Decisions about planetary protection of icy bodies and other solar system destinations must initially assess their habitability by considering environmental conditions that terrestrial microbes can tolerate and by evaluating empirical data for essential elements or other requirements (e.g., water, energy sources). The lack of water on dry rocky moons such as Io would mean that missions to this body would not require planetary protection. On the other hand, if the physical and chemical environment of an icy body might be compatible with the growth of terrestrial life, mission planners must assume it to be habitable. Knowledge acquired in areas of biological (primarily microbiological) science over the past 20 years provides important guidance for defining habitability for icy bodies. Researchers can define terrestrial life fairly precisely with regard to its composition and requirements for metabolic generation of energy. If the target site does not provide these basic needs, mission planners do not have to take special precautions normally associated with preventing forward contamination beyond the routine cleaning and monitoring of spacecraft. This approach restricts the number of bodies of concern for planetary protection requirements. Based on current understanding, the outer solar system icy bodies Europa, Enceladus, Titan, and Triton are most relevant to this discussion (see Chapter 4, and see Appendix B for a summary of exploration plans for icy bodies). It should be stressed that this committee’s designating a body as being habitable does not just refer to the surface of a body, but rather any microenvironments that might exist within the body (e.g., the subsurface, the atmosphere, and so on). The Decision Points 1-7 given in Figure 2.2 represent a hierarchical organization of environmental features that relate to habitability—from the most constraining to the least constraining. For example, since all terrestrial life requires liquid water, the complete absence of water would render all other considerations of habitability irrelevant for planetary protection.
Such considerations as those outlined in Chapter 2 and above led the committee to the definition of seven binary decision points. Subsequent subsections outline each of the decision points. A more detailed discussion of these decision points can be found in Chapters 4 and 5. The answers for different decision points will vary for different objects, as will the level of confidence. The framework’s language “do current data indicate …” makes the implicit statement that the preponderance of data supports a particular answer but that new information could strengthen or alter the outcome of the decision points.
Decision Point 1—Liquid Water
All life on Earth requires liquid water for protein-based enzymes to function properly. Even for those systems in which extracellular electron transport to an extracellular substrate occurs,1,2,3 liquid water remains an absolute requirement. Mission planners should consider any body that lacks liquid water to be non-habitable for terrestrial life.
Decision Point 2—Key Elements
All life on Earth requires carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and a large number of elements in trace concentrations: 70 in all are either required or influence the physiology and growth of various species.4 Specific transition metals often serve as electron acceptors and donors for catalytic activity or play a role in protein structure. Although the literature describes many of the biological functions of trace elements, researchers have far less information about minimum concentrations of the different trace elements required by organisms and their transport into the cell. In oligiotrophic aquatic environments, iron, molybdenum, and phosphorus limit the extent of primary production and thus other microbial autotrophic and heterotrophic metabolic activity. Because of its importance in all metabolic pathways, phosphate is likely the most important limiting nutrient for marine primary production.5 If mission planners can confidently demonstrate that the concentration of any one of these elements falls below minimal levels required for microbial growth, the icy body should be considered non-habitable.
Decision Point 3—Physical Conditions
Physical and chemical extremes restrict the distribution of life. Knowledge of how microbes solve the problems of growth in extreme conditions, such as temperature, pH, Eh, and other variables, expands as study of extreme environments develops. Nevertheless, physical extremes (e.g., temperatures above 122°C,6 or below −15°C) define the known temperature constraints for the replication of terrestrial (carbon-based) life, although metabolic activity can occur at temperatures as low as −20°C.7 The high temperature range relates to the stability of hydrogen bonds within liquid water at very high temperatures, whereas the low temperature range relates to the absence of available water molecules in the liquid state. If conditions outside the known limits exist throughout a target body, then it cannot support terrestrial life and should be considered non-habitable.
Radiation also presents a physical challenge to the survival of terrestrial organisms both during flight and on or near the surface of the target icy bodies. Radiation causes DNA double stranded breaks that must be repaired if an organism is to survive. However, as discussed below (see Decision Point 6) and in Chapter 5, these repair processes require complex organic compounds.
