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Planning Considerations Related to the Organic Contamination of Martian Samples and Implications for the Mars 2020 Rover

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Planning Considerations Related to the Organic Contamination of Martian Samples and Implications for the Mars 2020 Rover ASTROBIOLOGY Volume 14, Number 12, 2014 News & Views ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2014.1244 Planning Considerations Related to the Organic Contamination of Martian Samples and Implications for the Mars 2020 Rover 2014 Organic Contamination Panel: R.E. Summons1 and A.L. Sessions2 (co-chairs); A.C. Allwood,3 H.A. Barton,4 D.W. Beaty,5 B. Blakkolb,6 J. Canham,7 B.C. Clark,8 J.P. Dworkin,9 Y. Lin,6 R. Mathies,10 S.M. Milkovich,6 and A. Steele11 Table of Contents 1. Executive Summary 970 2. Introduction 972 2.1. Introduction to the Mars 2020 OCP 972 2.1.1. Mars and the potential for habitability 972 2.1.2. Charge to the OCP 972 2.1.3. Introduction to the proposed Mars 2020 Mission 974 2.1.4. Definition of ‘‘organic contamination’’ 975 2.1.5. A note about units 975 2.2. Previous work on organic contamination control of acquired samples 975 2.3. Key concepts 980 2.3.1. Terrestrial microbial life forms (alive or dead) as sources of organic molecular contaminants 980 2.3.2. Analytical method limits of detection and contamination limits 982 2.3.3. Not all contaminants are equal 982 2.3.4. Contamination control versus contamination knowledge 983 2.4. Science and PP objectives both drive need for organic analyses 983 3. Sample-Based Investigations and Measurements 984 3.1. Need for early survey measurements 985 3.2. Potential analytical methods for returned samples 985 3.3. Survey versus targeted analytical methods 987 3.4. Survey analytical methods 987 4. Sample-Based Contaminants of Concern 988 4.1. General considerations 988 4.1.1. Selection criteria for choosing contaminants of concern 988 4.1.2. S/N threshold for acceptable contamination 989 4.1.3. Limits for TOC, individual molecules, or particles 989 4.1.4. Alive versus dead microbial contamination 990 4.1.5. The possibility of reproduction of Earth-sourced microbial contaminants in sealed sample tubes 990 4.2. Considerations related to specific contaminants 990 4.2.1. Which contaminants? 990 4.2.1.1. Tier-I contaminants 991 4.2.2. Allowable levels of contamination 991 4.2.2.1. What analyte concentrations do we expect? 992 4.2.2.2. What concentrations can we measure? 996 4.2.2.3. What level of cleanliness can we achieve? 996 4.2.3. Conclusions for specific compound levels 996 1Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts. 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California. 3Planetary Science and 5Mars Exploration Directorate, 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California. 4Department of Biology, University of Akron, Ohio. 7ATK, Washington, D.C. 8Space Science Institute, Boulder, Colorado. 9NASA Goddard Space Flight Center, Greenbelt, Maryland. 10Department of Chemistry, University of California Berkeley, Berkeley, California. 11Geophysical Laboratory, Carnegie Institution for Science, Washington, D.C. 969 970 SUMMONS ET AL. 4.3. Considerations related to TOC 997 4.3.1. Allowable levels of contamination 997 4.3.1.1. What analyte concentrations do we expect? 997 4.3.1.2. What concentrations can we measure? 997 4.3.1.3. What level of cleanliness can we achieve? 999 4.3.1.3.1. Contamination pathways 999 4.3.1.4. Contamination transfer from sample-contacting surfaces 1000 4.3.2. Conclusions for TOC levels 1002 4.4. Considerations related to particulate organic matter 1002 4.4.1. Introduction 1002 4.4.2. Analytical approaches to measuring particulates on Earth 1002 4.4.3. Limits on organic particulates 1003 4.4.4. Conclusions and recommendations for particulates 1004 4.5. Implementation 1004 4.5.1. Strategy for implementing contaminant requirements 1004 5. Strategies for Recognizing and Characterizing Organic Contamination 1005 5.1. Introduction 1005 5.2. Witness plates 1005 5.3. Blanks and blank standards 1007 5.4. Archive of organic and trace biological materials 1008 5.5. Spatially resolved measurements on returned samples 1010 6. Discussion and Proposals for Future Work 1011 6.1. The case for cleaner 1011 6.2. Summary and conclusions 1012 6.3. Topics for future work 1013 Acknowledgments 1014 References 1014 A. Appendices and Supporting Files 1020 A.3. Appendix 3: Definitions of terms 1020 A.4. Appendix 4: Summary of instruments and measurements available as of 2014 for investigating organic molecules in rock and soil samples 1024 A.4.1. Notes regarding detection limits and capability of surface spectroscopic techniques 1024 1. Executive Summary Central to these objectives would be the ability to reliably differentiate indigenous martian organic molecules from ata gathered during recent NASA missions to terrestrial contamination in any future samples returned DMars, particularly by the Rovers Spirit, Opportunity, from Mars. and Curiosity, have provided important insights into the Early on during its deliberations, the OCP recognized past history and habitability of the Red Planet. The