Sesssion 4: Detecting Extinct Life



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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques Sesssion 4: Detecting Extinct Life

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques This page in the original is blank.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques FORMATION AND PRESERVATION OF BONA FIDE MICROFOSSILS Sherry L. Cady Department of Geology Portland State University Abstract The discovery of microfossil-like objects in the martian meteorite ALH84001 underscores the principal challenge facing astrobiologists in the search for fossilized evidence of microbial life beyond Earth: How can bona fide microfossils be distinguished from carbonaceous or mineral pseudofossils? The same challenge faces paleontologists searching for the earliest signs of life on Earth. Criteria for assessing whether ancient microfossillike objects are remnants of microorganisms have been established,1 and only permineralized and non-mineralized cellular remains are accepted as bona fide microfossils. Although the exact mechanism and precise chemistry of permineralization are not known, permineralized cells retain enough morphological fidelity to be recognizable and, by definition,2 are carbonaceous (composed of complex organic biopolymers). As such, permineralized and nonmineralized cellular remains harbor multiple biosignatures that include cellular morphology. To detect and confirm the biogenicity of bona fide microfossils,it is necessary to use analytical instruments that not only reveal their presence, but also reveal the biochemical nature of their organic compounds. Whether permineralized and nonmineralized microfossils harbor definitive biosignatures depends upon the intrinsic characteristics of the microorganisms, the extrinsic characteristics of their environment, and the diagenetic transformations that alter the microfossils and the fine-grained mineral matrix in which they are preserved. Discussed here are ways in which these factors affect the formation and preservation of bona fide microfossils in mineralizing ecosystems—environments that produce the types of sedimentary deposits targeted in the search for life on Mars.3 Life itself arose as a self-perpetuating product of physical processes, and it is likely that the characteristics of Earth’s earliest organisms—their size, shape, molecular composition, and catalytic properties—bore a close resemblance to the products of physical processes that gave rise to biology. For this reason, detecting the remnants of early life in terrestrial rocks is difficult. In martian or other extraterrestrial samples, it is doubly challenging. —Andrew H. Knoll4 Introduction Evidence of early microbial life on Earth is preserved in chemically precipitated deposits that form from mineralizing fluids (e.g., hydrothermal deposits, evaporites, carbonates, and silicified carbonates) and detrital sediments deposited by water (e.g., clays, volcanic ash, siliciclastics). Although fossil-bearing strata are sparse in ancient rocks of Precambrian age, a continuous fossil record exists from about 2 billion years ago to the present. 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. Permineralized microfossils retain the morphology the microbial cells had at the time of fossilization,as well as degradation-resistant (i.e., recalcitrant) cellular components such as cell walls and extracellular sheaths. Nonmineralized cells, while compressed, also retain morphological features and biochemical signatures. The structure and stereochemistry of individual organic

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques compounds that comprise the recalcitrant biomolecules of permineralized and nonmineralized cells may provide biomarker compounds that can be used to distinguish some of the major groups of microorganisms.5 The isotopic signatures of cellular remains may also reveal specific groups of organisms based on the degree to which they metabolically fractionated stable carbon isotopes.6 Some permineralized cellular remains also preserve anomalous concentrations of trace elements that were concentrated in vivo7 or post mortem. While the search for ancient microfossils has produced a relatively robust set of criteria for assessing the biogenicity of even the most ancient microfossil-like objects,810 proving that purported microfossils represent fossilized microorganisms is extremely difficult and time consuming, and numerous claims of the microfossils in ancient rocks remain contentious. In those cases where primary biomolecules (or their diagenetic equivalent) are not preserved with a microfossil (e.g.,when complete mineral replacement occurs), proving the biogenicity of such objects is often impossible. Microfossil-like objects composed entirely of minerals that do not harbor characteristics indicative of having formed uniquely via the replacement of cellular remains must be interpreted with the utmost caution. Almost all controvernial objects purported to be early Precambrian microfossils are mineralic.11,12 As noted by Buick,13 unless compelling reasons for accepting noncarbonaceous objects as microfossils exist (i.e., the morphology of the object matches the morphology of a distinctive microbe, and it is unlike the morphology of any known nonbiogenic structure), such objects should be discounted as bona fide microfossils. Only when attributes that distinguish the biogenicity of such objects are demonstrated, and the purported microfossils are confidently distinguished from possible carbonaceous or mineralic non-biologically produced objects, can they be considered bona fide evidence of life. Cherts, Important Paleobiological Repositories on Earth (and Mars?) Although silica deposits harbor less than one-half of the ancient permineralized microorganisms, those preserved in primary and early diagenetic cherts display some of the most exquisite morphological fidelity.14 Siliceous rocks known as cherts consist primarily of opaline, micro-and macrocrystalline silica varieties (e.g., opalA,opal-CT, opal-C, microquartz, fibrous quartz varieties, and macroquartz). Cherts form in a range of environments via a number of different processes.15 Primary chert deposits form where biogenically or nonbiogenically precipitated silica grains accumulate. Early diagenetic cherts form as a result of the replacement of preexisting sedimentary phases such as opals, carbonates, or evaporites. Regardless of the origin (biogenic or nonbiogenic) and nature (hydrated opaline or microcrystalline silica varieties)of the primary silica phase, all cherts, given enough time, will eventually recrystallize to quartz cherts. The inevitable transformation of all authigenic silica phases to quartz often masks the exact sequence of events that led to the formation of a quartz chert deposit —hence, the need to place the deposit within the context of its regional and local geology. The long crustal residence time of cherts reflects the thermodynamic stability of quartz at Earth’s surface temperatures and pressures. Few modern-day analogues exist for ancient primary Precambrian cherts, most of which precipitated nonbiogenically. The remains of silica-secreting eukaryotes (i.e.,sponges, radiolarians, diatoms) provide the source of silica for most younger, Phanerozoic cherts.Modern chert-forming environments that can be considered analogues for Precambrian chert-forming environments include silica-depositing thermal spring ecosystems.As mineralizing environments, hydrothermal systems have an enhanced potential to preserve biosignatures indicative of their prolific heat-loving microbial communities.1618 Hydrothermal systems are habitats for a variety of thermophilic and hyperthermophilic microbial communities.1921 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. The predominantly monomineralic composition of cherts enhances the probability that such deposits on Mars could be detected via remote spectroscopy, provided the surface area of the deposit exceeds or approximates the pixel dimension of the spectrometer utilized. Primary hydrothermal cherts could have formed on Mars as surficial siliceous thermal spring deposits23 and as shallow subsurface siliceous epithermal deposits.24 Silica leached from near-surface silicate rocks can be redeposited at depth as silcretes,

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques forming extensive silica-rich soil horizons, or it can be redeposited along the edges of play a lakes and evaporative shallow-water basins forming primary or replacement cherts.25 Targets in the search for life on Mars, therefore, include hydrothermal deposits, hardpan subsoils, evaporites, and paleolake basin deposits.26 On Earth, these types of deposits are relicts of ecosystems that produced microfossils predominantly via the encrustation of cells by minerals precipitated in situ (i.e., authigenically). If microbial life did emerge on Mars and thrived in any of the targeted mineralizing environments mentioned above, it would likely have been fossilized and some of its biosignatures preserved. 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. Differences in the distribution and composition of the various extracellular and cellular components, as well as the reactivity of the subcellular biomolecules within these components of the cells, can affect an organism’s susceptibility to fossilization. Modern analogue and experimental studies that utilize various types of electron microscopy techniques have revealed the types of preservational biases that occur during the earliest stages of microbial fossilization. The example shown in Figure 1 illustrates a type of preservational bias introduced during the silicification of FIGURE 1. Transmission electron microscopy photomicrograph. Cross-sectional view of a high-temperature hot spring microbial community partially entombed within an opaline silica matrix (dark matrix). This image illustrates how differences in the susceptibility of various microbial taxa to fossilization could lead to biases in paleobiological information during the earliest stages of preservation.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques a microbial biofilm at life’s upper temperature limit. A biofilm containing a climax microbial community, which developed over several months on a substrate deployed by the author along the edge of a chert-forming hydrothermal spring located in Yellowstone National Park, was exposed subsequent to development to mineralizing fluid. Different microorganisms in the specimen, sectioned to electron transparency, are distinguished in the transmission electron photomicrograph on the basis of their ultrastructural characteristics. Note the variable amounts of extracellular matrix surrounding the different microorganisms. Some cells appear to be isolated within the opaline silica matrix, while other, more closely spaced cells,appear to form cohesive biofilm communities surrounded by opaline silica. It is clear that the proximity of opaline silica to the cell walls of the various organisms (one of the more robust cellular components during fossilization) differs as a function of the amount of extracellular matrix they produced. How such intrinsic differences in the amount and nature of the extracellular matrix of the different microorganisms affect their preservation cannot be predicted from such an image. Since postdepositional degradation processes will alter all cells to some degree, it is likely that the intrinsic differences displayed at this early stage of fossilization will lead to preservational biases. Experimental silicification studies have revealed gross differences in the susceptibility of Gram-positive and Gram-negative bacteria.Gram positives silicify more rapidly than Gram negatives under controlled conditions.28 Gram positive and negative refer to the way in which the cell wall of a bacterium reacts to the Gram staining procedure (the reaction being a function of the structure and composition of the cell wall). A polymeric peptidoglycan layer, located between the plasma membrane and the outer cell wall membrane, contains numerous carboxyl and phosphoryl groups that serve as metal cation-binding sites that promote mineral nucleation and growth. The thicker peptidogly can 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.30,31 Investigations to date indicate that the exposed reactive chemical groups on microbial cell walls interact ionically, as a function of pH, with solutes in the milieu surrounding the cell. The organometallic complexes that precipitate at reactive biomolecular sites provide additional sites for the sorption of metal and nonmetal ions. Fortin et al. recently reviewed the principal types of biomolecules known to alter the reactivity of microbial cell walls and extracellular layers (e.g., capsules, S-layers).32 When microbial cells attach to surfaces, they produce exopolymeric substances containing reactive biomolecules that can bind a variety of ions, including metals. Anomalous concentrations of trace metals localized by microbial cells and biofilm matrices have been proposed as possible biosignatures, the distribution of metal ions revealing the former presence of microbial cells.33,34 Microbial taxa produce exopolymers under a range of environmental conditions, the composition of which can vary for various microbial species and for the same species under different environmental conditions.35 Important topics for future research include studies to determine how environmental perturbations alter the function and composition of cellular and extracellular components,and how these changes affect the susceptibility of microorganisms to fossilization. Extrinsic Environmental Factors and Microbial Fossilization Distribution of Microorganisms Most microorganisms immersed in aqueous fluids attach to surfaces and form biofilms that consist of distinct communities of cells immersed within a hydrated matrix of exopolymeric substances. Dynamic studies regarding biofilm architecture and composition, and the distribution of microorganisms within biofilms,36 indicate that biofilms cannot be represented by a single, universal model. At the present time, with limited understanding of the complexities of microbial communities, models for biofilms are best developed on the basis of actualistic studies in modern ecosystems. Biofilms develop in nearly every environment where water and available carbon and energy sources exist. Even at the upper temperature limit for life, hyperthermophilic biofilms develop on hydrothermal mineral precipi-

