The clarification of the biological potential of the martian surface had a major impact on planetary protection policies for Mars and, by extension, other solar system bodies. NASA’s current planetary protection requirements and those for Mars, in particular, derive from a policy adopted at COSPAR’s 25th General Assembly, held in Graz, Austria, in 1984,7 as refined in a 1992 NRC report on the topic.8 The key feature of COSPAR’s 1984 policy was the abandonment of the quantitative, statistical approach used in the Viking era and the adoption of a simpler, more straightforward methodology based on the type of mission (e.g., flyby, orbiter, lander, or sample return) and the degree to which the mission ’s destination is of interest to the process of chemical evolution.

The 1992 NRC report refined the COSPAR approach by drawing a distinction between Mars missions that carry instruments designed to search for evidence of life and those that do not carry them. Since terrestrial organisms are unlikely to grow on the martian surface, the report argued, they do not pose a significant contamination hazard. They could, however, confound the results from life-detection experiments. Thus, the report recommended that landers carrying instrumentation for in situ investigation of extant martian life “should be subject to at least Viking-level sterilization procedures” (see Box 1.1).9 Orbiters and landers without biological experiments, on the other hand, “should be subject to at least Viking-level presterilization procedures—such as clean-room assembly and cleaning of all components —for bioload reduction, but such spacecraft need not be sterilized. ”10 The NRC’s recommended distinction between Mars landers with and without life-seeking experiments was later codified and adopted by COSPAR.11 ,12

NASA’s implementation of these policies, described in Planetary Protection Provisions for Robotic Extraterrestrial Missions, involves adherence to the following procedures:13

  • Spacecraft that fly by or enter orbit around Mars are subject to planetary protection requirements designed to control contamination and to reduce the risk that spacecraft or its boosters will impact the planet. This is achieved by assembling the spacecraft in clean rooms rated at Class 100,000 or better (i.e., less than one particle in the size range 1 mm to 0.001 µm for every 100,000 cubic feet of air) and by ensuring that the probability of impact by the launch vehicle and the flyby spacecraft does not exceed 10 -4and 10-2, respectively. The lifetime of an orbiter must be such that it remains in orbit for a period in excess of 20 years from launch, and the probability of impact for the next 30 years must be no more than 0.05. If the lifetime requirements cannot be met, then the surface microbial bioburden must meet the Viking presterilization limit. Following bioassay, such spacecraft must be protected against recontamination.

  • Spacecraft that land on Mars but are not equipped with life-detection experiments are subject to planetary protection requirements designed to control the lander’s bioburden and to prevent accidental impact by hardware not intended to land. The total probability of any accidental impacts by any hardware other than the lander must be no more than 10-4. Bioburden control involves assembly in a Class 100,000 or better clean room, periodic microbiological assays, and maintenance of hardware cleanliness. Bioburden reduction to the Viking presterilization level is required. The mission team is also required to inventory, document, and archive samples of organic compounds used in the construction of the lander and associated hardware that might accidentally impact the planet. Finally, the locations of landing sites and impact points must be determined as accurately as possible, and the condition of the hardware at each site must be estimated to assist in determining the potential spread of organic compounds.

  • Spacecraft that land on Mars and are equipped with life-detection experiments are subject to all of the requirements outlined above and must, in addition, undergo a Viking-level sterilization process.

Although the bioburden reduction employed for all types of landers may be measured by any microbiological assay, it is incumbent on the project to prove the equivalency between its assay and that employed on Viking. Moreover, no allowance can be made for burden reduction in flight or associated with surface conditions on Mars.

The central question to be addressed in this report concerns the degree to which Europa can be incorporated into the planetary protection framework developed in light of 40 years of experience with the exploration of Mars. In other words, is our current knowledge of Europa and its ability to sustain terrestrial organisms analogous to our understanding of martian conditions before the Viking missions or after them? In an attempt to answer this question, Chapter 2 focuses on current understanding of Europa and Chapter 3 discusses the limits of terrestrial life.



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