levels (see Chapter 3).
1. 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., 1998.
2. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
3. Space Studies Board, National Research Council, Mars Sample Return: Issues and Recommendations, National Academy Press, Washington, D.C., 1997.
4. Space Studies Board, National Research Council, The Quarantine and Certification of Martian Samples, National Academy Press, Washington, D.C., 2002.
5. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.