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

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement