Report Series: Committee on Planetary Protection Evaluation of Bioburden Requirements for Mars Missions (2021) / Chapter Skim
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3 Considerations for Reducing Bioburden Requirements
3 Considerations for Reducing Bioburden Requirements
Pages 15-38
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The committee bases its assessment of criteria for permitting some missions to land with relaxed spacecraft bioburden requirements on assessment of the contamination risk for each mission and landing site. Missions whose objectives include the search for evidence of Martian life (i.e., Category IVb missions)
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, in contrast to most other life forms that are termed heterotrophs. According to the Oparin-Haldane theory for the origin of life on Earth, the prebiotic synthesis of organic matter on Earth most likely initially promoted heterotrophic growth, with oxygenic photolithoautotrophy appearing nearly 1 billion years after the appearance of the earliest forms of microbial life.
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. At the surface and shallow subsurface temperatures of Mars, the risk of harmful contamination is negligible, unless a contaminated environment is connected to other habitable sites.
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the UVC radiation, ionizing radiation, and desiccation resistance characteristics of environmentally robust (extremotolerant) prokaryotes and simple eukaryotes; (2)
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. Microbial Survivability at the Martian Surface Over the last three decades, numerous microorganisms have served as models considering the potential for survival and proliferation of life beyond Earth, as well as the ability of life to survive long periods of metabolic dormancy in high-radiation, frozen, and desiccated states (Holm-Hansen 1967; Slade and Radman 2011; Sharma et al.
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Microbial Survivability in the Martian Subsurface A critical issue for the preservation of contaminating microorganisms in the Mars subsurface is ionizing radiation.3 If an organism similar to D radiodurans were to be maintained as fully cryptobiotic, or simply frozen and desiccated in a subsurface environment of Mars, the theoretical accumulated dose maximally allowed without overwhelming an ecological population might approach 50 kGy.
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Therefore, time aloft in the wind is relevant to define the planetary protection level for a given landing site, based on its proximity to potential habitable environments (p.
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. Nutrient Conditions Terrestrial extremotolerant organisms, which have been cultured and are thus available for study, require a source of macronutrients for recovery from radiation and desiccation; therefore, organisms delivered to subsurface environments in either desiccated or radiation-compromised states would likely 22
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After considering all the conditions and locations for relaxed bioburden requirements discussed above, the committee reached the following conclusion: 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.
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Subsurface Cavities The 2019 National Academies of Sciences, Engineering, and Medicine Astrobiology Strategy report recommended emphasis on research and exploration of subsurface environments based on the astrobiological potential of extraterrestrial caves. Martian caves, particularly lava tubes, are of astrobiological interest (Carrieret al.
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and thus could harbor metastable water ice (Williams et al.
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. At the Insight landing site, Manga and Wright (2021)
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PLANETARY PROTECTION IMPLICATIONS: CONDITIONS AND LOCATIONS FOR RELAXED BIOBURDEN REQUIREMENTS Mars Conditions Where Limits for Microbial Growth Are Met Modelled Subsurface Temperatures The committee adopts a lower temperature limit for microbial growth of 245 K (−28°C) (cf., discussion, p.
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Subsurface temperatures for buried water ice would be warmer than these modeled values because of its higher heat capacity and thermal conductivity than soil. Better constraints on material properties with depth and crustal heat flow are needed to improve estimates of subsurface temperatures.
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FIGURE 3.3 Modeled mean annual surface temperatures. Contours define 10 K temperature gradients.
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Thus the committee concludes that temperature and water activity conditions that allow subsurface microbial growth (as adopted in this report, namely, T >245 K (−28°C)
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Subsurface Ice Distribution Existing radar, neutron spectroscopy, thermal and high-resolution imaging data sets provide incomplete knowledge on where permafrost and buried ice sheets exist, with each data set having its own set of limitations in vertical and spatial resolution. Sounding radar from SHARAD and MARSIS have vertical resolutions of ~10 and 100 m, respectively, permitting resolution of the base of ice sheets (Bramson et al.
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Lower latitudinal limit of modeling capability is ±~35° (gray regions = no model results included)
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Potential Locations for Relaxed Bioburden Requirements Given the conditions described above, the committee assesses some potential regions on Mars where bioburden requirements for future missions might be relaxed. Knowledge about the Martian subsurface is incomplete, which poses challenges for finding locations that will avoid conditions for growth with certainty (p.
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To summarize the points discussed above, environments that permit microbial growth could be present in the top tens of centimeters of the shallow subsurface and below ~tens of meters in the deeper subsurface. However, such environments are likely discontinuous in (1)
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Exceptions to this finding include buffer zones around subsurface access points and sites of astrobiological interest. Permafrost, Subsurface Ice Sheets, and Polar Ice Permafrost and buried ice sheets in the northern mid-latitudes and polar ice, which can be relatively continuous over km scales (Bramson et al.
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. Additional regions of astrobiological interest that should be avoided (for missions with relaxed bioburden requirements)
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The committee interprets a designation of relaxed bioburden standards for some Mars landed missions to be consistent with a Category IV mission for which the maximum surface bioburden is less stringent than specified in current NASA and COSPAR policy but not equivalent to Category II, because there will still be some explicit approach to ensuring appropriate spacecraft cleanliness. Finding 7: To minimize the risk of harmful contamination, pre-launch cleanliness provisions are still needed for missions landing in regions of Mars with lower bioburden requirements than under current Category IV requirements.
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For mission activities as deep as 1 m, the landing site is in a location where no ice is detected in neutron or thermal data; AND 2) For both cases above, the landing site is a conservative distance from any subsurface access point, to be determined considering wind conditions for the location and season, and best estimates of microorganism survival time in the surface UV environment.
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