Summary
For ages, scientists and philosophers have pondered the origin, prevalence, and nature of life in the universe. That quest for knowledge is manifest today in NASA’s goals for exploration of the solar system and the universe, especially concerning Mars. Evidence for persistent liquid water on ancient Mars, significant amounts of contemporary water ice, the planet’s proximity to Earth, and its similarities with Earth as a terrestrial planet, have made Mars important in the search for existing or extinct extraterrestrial life.
Since the 1980s, national and international planetary protection policies have specified that Mars lander missions should meet documentation, cleanroom assembly, partial sterilization, and bioassay monitoring requirements to characterize and reduce the bioburden delivered to Mars. These requirements seek to avoid contamination by terrestrial organisms that could compromise future investigations regarding the origin or presence of Martian life.
Over the last decade, the number of national space agencies planning, participating in, and undertaking missions to Mars has increased, and private-sector enterprises are engaged in activities designed to enable commercial missions to Mars. The nature of missions to Mars is also evolving to feature more diversity in purposes and technologies. As missions to Mars increase and diversify, national and international processes for developing planetary protection measures recognize the need to consider the interests of scientific discovery, commercial activity, and human exploration. The implications of these changes for planetary protection should be considered in the context of how much science has learned about Mars, and about terrestrial life, in recent years. There is now an opportunity to take a more nuanced, and in some cases, more permissive approach to activities on Mars.
This report responds to NASA’s request for a short report by the Committee on Planetary Protection to identify criteria for determining locations on Mars potentially suitable for landed robotic missions that satisfy less stringent bioburden requirements than current NASA policy imposes to manage the risk of forward contamination.1 In this report, the committee attempts to take the increasingly diverse set of mission objectives into account.
No scientific consensus exists about how to distinguish extraterrestrial life on another solar system body from contamination by terrestrial organisms carried by spacecraft. Thus, planetary protection protocols aimed at avoiding contamination remain necessary to prevent compromising future investigations of extraterrestrial life. In particular, minimizing proliferation by terrestrial microorganisms is critical for maintaining the viability of future searches for indigenous Martian life. In addition, despite the increase in scientific information about Mars, much about its surface and subsurface remains underexplored, creating the need for caution in avoiding contamination harmful to future scientific investigations of extinct or extant life on Mars.
Harsh conditions on much of the surface of Mars, including the ultraviolet (UV) radiation environment, paucity of persistent liquid water, and humidity-and-temperature cycles, make survival, growth, and proliferation of terrestrial organisms on the surface unlikely. (This report focuses on conditions affecting proliferation—including temperature, presence of water, dust, radiation environment and potential transport (namely wind)—individually; however, cumulative effects from the combination of these conditions may lead to complicated physiological outcomes, and may be even more biocidal than when considered individually). Likewise, portions of the Martian subsurface, down to a
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1 NASA statement of task to the committee, February 22, 2021 (see Appendix A for the full statement of task).
depth of approximately 1 m and lacking evidence of ice, are not environments where terrestrial organisms could proliferate. Thus, the presence of terrestrial organisms on the surface or in these portions of the subsurface are, by themselves, unlikely to compromise future investigations of Martian life.
However, terrestrial organisms delivered to the surface of Mars could survive and be transported by wind or robotic device to some regions of the Martian subsurface where such organisms could grow and proliferate. These subsurface regions include (1) locations such as caves where water ice deposits and salt and brine deposits might exist, and (2) the deep subsurface, where underground aquifers are hypothesized to exist. These areas might also be where evidence of indigenous Martian organisms is most likely to be found. These subsurface regions remain poorly identified, and those regions located by remote sensing are largely unexplored and uncharacterized scientifically.
The environment over much of the surface of Mars and in portions of the uppermost subsurface (upper 10 cm) is hostile to terrestrial organisms. This context creates a potential basis for changing the bioburden reduction requirements for robotic missions to Mars that (1) interact only with the Martian surface and uppermost subsurface; and (2) do not land or operate near subsurface areas that require continued protection from harmful contamination by terrestrial organisms. In addition, the development of a risk management approach similar to that currently used by NASA and the aerospace industry (though not yet for planetary protection) could provide a flexible, effective way to implement planetary protection requirements in order to comply with revised or existing planetary protection measures for robotic missions that land on Mars.