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques tates.37 It has been estimated that the numbers of prokaryotic cells in subsurface microbial populations exceed by at least an order of magnitude the numbers of cells found within soils and the open ocean.38 Subsurface biofilms in the deep subterranean biosphere have become the subject of intense study39 since microorganisms display a propensity to form biofilms on all available surfaces in environments that favor their proliferation. The presence of microfossil-like objects in the mineral-filled fractures of the martian meteorite ALH84001,40 a subsurface igneous rock, serves as a bellwether regarding the potential importance of subsurface paleobiological repositories. The need to search for microbial biosignatures in any structural discontinuities in rocks from Mars through which mineralizing fluid has passed freely is exemplified by the discovery on Mars of recently young surficial effluents.41 Sedimentary deposits associated with surficial effluents on Earth are extremely fine grained, having precipitated rapidly from solutions supersaturated most commonly by cooling,evaporation,or fluid mixing. Any particulate matter, including microorganisms, entrained in the subsurface fluids that escape from such effluents is often sequestered in fine-grained mineral deposits. Even if evidence of a subsurface biosphere was not discovered in the deposits that precipitated around the effluents discovered on Mars, their study would provide detailed information about the geochemistry of the planet’s near-surface aquifers. Effects of Solution Chemistry in Determining How Fossilization Occurs In order to retain high cellular fidelity, permineralized microfossils must be preserved intra-and extracellularly within a fine-grained mineral matrix prior to extensive cellular degradation. Fine-grained mineral matrices form when numerous, aqueously precipitated crystal nuclei reach, nearly simultaneously, the critical dimensions needed for energetically favored crystal growth. Mineral nuclei of such critical dimensions can form by the random collisions of ions or atoms within a supersaturated solution (i.e., homogeneous nucleation) or on preexisting surfaces (i.e., heterogeneous nucleation). Since reactive sites that promote surface sorption and chemical bonding occur on microbial cell surfaces, heterogeneous precipitation of mineral nuclei at these sites can result in mineral encrustation of the cell.42 Whether encrustation will lead to the formation of a permineralized cell depends upon whether, and when, minerals precipitate on the inside of the cell wall, obviously post mortem, but prior to complete degradation of the cell wall. A limited amount of microbial cellular decomposition, accompanied by early mineralization, appears to have enhanced the preservation of cellular remains in the geological record.43 Post mortem fossilization of microorganisms favors the formation of fine-grained mineral matrices, since more reactive sites for mineral nucleation become available once organisms begin to decay. Environments containing aqueous fluids likely to reach saturation for either heterogeneous or homogeneous mineral nucleation include (1) evaporated or rapidly cooled subaerial and subaqueous hydrothermal systems and the edges of play a lake basins, (2) locations where fluids of different composition mix (e.g., at the edges of all types of aquifers), and (3) sites where fluids are stratified across chemoclines and thermoclines (e.g., in stratified lake basins,in water-filled caves, and within biofilms and microbial mats). All of these environments should be considered in the search for paleobiological repositories on Mars. 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. Preservational biases resulting from variation in the amount of cellular degradation have been quantified by Bartley,45 who established a rating system for analyzing statistically the effects of degradation on cell morphology. Post-fossilization events can either enhance or limit the potential for long-term preservation of permineralized microfossils or nonmineralized cellular remains. For example, preservation is enhanced in detrital sediments if they are cemented

