Signs of Life A Report Based on the April 2000 Workshop on Life Detection Techniques (2002) / Chapter Skim
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Workshop Papers Session 4: Detecting Extinct Life
Workshop Papers Session 4: Detecting Extinct Life
Pages 147-210
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Session 4: Detecting Extinct Life 147
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This fossil record has demonstrated that three-dimensionally preserved permineralized microbial cells are found primarily in deposits that formed in mineralizing environments, whereas two-dimensional compressions of cells, flattened by compaction during burial, are found primarily in detrital sediments. Our understanding of early life on Earth is based upon the various types of biosignatures extracted from permineralized and nonmineralized cells and cellular remains.
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As mineralizing environments, hydrothermal systems have an enhanced potential to preserve biosignatures indicative of their prolific heat-loving microbial communities.~6~~8 Hydrothermal systems are habitats for a variety of thermophilic and hyperthermophilic microbial communities.~9~2~ As shown by Cady and Farmer,22 the microbial inhabitants of thermal springs and the sedimentary structures they produce are fossilized in metastable opaline silica by a variety of mechanisms. Quartz cherts, including those precipitated from hydrothermal fluids, are likely to have been stable at the surface of Mars throughout its history.
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Given the lack of extensive recrystallization of microfossil-like objects in the ancient carbonate globules found in the martian meteorite ALH84001,27 it appears likely that cell-sized objects, whether biological or nonbiological in origin, would have been preserved with higher morphological fidelity in ancient Mars rocks than their counterparts preserved in Earth's Precambrian geological record. Intrinsic Characteristics of Microorganisms and Fossilization The intrinsic characteristics of microbes that affect fossilization have been studied at various structural levels with a relatively limited number of techniques.
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The thicker peptidoglycan layer in Gram-positive cell walls presumably contains a greater number of reactive sites for mineral nucleation. Based on these observations, Westall proposed that Gram-positive bacteria are more likely to be preserved in the geological record.29 Other experimental studies have focused on quantifying the type, density, and distribution of biomolecules that compose the cell walls and various extracellular components of a variety of microbial taxa 30 3~ Investigations to date indicate that the exposed reactive chemical groups on microbial cell walls interact tonically, as a function of pH, with solutes in the milieu surrounding the cell.
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Relative Timing of Fossilization, Cellular Degradation, and Post-fossilization Events The amount of cellular fidelity displayed by permineralized and nonmineralized cells ultimately depends upon the amount of cellular degradation that occurs prior to fossilization. In their study of microbial preservation along outflow channels of mineralizing thermal spring ecosystems, Gerasimenko and Krylov demonstrated how the apparent paleobiodiversity preserved in the modern hot spring deposits resulted from the fossilization of morphologically similar taxa that were fossilized at various stages of cellular decomposition.44 The degree to which nonmineralized cells in detrital sediments retain their cellular fidelity depends upon whether they were buried to depths where anaerobic conditions prevailed prior to extensive cellular degradation.
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of the macromolecular biomolecules.48 The preservation potential of various types of organic biomolecules has been reviewed by Logan et al. and by Allison and Briggs.4950 Factors that degrade organic matter and determine their recalcitrance have been studied in great detail.5~ The organic remains of microorganisms source much of the world's petroleum and gas reserves.
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Although cellular permineralization is likely to occur in nearly all types of mineralizing environments, the rarity of permineralized microfossils in the geological record suggests that much remains to be learned about how diagenetic processes affect long-term preservation. As improvements are made in the resolution limits of analytical instruments that can simultaneously "image" microfossil-like objects and detect and analyze the biochemical structure of their minute concentrations of carbonaceous compounds, we may find that permineralized remains are more common than heretofore realized.
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, TEM, and EMP. Petrographic doubly polished thin sections of the rock samples are necessary for the preliminary examination by petrographic optical microscope.