In response to its charge from NASA, the committee offers the following specific findings:
Finding 1: The discovery of indigenous life on Mars would be a signal event in the development of human knowledge, with widespread impact and implications. Preserving unambiguous separation or distinguishability of terrestrial organisms from indigenous Martian organisms, by application of planetary protection protocols, or by other scientifically accepted means, is essential to realizing NASA’s solar system exploration goals and addressing profound questions that have long preoccupied humans.
Finding 2: The environment on Mars makes the survival, growth, and proliferation of terrestrial organisms on the surface, or suspended in the atmosphere, highly unlikely as a source of harmful contamination. However, transport of a viable terrestrial organism to potentially habitable subsurface environments, such as caves, creates a risk of harmful contamination.
Finding 3: Some regions of the Martian subsurface appear to be the most promising environments for (1) finding potential extant or extinct indigenous Martian organisms; and (2) providing terrestrial organisms with conditions that might support their survival and proliferation. However, the Martian subsurface remains largely unexplored and uncharacterized.
Finding 4: Microbial transport and proliferation are highly unlikely in disconnected subsurface environments. Thus, relaxed bioburden requirements could be appropriate for missions that do not access the subsurface, or for missions that access the subsurface (down to ~1 m)2 where no evidence
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2 Because ice is not detectable between ~1-10 m with existing thermal, neutron, or sounding radar subsurface reflection data, depths below 1 m could potentially harbor more continuous ice. The same connectivity concerns described below for permafrost and ice sheets (p. 40) apply to any putative ice environments below 1 m, hence the depth limit in this finding is restricted to 1 m. (The committee acknowledges that sounding radar surface returns can be used to search for low-density materials consistent with ice within the upper 5 m [Morgan et al. 2021]. However, uncertainty associated with determining the exact depth within the upper 5 m that any ice-like returns arise from, as well as the apparent spatial scatter in some of the data, presently introduces complexity and uncertainty in using the existing radar surface return data sets as a means to locate ice-free regions at depths below 1 m.) Although temperatures at depths below 1 m and down to several tens of meters are likely too cold for growth, uncertainty
of ice exists. Exceptions to this finding include buffer zones around subsurface access points and sites of astrobiological interest.
Finding 5: Estimates of habitat connectivity and of brine transport within subsurface ice are needed to evaluate the risk of harmful contamination via microbial proliferation as a result of subsurface mission activities within permafrost, ice sheets, or polar ice. Such estimates would be improved with additional observations of mid-latitude and polar ice structure, porosity, and salt content, at a range of scales.
Finding 6: To avoid contamination of subsurface access points and sites of astrobiological interest, a mission with relaxed bioburden requirements would need to land and operate at a conservative buffer distance from such locations. The buffer distance is to be determined considering wind conditions for the location and season, and best estimates of microbial survival time in the surface environment, especially considering UVC radiation.
Finding 7: To minimize the risk of harmful contamination, some pre-launch cleanliness provisions are still needed for missions landing in regions of Mars with lower bioburden requirements than under current Category IV requirements. Contamination risks remain given the current uncertainty in models, lack of observational coverage of surface and subsurface temperatures and subsurface access points, and possibilities of off-nominal landings.
Finding 8: Planetary protection requirements for Mars missions can be met using a risk management approach as an alternative approach to meeting current NASA planetary protection requirements found in NASA Procedural Requirements Document 8020.012D.
Finding 9: In situ bioburden reduction may present a cost-effective alternative or complement to prelaunch bioburden control and recontamination prevention measures. Appropriate validation methods for in situ bioburden reduction need to be developed. NASA’s planetary protection research and analysis program could develop such techniques for bioburden reduction and validation.
Box S.1 summarizes the committee’s principal conclusions in response to the charge from NASA. Box S.2 presents the findings on a risk management approach to comply with planetary protection requirements for Mars-bound missions.
Finally, the committee’s findings about meeting planetary protection objectives and requirements apply specifically to missions for which NASA has responsibility for planetary protection. For commercial missions in which NASA has no role or connection, the U.S. government still needs to designate a regulatory agency with responsibility to authorize and continually supervise the space activities of nongovernmental entities in accordance with the Outer Space Treaty.
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about habitat connectivity in notional ice raises concerns about the potential for transport to a more favorable location where growth and subsequent proliferation could occur (Morgan et al. 2021).
