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4 Biosignature Identification and Interpretation
Pages 64-86

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From page 64...
... In order to qualify as biosignatures, these "features must be sufficiently complex and/or abundant so that they retain a diagnostic expression of some of life's universal attributes" (Des Marais et al.
From page 65...
... The first steps toward developing a comprehensive framework that could be used to interpret potential biosignatures, abiosignatures, false positives, and false negatives, and increase confidence and consensus in interpretations. This work has progressed in parallel for both in situ biosignatures (e.g., those preserved in rock or ice that can be searched for on the surface of Mars or Europa)
From page 66...
... . While the current field of novel biosignature identification for in situ biosignatures focuses on structures believed to represent life as we know it -- particular classes of molecules or isotopic signatures, chirality, or molecular weight patterns for fatty acids or lipids -- the goal for the emerging field of agnostic biosignatures is to expand the ability to search for life as we do not know it through exploration of a broader definition of life based on activity, with less dependence on assumptions about structure and specific biogeochemistry (Johnson et al.
From page 67...
... If the goal of an in situ life detection experiment is to detect Earth-like life, or terrestrial contaminants thereof (Box 6.2) , then the powerful technologies of nucleic acid amplification and sequencing can be applied (Box 5.1)
From page 68...
... In addition to disequilibria, the community is now considering how biosignatures can be quantified using a Bayesian framework that can generalize search strategies beyond the biosignatures of known life. This Bayesian methodology will help quantitatively define the conditional probabilities and confidence of future life detection and may constrain the prior probability of life even without a positive detection (Walker et al.
From page 69...
... A cellular or acellular organism might exchange genetic information through a variety of mechanisms where the exchanged information itself becomes the object of evolutionary optimization and innovation, thereby contributing to the diversity of life. SURVIVABILITY OF BIOSIGNATURES Arguably, the question of survivability and preservation -- especially on a global planetary scale, in the rock record and in the atmosphere, and on planetary timescales -- that may extend to billions of years has received less attention than research into biosignature reliability and detectability.
From page 70...
... Indeed, the isotopic record of stable carbon isotopes in Earth's geological record, especially in carbonates, has been used to infer the oxidation state of this planet. Similarly, the pattern of sulfur isotopes has changed over geological time on Earth and has been used to infer when the atmosphere of Earth contained ozone, a gas that cannot exist in a planetary atmosphere without molecular oxygen.
From page 71...
... The deleterious effects of heating organic matter during burial or volcanism are relatively well understood, for example. Consistent with the relatively high degree of thermal metamorphism, there is no extractable organic matter preserved in Archean sediments known to have retained features that can be attributed to primary biological origins (French et al.
From page 72...
... 72 AN ASTROBIOLOGY STRATEGY FOR THE SEARCH FOR LIFE IN THE UNIVERSE FIGURE 4.2 Top: Plan-view of domal stromatolites from the 3.45 Ga Pilbara Strelley Pool Formation. B ­ ottom: Cross-sectional view of stromatolites from the Strelley Pool Formation.
From page 73...
... . DETECTABILITY OF BIOSIGNATURES Although biosignature preservation is an important consideration for molecular in situ biosignatures, "false negatives" can challenge remote-sensing and in situ biosignatures alike.
From page 74...
... Finding: Although suggestive of life and worthy of follow-on investigation, thermodynamic disequilibrium may result from a range of abiotic and biological processes and is therefore not always a biosignature. Recommendation: NASA should support expanding biosignature research to address gaps in understand ing biosignature preservation and the breadth of possible false positives and false negative signatures.
From page 75...
... Since the 2015 Astrobiology Strategy (NASA 2015) , there has been a growing realization of the importance of false positives, or abiosignatures -- abiotic planetary processes that can mimic biosignatures, even for biosignatures like O2 that were previously believed to be "robust" and have no known abiotic means of production.
From page 76...
... False Positives for the O2 Global Biosignature Since publication of the 2015 Astrobiology Strategy, multiple research groups have discovered mechanisms that could produce abiotic O2 and O3 in an exoplanet's atmosphere, especially in the atmospheres of those exoplanets orbiting M-dwarf stars. Each presents a potential false positive to different degrees.
From page 77...
... . Finally, O3 may be considered a proxy for O2 in a planetary atmosphere, and large abundances of abiotic O3 may build up in the massive O2-rich atmospheres, possible after ocean loss (Meadows et al.
From page 78...
... By understanding false positive mechanisms and their discriminants, observing strategies are now being developed that incorporate biosignature detection as well as stellar characterization, searches for false positive discriminants, and other environmental characteristics that can be used to enhance the interpretation of the biosignature. Comparative planetology can help identify relevant physical and chemical processes in planetary environments that could lead to the generation of a biosignature false positive or contribute to a false negative result.
From page 79...
... An analogous activity has been undertaken for exoplanet biosignatures, mediated by the community discussions at the 2016 NExSS Exoplanet Biosignatures Workshop. The template for this framework has been developed for O2, taking into account false negatives and their impact on target selection, and false positives and the observing strategy required to discriminate them from true biosignatures (Meadows et al.
From page 80...
... 2012. Sulfur isotopes of organic matter preserved in 3.450 billion-year-old stromatolites reveal microbial metabolism.
From page 81...
... 2015. Tracing biosignature preservation of geothermally silicified microbial textures into the geological record.
From page 82...
... 2017. Evolution of Earth-like extrasolar planetary atmospheres: assessing the atmospheres and biosphere of early Earth analog planets with a coupled atmosphere biogeochemical model.
From page 83...
... 2018b. "Agnostic Biosignatures: Towards a More Inclusive Life Detection Strategy." White paper submitted to the Committee on an Astrobiology Science Strategy for the Search for Life in the Universe.
From page 84...
... 2015. NASA Astrobiology Strategy 2015.
From page 85...
... 2017. False negatives for remote life detection on ocean bearing planets: Lessons from early Earth.
From page 86...
... Pp. 133-159 in Strategies of Life Detection (O.


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