As stated in Chapter 8, the proper design and implementation of planetary protection measures will ultimately be critical to the overall success of martian sample-return missions. Mission success can be most effectively promoted by integrating planetary protection measures into the engineering and design of a sample-return mission as early as possible in the planning phases. Implementing these measures, while preserving the scientific utility of the returned samples and containing overall mission costs, will pose a number of technical challenges. Research and development will be required to advance the available technologies for sample containment, sample sterilization methods that preserve geochemical and other data, in-flight verification of containment, and in-flight sterilization. Sterilization may be a particularly difficult issue. NASA will need to undertake research aimed at the definition and implementation of appropriate sterilization procedures for all phases of a Mars sample-return mission, up to and including the controlled distribution of returned material.
AVOIDING CONTAMINATION OF RETURNED SAMPLES WITH ORGANISMS OR ORGANIC MATERIAL OF TERRESTRIAL ORIGIN
It will be important to stringently avoid the possibility that terrestrial organisms, their remains, or organic matter in general could inadvertently be incorporated into sample material returned from Mars. Contamination with terrestrial material would compromise the integrity of the sample by adding confusing back-
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.
SAMPLE HANDLING AND PRESERVATION
It will be necessary to monitor and record the environment to which the sample material has been exposed from the time of acquisition until it is delivered to the ground-based receiving facility. Parameters to be recorded may include temperature, gas environment (pressure and composition), radiation exposure, exposure to magnetic fields, and exposure to shock and acceleration. Scientific considerations may dictate that some of the environmental parameters be controlled during the return flight and reentry. For example, it may be desirable to maintain the sample under ambient martian conditions (cold, dry) at all times. If in-flight sterilization of the returned sample becomes necessary and if heat is chosen as the means of effecting sterilization, it will be desirable to trap and contain the evolved gas products for subsequent analysis.
ENSURING SAMPLE CONTAINMENT
Canisters for returned samples must provide a physical barrier that prevents expulsion or migration of materials of martian origin, or materials exposed to the martian environment, outside of sealing points. This does not necessarily require that canisters be hermetically sealed (gas tight), as long as adequate filtration is provided to prevent transfer of biological entities.
As recommended in Chapter 4, specific measures should be taken to monitor the integrity of sample containment during all phases of a sample-return mission. If containment integrity cannot be verified by remote monitoring during the transit back to Earth, the sample, and any spacecraft components potentially exposed to it, should either be sterilized or not returned to Earth. One means of ensuring maximum safety would be to target the return vehicle away from Earth until, upon near approach to Earth, containment is verified, at which time the required trajectory corrections can be implemented.
Ensuring containment through reentry, descent, and landing will be technically challenging because of the potentially large accelerations and short reaction times that characterize this phase of a mission. All credible failure modes should be examined. For example, the possibility of parachute deployment failure should be accounted for in designing a rugged sample canister. All prudent precautions should be taken to maximize the likelihood of a mild entry event. Such precautions include improved targeting accuracy, tracking aids, and midair retrieval to avoid touchdown impact.
AVOIDING RETURN OF UNCONTAINED MARTIAN MATERIAL
To date, two different methods have been proposed to avoid the return of unsterilized material of extraterrestrial origin that is not strictly contained. One
approach involves aseptic transfer of the sample canister through a biobarrier to a receiving spacecraft that returns the sample to Earth. A possible variant of this approach, which avoids hand-off to a second craft, is to pass the sample canister through a biobarrier that fully encloses the Earth reentry vehicle on Mars. Once in space, the biobarrier would be opened like a cocoon to release the Earth reentry vehicle. Technical challenges include the selection of biobarrier materials, designing practical pass-through and sealing mechanisms, and validating the method.
In an alternative approach, all spacecraft surfaces that could be exposed to the martian surface would be coated with a pyrotechnic material that would be ignited in space during the return trip to Earth in order to heat the surfaces and sterilize any attached martian material. As with any method, extensive validation and testing would be required. Such testing would include analysis of the efficacy of heating across a steep gradient and avoidance of ablation or partial detachment of surface material that could defeat sterilization.
It must be assumed that putative martian organisms will be resistant to ultraviolet radiation and able to tolerate high vacuum. Possible mechanisms of biotic transfer within and on the return spacecraft must be considered. Such mechanisms include vibrations and shocks and micrometeorite erosion of surfaces, including a possibly dusty near-Mars environment owing to the proximity of two moons, Phobos and Deimos.