Planetary protection is the term given to the practice of protecting solar system bodies (i.e., planets, moons, comets, and asteroids) from contamination by Earth life (so-called forward contamination) and protecting Earth from possible life forms that may be returned from other solar system bodies (so-called back contamination). The 1967 United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Bodies states that all countries party to the treaty “shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination.” Internationally, technical aspects of planetary protection are developed through deliberations between space agencies and national and international scientific organizations, and the international consensus policy is maintained by the Committee on Space Research (COSPAR), an interdisciplinary committee of the International Council of Science, which consults with the United Nations in this area.
In COSPAR’s Planetary Protection Policy (COSPAR 2015), planetary protection requirements for each mission are categorized according to the nature of the target body (e.g., a planet, moon, comet, or asteroid) and the type of encounter the spacecraft will have with it (e.g., flyby, orbiter, or lander). Specific outbound mission target and/or mission type combinations are organized into four planetary protection categories (Category I to IV), depending on the degree to which the target body is likely to provide clues about the origins of life and chemical evolution. Planetary bodies of little interest to such studies (e.g., Mercury) are assigned to Category I, and no specific planetary protection requirements are levied. However, a spacecraft landing on a target body of interest to the origins of life and chemical evolution that has a significant chance of contamination by Earth life (e.g., Mars) is assigned to Category IV and must undergo stringent cleaning and bioload-reduction processes. Missions returning extraterrestrial samples to Earth are assigned to Category V, a planetary protection classification reserved for inbound missions.
NASA and ESA maintain planetary protection policies and administer associated procedures to ensure compliance with them. The planetary protection officers of both agencies oversee compliance with formal implementation requirements that are assigned to each mission. Agency policies are informed by the most current scientific information available about the target bodies and about life on Earth.
Planetary protection policies are not static but evolve over time based on the increasing knowledge and understanding of both planetary environments and the physical and chemical limits of terrestrial life. Conclusions and recommendations generated by internal and external advisory groups chartered by space agencies such as NASA and ESA are weighted and assessed in an iterative manner by COSPAR’s Panel on Planetary Protection (PPP). Consensus policy recommendations developed by the PPP are then forwarded for discussion and ultimate approval
by COSPAR’s Bureau and Council prior to becoming official COSPAR policy. The development of the concept of Special Regions on Mars is a good example of how planetary protection policies are developed and evolve as new information becomes available.
Observations conducted by NASA’s Mars Global Surveyor in the late-1990s and early-2000s led to the discovery of transient activity in martian gullies suggesting that liquid water may have flowed on the surface of Mars in recent times (see, for example, Malin and Edgett 2000). This discovery had an important impact on planetary protection, demonstrating that some regions may be more suitable to life than others (Meltzer 2011).
In April 2002, COSPAR and the International Astronomical Union convened a workshop in Williamsburg, Virginia, to discuss planetary protection policies (Rummel 2002). The workshop resulted in a revision of COSPAR’s policies and, in particular, established a new mission category—Category IVc—for spacecraft accessing a Special Region on Mars (COSPAR 2003, pp. 67-74). COSPAR defined a Special Region as a zone “within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant martian life forms. Given the current understanding, this is to apply to regions where liquid water is present or may occur. Specific examples include, but are not limited to: subsurface access in an area and to a depth where the presence of liquid water is probable, penetrations into the polar caps, [and] areas of hydrothermal activity.” (COSPAR 2003, p. 71).
In 2005, NASA adopted COSPAR’s concept of a Special Region within its planetary protection policy. In addition, NASA requested the National Research Council1 (NRC) to conduct a study to assess the body of policies, requirements, and techniques designed to protect Mars from Earth-originating organisms that could interfere with and compromise scientific investigations (NRC 2006, p. 1). The resulting NRC report, Preventing the Forward Contamination of Mars, concluded that there was insufficient data to distinguish between Special Regions on Mars and regions that are not special (NRC 2006, pp. 4 and 61-63). The committee proposed a new classification system, which would replace COSPAR’s Categories IVa through IVc, with Category IVn for Non-Special Regions and Category IVs for Special Regions (NRC 2006). In addition, the NRC committee commented: “Until measurements are made that permit distinguishing confidently between regions that are special on Mars and those that are not, NASA should treat all direct-contact missions (i.e., all category IV missions) as Category IVs missions” (NRC 2006, p. 118-119). In other words, the NRC recommended that all of Mars be considered a Special Region until additional observational data with better resolution can be obtained. If implemented, this recommendation required that all Mars landers be subjected to the most stringent—so-called Viking-level—bioload reduction procedures.
The programmatic consequences of subjecting all Mars landers to Viking-level bioload reduction led NASA to request that the Mars Exploration Program Analysis Group (MEPAG) charter a so-called Science Analysis Group (SAG) to look at Special Regions. In particular, the MEPAG group—SR-SAG—was asked “to develop a quantitative clarification of the definition of ‘special region’ that can be used to distinguish between regions that are ‘special’ and ‘non-special’” and to undertake “a preliminary analysis of specific environments that should be considered ‘special’ and ‘non-special’” (Beaty et al. 2006).
The SR-SAG found that COSPAR’s definition of Special Regions needed additional clarification; specifically, the uses of the words propagate and likely, which can have different meanings and interpretations (Beaty et al. 2006, p. 684). The SR-SAG also constrained physical variables that could be used to define a Special Region, such as the following: how long they exist (about 100 years), the maximum depth of penetration by a spacecraft (about 5 m into the crust), and the lower limit for the survival of terrestrial life in terms of temperature (–15°C or –20°C including margin) and water activity (0.62 or 0.5 including margin) (Beaty et al. 2006, pp. 684-691). The SR-SAG report concluded by proposing a new definition of Special Region that retained the original COSPAR definition and added to it a set a clarifications and implementation guidelines (Beaty et al. 2006, p. 719).
