NASA’s exploration of planets and satellites over the past 50 years has led to the discovery of water ice throughout the solar system and prospects for large liquid water reservoirs beneath the frozen shells of icy bodies in the outer solar system. These putative subsurface oceans could provide an environment for prebiotic chemistry or a habitat for indigenous life. During the coming decades, NASA and other space agencies will send flybys, orbiters, subsurface probes, and, possibly, landers to these distant worlds in order to explore their geologic and chemical context and the possibility of extraterrestrial life. Because of their potential to harbor alien life, space agencies will select missions that target the most habitable outer solar system objects. This strategy poses formidable challenges for mission planners who must balance the opportunity for exploration with the risk of contamination by terrestrial microbes that could confuse the interpretation of data from experiments concerned with the origins of life beyond Earth or the processes of chemical evolution. To protect the integrity of mission science and maintain compliance with the mandate of the 1967 Outer Space Treaty to “pursue studies of outer space, including the Moon and other celestial bodies…so as to avoid their harmful contamination,”1 NASA adheres to planetary protection guidelines that reflect the most current experimental and observational data from the planetary science and microbiology communities.2
The 2000 National Research Council (NRC) report Preventing the Forward Contamination of Europa3 recommended that spacecraft missions to Europa must have their bioload reduced by such an amount that the probability of contaminating a Europan ocean with a single viable terrestrial organism at any time in the future should be less than 10–4 per mission.4 This criterion was adopted for consistency with prior recommendations by the Committee on Space Research (COSPAR) of the International Council for Science for “any spacecraft intended for planetary landing or atmospheric penetration.”5 COSPAR, the de facto adjudicator of planetary protection regulations, adopted the criterion for Europa, and subsequent COSPAR-sponsored workshops extended the 10–4 criterion to other icy bodies of the outer solar system.6,7
In practice, the establishment of a valid forward-contamination-risk goal as a mission requirement implies the use of some method—either a test or analysis—to verify that the mission can achieve the stated goal. The 2000 Europa report recommended that compliance with the 10–4criterion be determined by a so-called Coleman-Sagan calculation.8,9,10 This methodology estimates the probability of forward contamination by multiplying the initial bioload on the spacecraft by a series of bioload-reduction factors associated with spacecraft cleaning, exposure to the space environment, and the likelihood of encountering a habitable environment. If the risk of contamination falls below 10–4, the mission complies with COSPAR planetary protection requirements and can go forward. If
the risk exceeds this threshold, mission planners must implement additional mitigation procedures to reach that goal or must reformulate the mission plans.
The charge for the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System called for it to revisit the 2000 Europa report in light of recent advances in planetary and life sciences and examine the recommendations resulting from two recent COSPAR workshops. The committee addressed three specific tasks to assess the risk of contamination of icy bodies in the solar system.
The first task concerned the possible factors that could usefully be included in a Coleman-Sagan formulation of contamination risk. The committee does not support continued reliance on the Coleman-Sagan formulation to estimate the probability of contaminating outer solar system icy bodies. This calculation includes multiple factors of uncertain magnitude that often lack statistical independence. Planetary protection decisions should not rely on the multiplication of probability factors to estimate the likelihood of contaminating solar system bodies with terrestrial organisms unless it can be unequivocally demonstrated that the factors are completely independent and their values and statistical variation are known.
The second task given to the committee concerned the range of values that can be estimated for the terms appearing in the Coleman-Sagan equation based on current knowledge, as well as an assessment of conservative values for other specific factors that might be provided to the implementers of missions targeting individual bodies or classes of objects. The committee replaces the Coleman-Sagan formulation with a series of binary (i.e., yes/no) decisions that consider one factor at a time to determine the necessary level of planetary protection. The committee proposes the use of a decision-point framework that allows mission planners to address seven hierarchically organized, independent decision points that reflect the geologic and environmental conditions on the target body in the context of the metabolic and physiological diversity of terrestrial microorganisms. These decision points include the following:
1. Liquid water—Do current data indicate that the destination lacks liquid water essential for terrestrial life?
2. Key elements—Do current data indicate that the destination lacks any of the key elements (i.e., carbon, hydrogen, nitrogen, phosphorus, sulfur, potassium, magnesium, calcium, oxygen, and iron) required for terrestrial life?
3. Physical conditions—Do current data indicate that the physical properties of the target body are incompatible with known extreme conditions for terrestrial life?
4. Chemical energy—Do current data indicate that the environment lacks an accessible source of chemical energy?
5. Contacting habitable environments—Do current data indicate that the probability of the spacecraft contacting a habitable environment within 1,000 years is less than 10–4?