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques by a fine-grained mineral assemblage prior to physical disruption, exposure to oxidants, or metamorphic alteration. On the other hand, preservation of cellular fidelity and long-term preservation are limited in mineralizing environments if intracellular and extracellular mineral growth continues unabated.46,47 Diagenesis and Preservation of Microbial Biosignatures Once microorganisms are fossilized, the potential for long-term preservation of their cellular components depends upon the recalcitrance (resistance to biodegradation and major chemical transformations such as hydrolysis and oxidation) 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.49,50 Factors that degrade organic matter and determine their recalcitrance have been studied in great detail.51 The organic remains of microorganisms source much of the world’s petroleum and gas reserves. Of the four principal classes of biomolecules, the various classes of lipids, especially glycolipids and lipopolysaccharides, are most resistant to degradation in all types of depositional environments. Lipids tend to resist chemical attack, are insoluble in water, and can be incorporated in kerogens, thereby increasing their preservation potential. Although the conversion of primary lipids to their diagenetic counterparts (i.e., geolipids) involves information loss through structural alteration (e.g., hydrogenation of double bonds, aromatization of rings, loss of functional groups), the original class of lipid can often be identified even after diagenetic changes.52 The diagenetic history of the minerals that entomb microfossils in paleobiological repositories ultimately determines the length of time microfossils can be preserved in the geological record. In general, diagenesis occurs when an increase in the temperature and/or pressure of a deposit rises to the point where it alters the structural configuration of the primary organic molecules and mineral phases. Microbial cells are usually preserved initially within thermodynamically metastable mineral assemblages, and their transformation to thermodynamically stable mineral assemblages occurs as a result of diagenetic processes that operate over time scales from tens to millions of years (e.g., burial and tectonic alteration). For example, mineral diagenesis alters chert deposits with time via the transformation of primary opaline silica phases to microcrystalline quartz varieties (microquartz, macroquartz, chalcedony, quartzine, “lutecite,” “moganite”) through intermediary cryptocrystalline opal phases.53,54 Importance of Chemical and Structural Discontinuities Regardless of the mechanism by which a microbe is fossilized or the degree to which it is altered from its primary state, the microfossil must differ either in composition or in structural organization from the mineral matrix that surrounds it in order to be detected. For example, chemical and structural discontinuities occur between nonmineralized cells or permineralized microfossils and their mineral matrix. Compositional differences between a microfossil and its mineral matrix also develop when microorganisms concentrate —actively or passively, intra-or extracellularly —heavy metals or other ions from solution. Environmental perturbations can produce gradational compositional differences between microorganisms and their mineral matrix. The geochemical changes that accompany the evaporation of fluids can be preserved in the laminated crusts that develop around microbial cells. The geochemically predictable sequence of minerals around the microfossils preserves not only a cast of the microorganisms, but also paleoenvironmental and paleogeochemical information. It is worth noting that if mineral-replaced microbial cells are preserved within a mineral matrix of the same composition, it will be extremely difficult to establish the biogenicity of the microfossil-like objects. Even when minerals display habits similar to the morphology of microbial cells, it will be necessary to demonstrate that such minerals could not have formed abiotically. As discussed by Buick,55 a certain degree of cellular elaboration must be displayed by a purported microfossil, although the level of morphological complexity continues to be debated. It could also be possible that structural defects or structural differences between the matrix minerals and microfossil-like objects exist. If so, however, they must be interpreted with caution —mineral microstructural characteristics may reflect only differences in the relative amounts of diagenetic processing.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques Conclusions At the present time, only bona fide microfossils provide the direct evidence needed to establish extraterrestrial and earliest Earth life. Nonmineralized and permineralized cells contain multiple biosignatures indicative of their biogenicity, thereby distinguishing them from pseudofossils. 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 ” micro-fossil-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. Acknowledgments The author gratefully acknowledges the NASA Exobiology Program (NASA Grant#NAG5-9579) and the National Science Foundation LExEn Program (NSF Grant #EAR-0096354) for support of basic research in areas described in this paper.