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An Astrobiology Strategy for the Exploration of Mars C Objectives for Developing a Further Understanding of Biosignatures The 2001 workshop on biosignatures organized by the NASA Biomarker Task Force established comprehensive objectives for developing a better understanding of biosignatures. These objectives represent an important starting point for future discussions. But, unfortunately, the results of the task group’s deliberations were never published in full. To correct this omission, the task group’s objectives are reproduced below:a Improve knowledge of recalcitrant biochemicals (those that are resistant to geological degradation) that are produced by bacteria and archaea. Many new organisms are identified on the basis of r-RNA sequences for which we have no knowledge of the lipid or other biomarker compositions. Develop and test of immunological or other tagging methods for recognition of specific organic biomarkers that might be more sensitive than mass spectrometry. Improve understanding of hydrocarbon distributions produced by abiogenic synthesis (FT, tholins) especially with regard to PAHs and alkanes. Measure experimentally both the magnitude of isotopic discrimination and the key factors that control it during isotopic exchange reactions involving metals utilized by biota. Explore effects of important parameters such as pressure, temperature, and composition of the exchanging medium. Broaden the database of measured isotopic discrimination by microbiota. Measurements are needed for a broad array of carbon-fixing enzymes (e.g., RUBISCO, PEP-carboxylase). Enzymes that affect the discrimination of sulfur, nitrogen, and biologically important metals are largely unstudied and thus merit investigation. Characterize how microbial ecosystem processes can modify the expression of stable isotopic discrimination by the constituent biota. Understand how isotopic patterns observed among the fossilizable products of an ecosystem can help to characterize the biota, their mutual interactions, and also the effects of environmental parameters such as concentrations of key solutes, temperature, and the nature and availability of energy sources. In ancient, yet well-characterized fossiliferous rocks, catalog the diversity of the isotopic compositions of both microfossils and minerals (e.g., carbonates, sulfides, etc.) affected by biological processes. The source(s) of this variability should be defined. Characterize the nature and extent of post-depositional alteration (by thermal processes, etc.) of minerals a Michael Meyer, NASA Science Mission Directorate, personal communication, 2006.
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An Astrobiology Strategy for the Exploration of Mars and biogenic features at the submicron scale. Understand the relationship between metamorphic grade and isotopic patterns among minerals and organic carbon, as a basis for assessing the syngenicity of these components. In order to utilize isotopic data for coexisting minerals as indicators of environmental conditions (e.g., temperature of mineral formation), establish criteria for determining whether measured isotopic patterns reflect kinetically controlled versus thermodynamically equilibrated reactions. Can such estimates of environmental conditions be achieved by analyzing coexisting phases at the millimeter to submicron scale? By analyzing isotopic compositions of minerals formed in the surface environment, characterize the range of isotopic compositions (e.g., for carbon, nitrogen, and sulfur) witnessed by the environments in which the martian meteorites were formed. This analysis will help to define the background “noise” of isotopic variability against which an isotopic signal of life might ultimately have to be discerned. Document the range and systematics of isotopic compositions associated with habitable subsurface environments, including hydrothermal systems. Can “abiotic” versus “biotic” patterns be resolved? Document the ancient fossil record of subsurface life on early Earth. Assess the isotopic consequences of aqueous alteration of rocks over a wide range of water/rock ratios, particularly low ratios. An essential objective will be to characterize the nature and extent of the aqueous alteration of the martian crust. Improve knowledge about processes that alter the fidelity of morphological biosignatures during and after fossilization. Emphasis should be placed on comprehensive taphonomic and diagenetic studies of extreme ecosystems of relevance to potentially important Mars biotopes. These would include hydrothermal systems (volcanogenic and impact-related), evaporative basins, dry valleys (hot and cold), and subsurface aquifers. Experimental studies are required in order to further characterize abiotically produced morphological mimics of microorganisms. Interlaboratory evaluation of samples that contain various morphological biosignatures would also help refine criteria for detecting/assessing morphological biosignatures. Before mineralogical biosignatures can be determined for small quantities of martian minerals, the database of unambiguous terrestrial biomineralogical signatures must be greatly expanded. It is also essential to consider how mineral-based biosignatures change due to heat, time, and other factors (phase transformation, particle morphology evolution). Specifically, the morphological consequences of crystal growth and transformation to other stable, and possibly anhydrous, phases (e.g., hematite, and magnetite) are required. BIF formations are prevalent on Earth but we don’t have criteria that unequivocally demonstrate that such deposits preserve life signatures. Much work remains to be done in cataloging shapes and compositions of submicron magnetites, and terrestrial analogs. Characterization of sulfide minerals and morphologies together with reliable information about the thermal history of the specimen is needed to provide useful information about possible bacterial origin of Fe sulfides in extraterrestrial samples. Explicit consideration of probable differences between Mars hydrothermal geological and Earth geological systems is required. Potential biosignatures should be evaluated by means of a systematic testing program utilizing extraterrestrial materials, such as some of the large number of unstudied meteorites. This should include the use of martian meteorites to hone sample-handling and life-detection techniques. Methods need to be refined for the characterization and quantification of contamination of returned samples, utilizing, among other approaches, the use of highly characterized witness plates, screening of all materials and methods employed with returned samples, deliberate contamination experiments, and standardization of quality control. Objectives for flight experiments relevant to biosignatures and prebiotic chemistry include the following: Establish primordial isotopic abundances for CHNO; and Study martian geological processes that might distinguish biospheric fractionations from geological processes for detection of active metabolism via redox-specific electrodes.
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