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4. Similarly, some polished thin sections may be etched in an ion beam at low angle to bring out textures related to ion erosion resistance.56 57 Ion etching may bring out extremely delicate textures not visible in optical, SEM secondary, or SEM backscattered images of polished samples.
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Laser Raman spectroscopy provides some information on molecular species present. Three-dimensional relationships within the 30-,um polished thin sections can be determined using laser confocal microscope techniques.
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The medium-gray crystals are olivines, and the bright crystals are spinets. Ion etching was used to characterize the mineralogy, textures, and cooling history of these volcanic particles60 and to reveal subtle matrix textures in impact-breccias.6~ This technique may reveal structures below the resolution limits of ordinary backscattered electron mapping, and enables much higher spatial resolution secondary electron imaging to be used on polished thin sections.
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The chemical zoning is apparent for all elements, but in particular for the two magnetite-rich zones. Most investigators now agree that the complex zoning is consistent with precipitation of the carbonates from liquid water or brine at relatively low temperatures.
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It has only recently been realized that biofilm may fossilize and be preserved, often better than microbial cells.
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Backscatter electron (BSE) image of a polished thin section of the martian meteorite Nakhla.
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(a) SEM view of a complex fracture surface from a chip of the martian meteorite, Nakhla.
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This report discusses what is known about the organic matter in martian samples and further addresses techniques and strategies for future missions to Mars and beyond. The Viking Missions One of the major objectives of the 1976 Viking missions was to search for extinct organic compounds preserved in soils on the surface of Mars and to determine their structure and abundances.63 The determination of the structure of organic compounds would provide the information needed to assess both "abiotic" (meteoritic)
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In fact, the techniques used to evaluate the origin of organic matter in ALH84001 and other martian meteorites are central to any discussion of the detection of extinct organic matter on Mars. Much of the controversy surrounding ALH84001 centers on stable carbon isotope studies (DISC)
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. The study of extinct organic matter in martian meteorites brings some new considerations to bear on the selection of analytical techniques for future life detection missions.
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Methods for the Detection of Extinct Organic Matter in Martian Samples Raman Spectroscopy and Biomineralization A major improvement in the study of complex biological systems has been the use of resonance Raman spectroscopy.7i Due to the rapid advancements in laser technology, most wavelengths between 190 and 1064 nm are available and easily implemented in a flight-compatible package. Although fluorescence can interfere with Raman spectra, most bacterial taxonomic markers (pigments)
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.. Laser Desorption Mass Spectrometry One of the methods used for the detection of extinct organic matter in martian meteorites has been laser Resorption mass spectrometry (LDMS)
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Iron Isotopes as Biomarkers As we have learned from examining the martian meteorites, isotopic fractionation studies of the lighter elements (i.e., carbon, sulfur, etc.) are extremely useful in determining the origin of extinct organic compounds (biogenic versus abiogenic)
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Nevertheless, the research to date clearly shows the potential for the use of Fe isotopes in biogeochemical life studies and should be closely watched as, perhaps, a unique and novel approach in the search for life signs on Mars and beyond. Extinct Fossil Life (Electron-beam TechniquesJ Evidence of extinct fossil structures can be sought in detail at various detection levels ranging from macroscopic stromatolitic structures, to microfossils, to the intermolecular distribution of carbon isotopes and organic compounds of the associated organic matter.
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Thus, sensor web represents the type of new technology that can be used for future life detection missions. Earth Analogues and the Search for Extinct Organic Matter Beyond Mars Europa If liquid water exists beneath the surface ice layer on Europa, then one of the environmental requirements for life as we define it will have been met.
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Studies of Earth analogues such as Lake Vostok may be necessary to prepare for future missions to moons like Europa, which may have a liquid ocean beneath a thick ice cap.