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques ELECTRON-BEAM TECHNIQUES FOR MICROFOSSIL CHARACTERIZATION David McKay NASA Johnson Space Center Abstract Electron-beam techniques are extremely valuable in searching for and characterizing microfossils of all kinds. Electron-beam techniques combined with optical microscopy, ion probe, and organic microanalysis techniques can usually provide definitive information on whether a candidate feature is truly a microfossil, even if it has been considerably altered by diagenesis. In our laboratory, we use scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with detailed electron microprobe (EMP) analysis. Specialized techniques that may enhance textural data include ion etching of polished surfaces and small grains, acid etching of polished surfaces, and oxygen plasma etching of polished surfaces and small grains. Chemical mapping of polished surfaces with EMP can reveal subtle differences in chemistry and textures that may be associated with original biogenic features, even if the original biogenic features have been destroyed. Our general sequence of analysis is optical microscopy, followed by SEM petrography, EMP chemical mapping, quantitative EMP analysis of specific phases and areas, and TEM analysis. Examples for some of these techniques are illustrated.Some samples may then be analyzed in other laboratories by ion microprobe, time-of-flight (TOF) secondary ion mass spectroscopy (SIMS), or Raman spectroscopy. Introduction Indigenous microfossils in rocks are absolute proof that life was once active in the environment from which the rock came. Other than finding living organisms, finding true microfossils is perhaps the most reliable proof that life was present in the environment represented by the sample. However, some important pitfalls must be considered. The purported microfossil must be shown to be a true fossil of an organism or trace of an organism. This requires detailed textural information and often requires chemical information, sometimes including isotopic ratios. Next, the microfossil must be shown to be indigenous and not contamination. For some purposes, the microfossil must be shown to be contemporaneous with the enclosing rock, but for other purposes, finding microfossils added to the rock at a later time may be quite useful. Microfossils found in igneous rocks, for example, were clearly not present during the initial cooling and crystallization of the melt,but were added after the rock cooled,generally into cracks and pores of the original rock or by alteration of its phases. Finding and characterizing microfossils in rocks can be greatly facilitated by using electron-beam techniques, particularly in combination with other techniques. Basic instruments used in our laboratories include SEM including attached energy-dispersive x-ray analyzer (EDXA), TEM, and EMP. Petrographic doubly polished thin sections of the rock samples are necessary for the preliminary examination by petrographic optical microscope. Images from this microscope serve as maps for the more detailed mapping and analysis by SEM or TEM. For samples that will be prepared for SEM or TEM, a polished thin section is not always necessary and small chips can be used. It is usually helpful to study the chips with a good binocular microscope. The basic information desired is the texture of the rock and its included microfossils, the location of the microfossils, their morphology, and their dimensional measurements. Basic chemical information includes maps of major and minor elements and quantitative analyses of minerals, glasses, cements, and microfossils. Ideally, each suspected microfossil feature should have well-documented texture or fabric and feature morphology in two or three dimensions, quantitative chemistry, and chemical maps showing variation from place to place of each major and minor element. The chemistry of true microfossils is not likely to resemble the chemistry of the original organism; replacement or void filling may produce a composition such as iron oxide or SiO2 totally unlike the original reduced carbon, water, and other components of the living organism. Consequently, it is necessary to document and understand the chemical effects of diagenesis and replacement that create the fossilized version of the original organism. This, in turn, requires considerable experience, familiarity with the literature, or even experimental laboratory studies of fossilization.