Decision Point 4—Chemical Energy
Life requires a source of chemical or solar energy. Typically electron donors (reductants) coupled with acceptors (oxidants) form electron transport chains that provide chemical energy for living cells. The discovery of microorganisms that use novel redox couples and are capable of surviving chemical and physical extremes previously thought to be inhospitable to life has widened the range of recognized habitable environments.8 In the terrestrial deep subsurface, some sources of electron donors and acceptors, such as the production of hydrogen by radiolysis of water, show that extreme geophysical environments analogous to those on icy bodies in the outer solar system can be the source of half reactions. In contrast to potential sources of chemical energy, light capture would not provide a useful energy source for terrestrial life forms on the surfaces of, or beneath the thick ice shells of, planetary bodies in the outer solar system. This latter scenario would require the unlikely evolution of a photosynthetic apparatus tuned to the spectral qualities of the subsurface photon source. The former scenario contamination is equally unlikely because the liquid water necessary for the survival of photosynthetic life forms would freeze, and such organisms would potentially be exposed to unsurvivable radiation fluxes. For these reasons, the subsequent discussion focuses on chemical energy sources.
Although there is no conclusive information available concerning the presence of the chemical energy and elemental sources necessary to support the growth of potential contaminating organisms on icy bodies, the committee assumes they are available. Electron donors (e.g., Fe2+, SH–, organic carbon) and electron acceptors (e.g., CO2, SO42−, O2, H2O2)9-13 might be present on some icy bodies. If new data from future missions unequivocally demonstrate the absence of electron donor-receptor pairs on a targeted icy body, then that body cannot support terrestrial life and should be considered non-habitable.
Decision Point 5—Contacting Habitable Environments
If a target site cannot be designated as non-habitable according to criteria outlined by Decision Points 1-4, then mission planners must consider the probability of the spacecraft coming into contact with potentially habitable regions (see Chapter 4). The decision framework does not differentiate on the basis of mission mode, i.e., flybys versus landers versus orbiters in orbits that are either stable or unstable. Instead, Decision Point 5 focuses on the geophysical features of the target body. If the probability of the spacecraft, spacecraft parts, or contents contacting a potentially habitable region as defined by Decision Points 1-4 is less than 10–4 within 1,000 years (i.e., over the time period of biological exploration), then no bioload reduction for planetary protection is required. Each mission must calculate the probability of contacting a habitable environment over the time period of biological exploration, based on the design and architecture of the mission, and based on the geophysical properties of the target body.
Decision Point 6—Complex Nutrients
If nutrient conditions available in liquid environments of an icy body are deemed insufficient to support growth and/or recovery from irradiation and/or desiccation (Chapter 5), then that body cannot support terrestrial life and should be considered non-habitable.
Decision Point 7—Minimal Planetary Protection
If nominal heat treatment (e.g., 60°C for 5 hours) or other bioload-reduction technologies cannot eliminate those physiological types that might have the capacity to grow on the target body (Chapter 5), mission planners must meet NASA’s Viking-level, terminal bioload specification (see Chapter 1). Failure to meet this final decision point would require total redesign or cancellation of the mission.
A series of decision points based on constraints defined by the preponderance of available scientific data or new information from future missions and research provide a robust mechanism for evaluating planetary protection requirements. The first and most critical decision point must consider whether liquid water is not available, followed by decision points describing the lack of availability of building blocks including the key elements carbon, nitrogen, phosphorus, and so on—the absence of environmental parameters known to be compatible with the growth of terrestrial life—and finally the lack of available energy sources required for terrestrial life. If negative answers to the initial Decision Points 1-4 fail to eliminate a requirement for planetary protection, mission planners must either demonstrate that the probability of a mission coming into contact with a habitable region is less then 10–4 over a 1,000-year time frame or that nutrient conditions will not support microorganisms’ growth and/or recovery from irradiation and desiccation. Finally, if nominal heat treatment at 60°C for 5 hours will not eliminate microorganisms that are likely to grow on the target body, then Viking-level terminal bioload reduction will be required.
Recommendation: NASA should adopt a binary hierarchical decision-making framework whereby affirmative answers to any decision point indicating the absence of a factor critical to life as currently known would eliminate further requirements for planetary protection measures.
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