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Mars Sample Return: Issues and Recommendations
ground to potential discoveries related to extinct or extant life on Mars. DNA and proteins of terrestrial origin could likely be unambiguously identified, but other organic material might not be so easily distinguished. The search for candidate martian organic biomarkers would be confounded by the presence of terrestrial material. Because the detection of life or evidence of prebiotic chemistry is a key objective of Mars exploration, considerable effort to avoid such contamination is justified.
Because the martian surface is so hostile to terrestrial life, forward contamination protection protocols specify a low bioburden rather than strict sterility. The measures required to avoid terrestrial contamination of returned sample material exceed those required to avoid forward contamination of Mars. Precautionary measures could include technologies used during the Viking missions, such as stringent cleaning with disinfectants and solvents, encapsulation of critical items with covers that can be removed on Mars, and general protection of sample pathways to isolate sample material from potentially contaminated surfaces.
The 1992 Space Studies Board report Biological Contamination of Mars: Issues and Recommendations (SSB, 1992) states that ''[l]anders … for … investigation of extant martian life should be subject to at least Viking-level sterilization procedures. Specific methods for sterilization are to be determined" (p. 47). The intent of this statement was to cite the Viking mission as an example of the successful application of techniques of bioburden reduction. It should not be interpreted as requiring the same whole-vehicle heat sterilization protocol for any lander carrying a life detection experiment. Indeed, other techniques may be both more effective and less costly.
The capacity for in-flight sterilization may be required for two reasons: (1) to decontaminate exterior portions of the canister, spacecraft, or other hardware and/or (2) to provide contingency sterilization in the event that sample containment cannot be verified. Candidate sterilization technologies include the use of heat, radiation, or chemical treatment.
Heat sterilization has been the most widely investigated technique (Hochstein et al., 1974), and various time and temperature protocols have been suggested, ranging from 24 hours at 150°C to 1 second at 500°C. Ionizing radiation may afford a less destructive route to sterilization, but implementation could be problematic. Chemical treatments that would ensure the destruction of unknown organisms would likely alter the sample material in ways that would reduce its value for subsequent scientific analysis. These candidate technologies will require further testing and development before they are ready for deployment on a sample-return mission.