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques Sample Preparation In our laboratory, we typically prepare samples using the following processes: A representative rock chip (generally 1 to 10 mm)is used for making one or more polished petrographic thin sections, typically 30 μm thick. We make these sections in the Curatorial Thin Section Laboratory here at Johnson Space Center. Such thin sections are used for optical petrographic mapping and mineral identification, followed by SEM petrographic mapping and EDXA analysis, EMP compositional mapping, and quantitative EMP mapping of particular phases, traverses, or features. A small chip (0.1 to 2 mm) containing interesting features identified in a binocular microscope is mounted on an SEM stub and coated with a conductive coating of Cr, C, or Pt for SEM examination and photography. For some purposes, the sample is left uncoated and examined at low voltage. Additional small chips (0.05 to 0.1 mm) are embedded in epoxy for microtome thin-section making, and several thin sections of ~50 to 80 nm are produced for TEM examination. The objective in TEM work is to document the mineralogy with electron diffraction, high-resolution imaging, and EDXA chemistry, and to determine the ultrastructure with low-to high-magnification images. Many TEMs have electron energy-loss spectrometers (EELS) attached, which can help with chemical and mineral identification. 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. This technique is particularly useful for detecting amorphous material such as glass or polymerized gels that develop distinctive bumpy textures in the ion etching device. For some applications, one of the polished thin sections is lightly etched in hydrofluoric acid fumes for a few minutes to bring out hidden textures.58 Another useful technique is to place the section or chip in an oxygen plasma-etching device for a few minutes. This technique removes many kinds of organic material from the exposed surface and may differentially etch away organics creating a multilevel surface. Before-and-after SEM images can then be used to locate etched organic material. Analysis Objectives Generally, we want to find features that might be the remains of microbes or might have been formed or influenced by microbial life. We wish to relate these features to their environment or surroundings. If the features are fossilized microbes, we try to determine the changes in texture, morphology, and chemistry that have taken place during diagenesis and fossilization. Next we want to relate the microbial remains or traces to the original environment and the original form and type of microbe. We try to determine whether nonbiologic processes that mimic the results of biologic processes could have made the feature. The value of a biomarker such as a microfossil is greatest for forms and features that cannot easily be made by inorganic processes. Finally, we try to determine whether features are indigenous or are contaminants added later. Every effort should be made to interpret the features as artifacts, inorganic products, and contamination. Only after such interpretations are eliminated can a microfossil origin be seriously considered. Examination Steps Petrographic optical microscopy of the polished thin section is generally the first step. This technique can rapidly locate areas of interest that may contain potential microfossils and can provide maps for subsequent, more detailed study with an electron microscope or electron microprobe. This technique can rapidly identify many minerals. Petrographic microscope examination can also show the relationship of the potential microfossils to the rest of the rock. We use optical images as maps to guide initial study by electron backscatter petrography in the SEM or EMP.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques FIGURE 1. The MOD 2005 instrument. A sample collected by a drill or a scoop is dropped into the rock crusher, which pulverizes the sample and then drops it into the oven. After closing the oven at Mars ambient pressure, the crushed sample will be heated stepwise to 950°C. Amino acids and PAHs in the sample will be sublimed and collected on the sample wheel (right end of the oven), which is cooled to Mars nighttime temperatures (around –100°C). The condensed compounds are detected by fluorescence using laser-based sensors. A TDL spectrometer (not shown), which is on the far side of this view, measures the released amounts and isotopic composition of water as well as carbon dioxide produced from the decomposition of various carbonate minerals. 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.186 Amino acid homochirality (enantiopurity) provides an unambiguous way of distinguishing between abiotic and biotic origins. Proteins made up of both D- and L-amino acids would not likely have been efficient catalysts in early organisms because they could not fold into bioactive configurations such as the α-helix. However, enzymes made up of all D-amino acids function just as well as those made up of only L-amino acids, but the two enzymes use the opposite stereoisomeric substrates. There are no biochemical reasons why L-acids would be favored over D-amino acids. On Earth, the use of only L-amino acids by life is probably simply a matter of chance. We assume that if proteins and enzymes were a component of extinct or extant life on Mars, then amino acid homochirality would have been a requirement. However, the possibility that martian life was (or is) based on D-amino acids would be equal to that based on L-amino acids.