Mars that the scientific and planetary protection (PP) objectives science community, via input through the National Re- of MSR are intimately linked, in that both rely heavily on search Council (NRC) Planetary Science Decadal Survey measurements of organic molecules in the returned sam- Committee, also identified the prime importance of a Mars ples. In each case, a key aspect of the problem is being sample return (MSR) mission to further exploration of able to recognize and interpret organic molecules as in- the Red Planet. In response, the Mars 2020 Mission (Mars digenous to Mars against a potential background of Earth- 2020) Science Definition Team (SDT) (Mustard et al., sourced contamination. It was within this context that the 2013) was chartered by the NASA Mars Exploration Pro- OCP committee considered the structure for a set of gram to formulate a new rover mission that would take measurement goals related to organic molecules in the concrete steps toward an eventual sample return. The SDT returned samples that would be of common interest to recommended that the 2020 rover should select and cache science and PP. scientifically compelling samples for possible return to The following is a summary of the most significant Earth. They also noted that organic contamination of the findings of the OCP regarding organic geochemical mea- samples was a significant and complex issue that should be surements that would be shared for both science and PP in independently investigated by a future committee. Ac- relation to potential future MSR. cordingly, NASA chartered the Mars 2020 Organic Con- tamination Panel (OCP). Rationale The OCP was charged with evaluating and recommend- ing sample contamination requirements for the proposed A key subset of both scientific and PP objectives could Mars 2020. A further task was to assess implementation be met by a common set of organic geochemical approaches in support of the investigation of broad scientific measurements of returned samples. The science and PP questions concerning the history and habitability of Mars. teams would need to work as a fully integrated entity. ORGANIC CONTAMINATION OF MARTIAN SAMPLES 971 Detection and characterization of indigenous organic considered particularly problematic. Tier-II compounds compounds are of fundamental and critical importance in comprise all other organic molecules. the search for ancient and extant life in martian samples. Much of the organic contamination that accumulates on Because of the sensitivity of modern analytical instru- the collected samples would be delivered from space- ments, we must accept that we would not be able to craft surfaces by direct contact. reduce all organic contaminants to non-detectable lev- In the hypothetical case of a system with sample contact els by all analytical techniques. surface area of 30 cm2 and contaminated with 20 ng/cm2 Our ability to correctly interpret data from partially organic carbon, collected samples would have a theo- contaminated samples would depend on three major retical maximum of 40 ng/g (i.e., 40 ppb) organic con- factors: (1) minimizing contamination at the start, (2) taminants. The actual contaminant concentration may be characterizing and understanding residual contamina- less than 40 ng/g, depending on transfer efficiency. tion throughout the mission, and (3) minimizing re- In order to achieve contamination levels for sample contamination back on Earth. contact surfaces lower than 20 ng/cm2, a more effective strategy for avoiding recontamination after initial Recommended approach cleaning than that used by the Mars Science Laboratory Maintaining the original physical structure and geom- (MSL) would need to be implemented. 2 etry of the samples (e.g., layering, gradients, grain Cleaning spacecraft surfaces to levels of 10–20 ng/cm boundaries, and cross-cutting relationships) and mini- has been achieved in prior missions. Significantly lower mizing vibration and fracturing are critical to inter- levels of cleanliness are technologically feasible, and preting indigenous organic geochemistry since the advisable, but would require engineering solutions to limit spatial distribution patterns of molecules can be espe- recontamination and the maintenance of these levels. cially informative. Methods used for assessing hardware surface contam- A huge diversity of techniques for organic analysis ination should be demonstrated to have a known, re- exists as of 2014. More will be invented. Not all po- producible efficiency in detection of the target Tier-I tential measurements would be possible on returned compounds. Mars samples given limited sample mass, nor would all We propose a threshold for total organic carbon (TOC) be needed. Containment requirements may also limit contamination of geological samples of 40 ng/g. access to some potential analytical methods. Accord- Molecular measurements provide a proxy for estimat- ingly, a two-step process, comprising initial survey ing total cellular/microbial particulate contamination measurements followed by more targeted analyses, is that is much more robust and universal than microbial recommended.
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