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Our future missions to Mars should begin with the examination of the latest MGS images that will aid in the assessment of the proper landing sites and will maximize the probability of detecting any record of a past martian biosphere. The instrumentation selected for the detection of extinct organic compounds must be capable of differentiating between mechanisms of formation (biogenic versus abiogenic)
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Relatively mild diagenetic conditions can result in racemization of molecules with only tertiary asymmetric carbon atoms, such as acyclic isoprenoids and amino acids. Distributions of non-biomarker compounds can amount to a biosignature.
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A large component of these are biosynthetically produced as functionalized compounds, such as fatty acids. Diagenetic processes convert them to n-alkanes.
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Three domains of extant life, Archea, Bacteria, and Eukarya, carry signature membrane lipids head-to-head linked isoprenoids, hopanes, and sterols respectively. Squalene (shown as squalane without double bonds)
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Stereochemical configuration is frozen at quaternary carbon atoms. For C3~ hopane, stereochemically frozen quaternary carbon atoms and those tertiary carbon atoms known to epimerize are indicated by *
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Labeled peaks are as follows: Numbers refer to number of carbon atoms in the chemical structures. Ts are tricyclic terpanes (cheilanthanes)
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Thus, ancient organic matter that still contains amino acids may tend to show a racemic mixture (50:50 mixture) of L- and D-amino acids.
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and N isotopic compositions among the organic components exhibit characteristic abiotic patterns distinctly different from those of terrestrial biochemistry. Together, these patterns provide criteria that can be used with cautious confidence to distinguish between organic matter of biological and nonbiological origins in extraterrestrial samples.
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Data for amino acids and hydroxy acids are chosen to illustrate these patterns, the former for their centrality to terrestrial biochemistry, and both for their wide range of possible structural variations and the availability of relevant meteoritic data. Undoubtedly, future analysts of extraterrestrial organic matter will target amino acids because an alien biochemistry is likely to evolve catalytic polymers consisting of similar multifunctional building blocks.
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Cronin and S Chang, "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite," in The Chemistry of Life's Origins, J.M.
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Future researchers will need to be cautious in using chirality to assign biotic origins to extraterrestrial compounds. Isotopic Compositions of Carbon, Hydrogen, and Nitrogen in Murchison Organic Matter Although bulk analyses of the isotopic compositions of C, H
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also carried out intramolecular carbon, hydrogen, and sulfur isotopic measurements on individual Murchison sulfonic acids.~36 Measurements at similar levels of molecular detail may prove invaluable in establishing sources and production mechanisms; they should certainly be included in the isotopic arsenal for future studies of extraterrestrial organic matter. Organic Survey Analysis of the Murchison Meteorite Survey analyses for volatile and organic components will be among the key preliminary measurements carried out on rocks returned from Mars.
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A pyrolysis-GC-isotope ratioMS, however, could have potential for life detection. Conclusions Analyses of Murchison organic matter reveal characteristic patterns of variation in molecular composition.
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Therefore, these criteria may be used with cautious confidence to distinguish nonbiological from biological organic matter. The discovery of L-enantiomeric excesses of abiotic origin in some Murchison amino acids means that chirality is not a foolproof molecular indicator of biology.
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Natural variations in the isotopic compositions of Fe (masses 54, 56, 57, 58)
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It is immediately apparent that while the values for transition metals are generally smaller, they are of comparable magnitude to values for other elements. Hence, it is reasonable to expect that transition metals, including Fe, could show natural variations in isotopic composition similar to those of lighter isotopes of order 1 to 10 parts per thousand (1 to 10%o)
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Natural Variations Variations in the isotopic composition of Fe have been determined in a range of terrestrial and lunar igneous rocks, iron meteorites, and loesses, as well as in ancient and recent sediments.~54 i55 These measurements provide unequivocal evidence that the isotopic composition of Fe varies in nature. Beard and Johnson first observed that 656Fe in marine sediments is more variable than in igneous rocks.~56 Fe isolated from Pacific and Atlantic ferromanganese nodules is shifted by ~ -1.5%o compared to the range for
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algae is shifted ~ -1.3%o compared to the initial isotopic composition of the ferrihydrite substrate. These data provide evidence that Fe isotopes are fractionated during microbially mediated reduction offerrihydrite.
