Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and Earth. Planetary protection reflects both the frequently unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of planetary bodies until they can be studied in detail. The planetary protection policy maintained by the Committee on Space Research (COSPAR 2015) defines guidelines and specific requirements depending on the mission target and mission type based on the actual state of knowledge. New findings and new technology developments require the COSPAR planetary protection policy to be updated on a regular basis.
High-priority science goals, such as the search for life and the understanding of the martian organic environment, may be compromised if Earth microbes—that is, prokaryotic or eukaryotic single-cell organisms—carried by spacecraft grow and spread on Mars. This has led to the definition of “Special Regions” on Mars where strict planetary protection measures have to be applied before a spacecraft can enter these areas. The concept of a Special Region was developed as a way to refer to those places where the conditions might be conducive to microbial growth as we understand this process. In particular, this refers to places that might be warm and wet enough to support microbes that might be carried by spacecraft from Earth. COSPAR’s planetary protection policy defines a Mars Special Region as a “region within which terrestrial organisms may be able to replicate, OR a region which is interpreted to have a high potential for the existence of extant martian life. Given current understanding, Special Regions are defined as areas or volumes within which sufficient water activity AND sufficiently warm temperatures to permit replication of terrestrial organisms may exist. In the absence of specific information, no Special Regions are currently defined on the basis of martian life.”
The physical parameter space defined in COSPAR planetary protection policy (COSPAR 2015) for Special Regions is constrained by the following:
- Water activity: lower limit, 0.5; upper limit, 1.0;
- Temperature: lower limit, –25°C; no upper limit defined; and
- Timescale within which limits can be identified: 500 years.
In 2014, NASA requested the Mars Exploration Program Analysis Group (MEPAG) to review the definition of Special Regions. In particular, the MEPAG group SR-SAG2 (Special Regions Science Analysis Group 2) was asked to address a number of topics including the following:1
- “Reconsider information on the known physical limits of life on Earth . . .”
- “Evaluate new (i.e., since 2006) observational data sets and models from Mars that could be relevant to our understanding of the natural variations on Mars of water activity and temperature;” and
- “Reconsider the parameters used to define the term ‘special region;’ propose updates to the threshold values for temperature and water activity, as needed . . . ”
The result of this analysis was published as a journal article (Rummel et al. 2014). In response to parallel requests from the European Space Agency (ESA) and NASA, the European Science Foundation and the U.S. National Academies of Sciences, Engineering, and Medicine initiated a joint review of the SR-SAG2 report by an international group of experts, the Committee to Review the MEPAG Report on Mars Special Regions (hereafter the “review committee”).
The SR-SAG2 report provides findings about the Mars-relevant physical and chemical limits of life (as we know it), the various phenomena observed on Mars that might be indicative of a Special Region and possible mechanisms for their formation, and the considerations related to spacecraft-induced Special Regions. The findings are followed by a discussion of human spaceflight and, in particular, the resources needed to support humans on Mars. The report also discusses the findings and makes recommendations to COSPAR for consideration in updating the Special Regions definition in the COSPAR planetary protection policy.
The review committee discussed the SR-SAG2 report during two face-to-face meetings, via conference calls, and by email exchange. The committee notes that its statement of task (see the Preface) could be interpreted as requiring a review and update of the requirements levied on a spacecraft venturing into a Special Region. However, discussions with the planetary protection officers from NASA and ESA confirmed that the committee’s task was limited to a review of the definition of a Mars Special Region and related revisions to COSPAR’s planetary protection policy as proposed in the SR-SAG2 report. The review committee understands that its report, like the SR-SAG2 report, will inform the process by which COSPAR will revise and update its planetary protection policies.
The findings from the SR-SAG2 report were discussed by the committee in view of additional information from scientific publications not addressed by the SR-SAG2 report and from new knowledge obtained by ongoing space missions, field studies, and laboratory experiments. This included discussions about the breadth and depth of SR-SAG2 analysis with respect to survivability of life forms singularly versus in communities and SR-SAG2 approach to defining geographical areas as Special Regions. The review committee agreed with many of SR-SAG2’s individual findings, including retaining the current limits for life specified by COSPAR, but arrived at different conclusions in some cases and is of the opinion that a more detailed consideration is necessary (see Chapters 2 to 5). The review committee summarizes its comments concerning the findings and presents a new definition of Special Regions that changes the way geographical features are designated as Special Regions in Chapter 6. In Chapter 7, the review committee revisits the scientific basis of the bioburden assays used to assess the microbiological contamination of spacecraft and comments on the necessity of updating planetary protection requirements so that they are based on the latest scientific facts concerning the probability of life surviving in the martian environment.
This report concludes with a series of appendices containing the following information: Suggestions for future research that could reduce uncertainties in the identification of Special Regions on Mars (Appendix A); a complete listing of the findings from the SR-SAG2 report and, where appropriate, the review committee’s comments thereon (Appendix B); the letter from NASA requesting the Academies’ participation in this study (Appendix D); and brief biographies of committee members and staff (Appendix E).
