An Astrobiology Strategy for the Exploration of Mars (2007) / Chapter Skim
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3 Biosignatures and Abiotic Chemistry
3 Biosignatures and Abiotic Chemistry
Pages 41-56
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From page 41... ...
These organic materials are pervasive in Earth's crust and constitute an extensive chemical and isotopic record of past life that far exceeds what is recorded by visible fossils. 1 The ubiquity of coal, organic-rich black shales, and petroleum hydrocarbons, for example, is one manifestation of life's activities that extends deep into the geological record and can be used to observe past biological activity and events.
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From page 42... ...
It is safe to assume that organic compounds that might have contributed to the prebiotic potential of the planet could have been synthesized elsewhere in the solar system or in interstellar space and then carried to the surface of Mars via carbonaceous chondrites and interplanetary dust particles. Since there is no consensus about the past history of prebiotic processes on Mars, it is more constructive to first consider the availability of the elements that constitute organic matter.
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From page 43... ...
combined with the key role it plays in biological processes suggests that organic nitrogen compounds would be an important potential biosignature. 14 Organic geochemists coined the term "biological marker compound" or "biomarker" to describe individual organic compounds that serve as molecular biosignatures.15–17 Biomarkers comprise a spectrum of biomolecules spanning those that are present in living systems (biomarkers for extant life)
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From page 44... ...
21,22 Another important set of molecular biosignatures can be identified, based on the observation that all known organisms utilize a universal subset of small metabolites as generic building blocks for constructing biomass and more complex biomolecules.23 The 20 amino acids of proteins, the four nucleotides of DNA, and the acetate precursor of most lipids are prime examples of generic building blocks. This simple fact, so fundamental to life on Earth, leads to patterns in the molecules of life and in the molecular remains of past life.
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From page 45... ...
Macko, eds., Organic Geochemistry Principles and Applications, Plenum Press, New York, 1993. FIGURE 3.1.1 The atoms in the α-amino acid alanine can assume two different configurations in three-dimensional space.
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From page 46... ...
24–27 Although not likely to yield unambiguous biosignatures in the near future, isotopic analyses of martian sediments and atmospheric gases will be important for discerning their evolution and for establishing comparative data, as they do on Earth. Identification of a suite of supporting isotopic data in a reaction pathway, and its environmental context, is the most effective approach to identifying an isotopic biosignature.
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From page 47... ...
Acetate provides one of the best examples because it shows very significant differences in the 13C contents of its methyl and carboxyl carbons.32 The most overt consequences are isotopic ordering in fatty acids and a major isotopic difference between acetogenic and polyisoprenoid lipids. In a single organism, the isotopic differences between acetogenic and polyisoprenoid lipids depend on how many of the polyisoprenoid carbon atoms arise from acetate versus carbohydrate metabolism.33 Morphological Biosignatures Morphological biosignatures represent the class of objects that can be interpreted as indicative of life based on their size, shape distribution, and provenance.
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From page 48... ...
Such objects and structures include intact microbes, metazoa and metaphytes, stromatolites, microbial mats, and other large-scale structures composed of aggregates of cells, as well as component parts of multicellular organisms such as cysts, pollen, embryos, organs, and so on. On Earth, these objects are pervasive in surface environments and in the deep subsurface and leave no doubt about how abundant and tenacious life is.
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From page 49... ...
Poor preservation and ambiguity about what constitutes a biosignature have confounded the search for visible evidence of early microbial life on Earth38–45 and in the martian meteorite ALH 84001 in particular.46 Related reports, and some of the controversies stemming from them, teach researchers that drawing an inference of biogenicity based on morphology is fraught with difficulties. If the feature being observed is demonstrably syngenetic with the host rock and displays a limited size (length and width)
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From page 50... ...
Snape, and I. Gilmour, "Hydropyrolysis of Insoluble Carbonaceous Matter in the Murchison Meteorite: New Insights into Its Macromolecular Structure." Geochimica et Cosmochimica Acta 68:1385-1393, 2004.
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From page 51... ...
These effects are related to the biological ability to nucleate minerals onto organic templates, or to the production of organic ligands that solubilize elements, affect growth mechanisms, or precipitate as salts. The inclusion of organic molecules or micronutrient impurities in mineral precipitates could also conceivably be indicative of biological activity.
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From page 52... ...
McCollom, and J.S. Seewald, "Carbon Isotope Composition of Organic Compounds Produced by Abiotic Synthesis under , Hydrothermal Conditions," Earth and Planetary Science Letters 243:74-84, 2006.
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From page 53... ...
Maruyama, "Ion Microprobe Analysis of Graphite from ca. 3.8 Ga Metasediments, Isua Supracrustal Belt, West Greenland: Relationship between Metamorphism and Carbon Isotopic Composition," Geochimica et Cosmochimica Acta 66:1257-1268, 2002.
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From page 54... ...
Treiman, "Submicron Magnetite Grains and Carbon Compounds in Martian Meteorite ALH 84001: Inorganic, Abiotic Formation by Shock and Thermal Metamorphism," Astrobiology 3:369-392, 2003.
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