1 Effective July 1, 2015, the institution is called the National Academies of Sciences, Engineering, and Medicine. References in this report to the National Research Council (NRC) are used in a historical context to refer to activities before July 1.
In 2007, COSPAR held a Mars Special Regions Colloquium, with the goal of reviewing the conclusions and recommendations contained in both the 2006 NRC and MEPAG (Beaty et al. 2006) reports and devising a consolidated definition of Special Regions. The report of the COSPAR Colloquium (Kminek et al. 2010) disagrees with the NRC 2006 report by stating that there is sufficient data to distinguish between “special” and “non-special regions” and it differs from SR-SAG report by reducing the lower temperature limit for the survival of terrestrial life from –20° C to –25° C (Kminek et al. 2010). The colloquium report also recommended that the definition of a Special Region and the list of terrains classified as “special” be reviewed every 2 years (Kminek et al. 2010).
MEPAG empaneled a new science analysis group (SR-SAG2) in the latter part of 2014 to revisit the concept of Special Regions on Mars following the recommendation of the COSPAR colloquium to review the standards every 2 years. The SR-SAG2 used the following general approach (Rummel et al. 2014):
- Clarifying the terms in the existing COSPAR definition;
- Establishing temporal and spatial boundary conditions for the analysis;
- Reviewing the data sets on the limits of microbial life and the availability of water on Mars;
- Identifying applicable threshold conditions for propagation;
- Evaluating the distribution of the identified threshold conditions on Mars;
- Analyzing on a case-by-case basis those purported environments on Mars that could potentially meet or exceed the biological threshold conditions;
- Describing conceptually the possibility for spacecraft-induced conditions that could exceed the threshold levels for propagation; and
- Considering the impact of special regions on potential future human missions to Mars.
The resulting SR-SAG2 report provided a comprehensive distillation of the current understanding of the limits of terrestrial life and relevant martian conditions and presented an analytical approach for considering special regions using current and future improvements in knowledge. The SR-SAG2 report determined that the lower limit for temperature should be –18°C and water activity (aw) above 0.60 (Rummel et al. 2014, pp. 894-898) and updated the list of features on Mars that should be classified as “special,” “non-special,” and “uncertain” regions.2 In reference to human missions, the SR-SAG2 found that although these locations would be preferable for potential resources, human missions should not contaminate special regions, and precautions should be taken to avoid converting non-special regions to special regions (Rummel et al. 2014).
The review committee understands that its report and SR-SAG2 report will be formally presented to and discussed at an international workshop, organized by COSPAR’s PPP, to be held in Bern, Switzerland, on September 22-24, 2015. The workshop and successor activities are part of the process COSPAR is using to revise and update its planetary protection policies. Recommendations from the PPP will ultimately be forwarded to COSPAR’s Bureau and Council for action and potential incorporation in COSPAR planetary protection policy.
The review committee notes that its statement of task (see the Preface) appeared to contain two separate items: to “review the current planetary protection requirements for Mars Special Regions” and to review “their proposed revision as outlined in” the SR-SAG2 report. The first of these items could be interpreted as reviewing the requirements levied on a spacecraft venturing into a Special Region. However, extensive discussions between the committee and the planetary protection officers from NASA and ESA confirmed that the task was to discuss the requirements defining a Mars Special Region and modifications to those requirements as proposed in the SR-SAG2 report.
2 According to the SR-SAG2 report (p. 888), uncertain regions are defined as follows: “Uncertain Regions. If a martian environment can simultaneously demonstrate the temperature and water availability conditions identified in this study, propagation may be possible, and those regions would be identified as Special Regions. Nonetheless, because of the limited nature of the data available for regions only sensed remotely, it may not be possible to prove that such environments are capable of supporting microbial growth. Such areas are therefore treated in the same manner as Special Regions until they are shown to be otherwise.”
Given this understanding of the task, the review committee’s assessment of the SR-SAG2 report revealed that the major items needing additional consideration and deliberations fell into three broad categories.
- The potential for terrestrial organisms to survive and proliferate when subjected to environmental conditions likely to be found on Mars. Important aspects of this topic include the following: current understanding of the physical and chemical limits for the survival of life on Earth, life in extreme environments, properties of multispecies communities, advantages of cells living in biofilms over planktonic single cells, the detectability of small-scale microbial habitats, and the processes likely to transport terrestrial contamination from a spacecraft landing site into a Special Region. Discussion of these topics can be found in Chapter 2.
- The relationship between martian geological, hydrological, and mineralogical features and Special Regions. Important aspects of this topic include the following: biotic and abiotic sources of methane on Mars; gullies, polar slope streaks, recurring slope lineae, and related features; snow and ice deposits; subsurface environments, caves, and cavities; and the phenomenon of deliquescence. Discussion of these topics can be found in Chapter 3.
- Issues not falling into the two previous categories. Such topics include considerations relating to human spaceflight (Chapter 4), the utility or otherwise of maps to delineate Special Regions and the implicit buffer zones around them (Chapter 5), new considerations relating to the definition of Special Regions (Chapter 6), and aspects of planetary protection not discussed in the SR-SAG2 report (Chapter 7).
The main text of this report is complemented by appendixes containing the following: suggestions for future research (Appendix A), the findings from SR-SAG2 and the committee’s suggested revisions and updates (Appendix B), a glossary of technical terms and acronyms (Appendix C), the letter from NASA requesting the Academies’ participation in this study (Appendix D), and biographical information on the committee and staff (Appendix E).