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Synthesis The evidence accumulated to date (Figure 1) demonstrates that the isotopic composition of Fe in ancient and modern Fe-rich sediments varies over a range of ~2.5%o.
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SESSION 4: ANBAR 193 Acknowledgments The author thanks S Brantley and T
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Geophysical Laboratory Carnegie Institution of Washington Abstract A "Grand Challenge" Research Program has been instituted at Caltech's Jet Propulsion Laboratory (JPL) to develop in situ measurement techniques that are capable of detecting the chemical signatures of life.
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This topic was triggered by discoveries in biology, planetary exploration, and astronomy that have led to the impression that life might be more commonplace in the universe than previously expected. The topic is also perfectly complementary to the ongoing Grand Challenge in in situ planetary exploration and robotics.
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isotope ratios Kanik Bearman Sinha George Ion mobility spectrometer Chemical imaging Fe isotope mass spectroscopy X-ray fluorescence. JPL has uncharacteristically chosen a nonresident scientist to manage this Grand Challenge Program, Wesley Huntress of the Geophysical Laboratory, Carnegie Institution of Washington; the local project scientist is Kenneth Nealson of the Jet Propulsion Laboratory.
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2005 Mars Explorer Program mission, uses sublimation-based extraction coupled with fluorescence detection to carry out highly sensitive analyses for amino acids or amines and polycyclic aromatic hydrocarbons (PAHs) directly on the martian surface.
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Unfortunately, contamination of martian meteorites by terrestrial-derived organic compounds greatly compromise these investigations.~78 This contamination problem underscores the importance of doing in situ organic compound analyses on Mars before samples are returned to Earth, where even under the best of circumstances they will be exposed to some level of terrestrial contamination. Because amino acids are the building blocks of proteins in terrestrial organisms, they are excellent target compounds in the search for life elsewhere.
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The quantities of water and carbon dioxide, and their isotopic compositions, evolved during the stepheating cycle are determined using the TDL spectrometer. Even if no organic compounds are detected, the TDL spectrometer provides information on the amounts of bound water and carbonate minerals contained in the Mars regolith.
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The entire instrument weighs ~2 kg and fits in the palm of your hand. MOD The Next Generation A central problem in amino acid analyses of martian samples is not only identifying and quantifying which compounds are present, but also distinguishing those produced abiotically from those synthesized by extinct or extant life.l86 Amino acid homochirality (enantiopurity)
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This requires that amino acid analysis data be acquired as early as possible in the Mars exploration program in order to provide a useful baseline data set for comparison with future analyses. A long-range monitoring program would be critical in assessing forward contamination during the eventual human exploration of Mars.
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ada, and R.A. Mathies, "Microfabricated Capillary Electrophoresis Amino Acid Chirality Analyzer for Extraterrestrial Exploration," Anal.
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Maechling, and R.N. Zare, "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001," Science 273:924-930, 1996.
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R.E. Summons, "Biogeochemical Cycles: A Review of Fundamental Aspects of Organic Matter Formation, Preservation, and Composition," in Organic Geochemistry, Principles and Applications, M.H.
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Rust, and J.L. Bada, "The Origin of Organic Matter in the Martian Meteorite ALH84001," Earth Planet.
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Chang, "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite," in The Chemistry of Life's Origins, J.M. Greenberg, C.X.
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Chang, "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite," in The Chemistry of Life's Origins, J.M. Greenberg, C.X.
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M Rehkamper, "The Precise Measurement of Tl Isotopic Compositions by MC-ICPMS: Application to the Analysis of Geological Materials and Meteorites," Geochim.
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Chang, "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite," in The Chemistry of Life's Origins, J.M. Greenberg, C.X.
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