In summary, the review committee reached the following conclusions:
1 See Rummel et al. (2014, Appendix A, pp. 945-946). Note that the identifiers “SR-SAG2 report” and “Rummel et al. 2014” are used interchangeably in this document.
1. The authors of the SR-SAG2 report are to be commended for their comprehensive review of the issues associated with Special Regions and the factors used to define them. The SR-SAG2 report contained 45 specific findings. Of these, the review committee does not support one (3-14), supports 13 in revised form (2-1, 2-4, 3-1, 4-1, 4-2, 4-8, 4-9, 4-14, 4-16, 5-3, 5-4, 5-7, and 5-9), suggests that two (4-6 and 4-7) be combined, proposes no changes for the remaining 29, and adds one new finding (6-1). The specific list can be found in Appendix B.
2. The environmental parameters used to define Special Regions (currently in the COSPAR policy and agreed upon in the SR-SAG2 report) of temperature and water activity are still appropriate. However, the review committee believes that if the detection of methane in the martian atmosphere—which may indicate biogenic activity—is confirmed, that may demand that the source region—that is, the location where methane is being produced—be designated as a Special Region.
3. The identification of Mars Special Regions is problematic for several reasons. First, detailed knowledge of the physical and chemical conditions of the surface and sub-surface of Mars at various scales is lacking, particularly the microscale. Second, current understanding of the ability of life to propagate is limited. It is not known if one, ten, or a million cells from a single species are required for propagation in an extraterrestrial environment. Alternatively, propagation may only be possible for microbial communities (i.e., collections of many different species). In view of the rapid development of powerful new techniques in biology and the increase in knowledge of the martian environment by ongoing and future space missions, the current practice of reassessing the concept of a Special Region and its definition every 2 years is both appropriate and essential.
4. The specific terrains identified as Special Regions in both the COSPAR policy and in the SR-SAG2 report (i.e., “gullies, and bright streaks associated with gullies, pasted-on terrains, subsurface below 5 meters, others, to be determined, including dark streaks, possible geothermal sites, fresh craters with hydrothermal activity, modern outflow channels, or sites of recent seismic activity” and “spacecraft-induced Special Regions”) are best regarded as “Uncertain Regions.” The final determination of a Special Region would depend on the review of the latest scientific knowledge about a specific site in order to verify if it is within the environmental parameters defining Special Regions, taking into consideration the potential existence of microscale habitats.
In addition, the review committee makes one recommendation.
Recommendation: Maps should only be used to illustrate the general concept of Special Regions and should not be used to delineate their exact location. Uncertain Regions in planned landing ellipses should be evaluated on a case-by-case basis as part of the site selection process. The goal of such an evaluation is to determine whether or not the landing ellipse contains water, ice, or subsurface discontinuities with a potential to contain hydrated minerals that could be accessed via a landing malfunction or by the operation of subsurface-penetrating devices (e.g., drills). As an example, landing site analysis will likely include a geological analysis, drawing on the Mars geologic literature (covering a broad range of relevant topics, including ground truth at previous lander locations) as well as orbital imaging, infrared spectroscopy, gamma-ray spectroscopy, and ground-penetrating radar sounding of the specific region.
Finally, the review committee proposes the following update to the definition of a Special Region (COSPAR 2015): A Special Region is defined as a region within which terrestrial organisms are likely to replicate. Any region which is interpreted to have a high potential for the existence of extant martian life forms is also defined as a Special Region.
Given current understanding of terrestrial organisms, Special Regions are defined as areas or volumes within which sufficient water activity AND sufficiently warm temperatures to permit replication of Earth organisms may exist. The physical parameters delineating applicable water activity and temperature thresholds are given below:
- Water activity: lower limit, 0.5; upper limit, 1.0;
- Temperature: lower limit, −25°C; no upper limit defined; and
- Timescale within which limits can be identified: 500 years.
Observed features for which there is a significant (but still unknown) probability of association with liquid water, and which should be considered as Uncertain Regions and treated as Special Regions until proven otherwise:
- Sources of methane (if identified);
- Recurring slope lineae;
- Gullies and bright streaks associated with gullies;
- Pasted-on terrains;
- Caves, subsurface cavities and subsurface below 5 meters; and
- Others, to be determined, including dark slope streaks, possible geothermal sites, fresh craters with hydrothermal activity, modern outflow channels, or sites of recent seismic activity.
Spacecraft-induced special regions are to be evaluated, consistent with these limits and features, on a case-by-case basis.
Organizations proposing to investigate any region that may meet the criteria above, have the responsibility to demonstrate, based on the latest scientific evidence and mission approach, whether or not their proposed landing sites are or are not Special Regions.
In the absence of specific information, no Special Regions are currently identified on the basis of possible martian life forms. If and when information becomes available on this subject, Special Regions will be further defined on that basis.