6. Complex nutrients—Do current data indicate that the lack of complex and heterogeneous organic nutrients in aqueous environments will prevent the survival of irradiated and desiccated microbes?
7. Minimal planetary protection—Do current data indicate that heat treatment of the spacecraft at 60°C for 5 hours will eliminate all physiological groups that can propagate on the target body?
Positive evaluations for any of these criteria would release a mission from further mitigation activities, although all missions to habitable and non-habitable environments should still follow routine cleaning procedures and microbial bioload monitoring. If a mission fails to receive a positive evaluation for at least one of these decision points, the entire spacecraft must be subjected to a terminal dry-heat bioload reduction process (heating at temperatures >110°C for 30 hours) to meet planetary protection guidelines.
Irrespective of whether a mission satisfies one of the seven decision points, the committee recommends the use of molecular-based methods to inventory bioloads, including both living and dead taxa, for spacecraft that might contact a habitable environment. Given current knowledge of icy bodies, three bodies present special concerns for planetary protection: Europa, Jupiter’s third largest satellite; Enceladus, a medium-size satellite of Saturn; and Triton, Neptune’s largest satellite. Missions to other icy bodies present minimal concern for planetary protection.
The advantage of the decision framework over the Coleman-Sagan approach lies in its simplicity and in its abandoning of the multiplication of non-independent bioload reduction factors of uncertain magnitude. At the same
time, the framework provides a platform for incorporating new observational data from planetary exploration missions and the latest information about microbial physiology and metabolism, particularly for psychrophilic (i.e., cold-loving microbes) and psychrotolerant microorganisms.
The committee’s third task concerned the identification of scientific investigations that could reduce the uncertainty in the above estimates and assessments, as well as technology developments that would facilitate implementation of planetary protection requirements and/or reduce the overall probability of contamination. The committee recognizes the requirement to further improve knowledge about many of the parameters embodied within the decision framework. Areas of particular concern for which the committee recommends research include the following:
• Determination of the time period of heating to temperatures between 40°C and 80°C required to inactivate spores from psychrophilic and psychrotolerant bacteria isolated from high-latitude soil and cryopeg samples, as well as from psychrotolerant microorganisms isolated from temperate soils, spacecraft assembly sites, and the spacecraft itself.
• Studies to better understand the environmental conditions that initiate spore formation and spore germination in psychrophilic and psychrotolerant bacteria so that these conditions/requirements can be compared with the characteristics of target icy bodies.
• Searches to discover unknown types of psychrophilic spore-formers and to assess if any of them have tolerances different from those of known types.
• Characterization of the protected microenvironments within spacecraft and assessment of their microbial ecology.
• Determination of the extent to which biofilms might increase microbial resistance to heat treatment and other environmental extremes encountered on journeys to icy bodies.
• Determination of the concentrations of key elements or compounds containing biologically important elements on icy bodies in the outer solar system through observational technologies and constraints placed on the range of trace element availability through theoretical modeling and laboratory analog studies.
• Understanding of global chemical cycles within icy bodies and the geologic processes occurring on these bodies that promote or inhibit surface-subsurface exchange of material.
• Development of technologies that can directly detect and enumerate viable microorganisms on spacecraft surfaces.
1. United Nations, Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, U.N. Document No. 6347, Article IX, January 1967.
2. M. Meltzer, When Biospheres Collide: A History of NASA’s Planetary Protection Programs, NASA SP-2011-4234, NASA, Washington, D.C., 2011.
3. National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
4. National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
5. The recommendation to accept the 10–4 criterion was made at the 7th COSPAR meeting in May 1964 (see COSPAR, Report of the Seventh COSPAR Meeting, Florence Italy, COSPAR, Paris, 1964, p. 127, and, also, COSPAR Information Bulletin, No. 20, November, 1964, p. 25). The historical literature does not record the rationale for COSPAR’s adoption of this standard. Subsequent policy changes restricted the 10–4 standard to Mars missions (COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. A1, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
6. COSPAR Panel on Planetary Protection, COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies, European Space Policy Institute, Vienna, Austria, 2009.
7. COSPAR Panel on Planetary Protection, COSPAR Workshop on Planetary Protection for Titan and Ganymede, COSPAR, Paris, France, 2010.
8. C. Sagan and S. Coleman, Spacecraft sterilization standards and contamination of Mars, Astronautics and Aeronautics 3(5), 1965.
9. C. Sagan and S. Coleman, Decontamination standards for martian exploration programs, pp. 470-481 in National Research Council, Biology and the Exploration of Mars, National Academy of Sciences, Washington, D.C., 1966.
10. J. Barengoltz, A review of the approach of NASA projects to planetary protection compliance, IEEE Aerospace Conference, 2005, doi:10.1109/AERO.2005.1559319.