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques The detection of a nonracemic mixture of amino acids in a martian sample would be strong evidence for the presence of an extinct or extant biota on Mars. The finding of an excess of D-amino acids would provide irrefutable evidence of unique martian life that could not have been derived from seeding the planet with terrestrial life (or the seeding of Earth with martian life). In contrast, the presence of racemic amino acids, along with abiotic amino acids such as α-aminoisobutyric acid, could be indicative of an abiotic origin or, alternatively, the racemization of biotically produced amino acids.187 A potential impediment to the search for life on Mars is the forward contamination of the planet with either terrestrial organisms or, more likely, terrestrial biomolecules. This problem would be of great importance in assessments of whether there are any amino acids indigenous to Mars. Because of the distinctive L-enantiomer signature of amino acids associated with terrestrial life, chiral amino acid analyses can be used to monitor the level of forward contamination of Mars that occurs during the course of planetary exploration. 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. A relatively new technology that shows promise for spacecraft-based amino acid enantiomeric analysis is microchip-based capillary electrophoresis (μCE). With μCE, both the identity and the enantiomeric composition of amino acids can be determined at sub-part-per-billion levels. The μCE-based analyses are about an order of magnitude faster than analytical methods such as conventional capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC). Such short analysis times are an inherent advantage for robotic in situ measurements carried out from a spacecraft. In addition, μCE has a detection limit more than three orders of magnitude better than conventional HPLC. Thus, proportionally smaller samples (~100 pl or 10–10 l) can be analyzed, another important advantage for in situ spacecraft-based instruments. A μCE chip system has been used to explore the feasibility of using such devices to analyze for amino acid enantiomers in extraterrestrial samples.188 The test system consisted of a folded electrophoresis channel (19.0 cm long × 150 μm wide × 20 μm deep) that was photolithographically fabricated in a 10-cm-diameter glass wafer sandwich, coupled to a laser-excited confocal fluorescence detection apparatus providing subattomole (<10–18 mole) sensitivity. The μCE analysis system consists of a stack of wafer-scale components that individually provide the liquid flow channels, the capillary separation zones, the electrophoretic controls, the fluid logic, and the detection system.This μCE system is more than an order of magnitude smaller in size than conventional laboratory bench top amino acid analytical instruments. A critical aspect is that enantiomeric ratios can be rapidly and accurately determined using the microfabricated μCE chip instrument. Using a sodium dodecyl sulfate/γ-cyclodextrin pH 10.0 carbonate electrophoresis buffer and a separation voltage of 550 V/cm at 10°C, baseline resolution is observed for the enantiomers of valine, alanine, glutamic acid, and aspartic acid in only 4 minutes (Figure 2). Enantiomeric ratios of amino acids extracted from the Murchison meteorite using this μCE chip system closely matched values determined by HPLC. The reduced time, resources, and sample requirements for microfabricated μCE translate into a significant reduction in mass, power, and volume. With an estimated mass of ~1 kg, a volume of ~1000 cm3, and a power requirement of ~2 W, the μCE chip system provides a compact, low-mass instrument suitable for a wide variety of in situ exobiology applications. For spacecraft-based analyses, a microfluidics-based sample processing system will be needed in order to deliver an amino acid extract suitable for analysis by a μCE system. The design of such a system is presently under way. Conclusions This discussion has focused on amino acid detection systems tailored for missions to Mars. However, other solar system bodies, such as Europa, Saturn’s moon Titan, asteroids, and comets, likely hold information about natural abiotic synthetic processes and the conditions necessary for synthesis of the organic compounds needed for the origin of life. In the case of Europa, compounds derived from living entities could possibly be present. In situ analyses carried out on these solar system bodies could thus potentially provide information about the suite of organic compounds that may have been present on prebiotic Earth and how the organic compounds used by

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques FIGURE 2. Baseline resolution of several amino acid enantiomers using the μCE chip system. SOURCE: L.D. Hutt, D.P. Glavin, J.L. Bada, and R.A. Mathies, “Microfabricated Capillary Electrophoresis Amino Acid Chirality Analyzer for Extraterrestrial Exploration,” Anal. Chem. 71:4000-4006, 1999. extraterrestrial life compare with those used by terrestrial organisms. The MOD instrument concept described here for investigations on Mars could easily be tailored for use on other bodies of interest in the solar system. Acknowledgments The development of MOD has been funded by the Planetary Instrument Definition and Development Program (PIDDP) and the Mars Instrument Development Program (MIDP) of NASA and the NASA Specialized Center of Research and Training in Exobiology at the Scripps Institution of Oceanography. The following members of the “MOD squad ” contributed to this work:: C. LaBaw, C. Mahoney, G. McDonald, O. Serviss, C.R. Webster, and F. Grunthaner (NASA-JPL); R. Mathies and L. Hutt (UC Berkeley); and G. Kminek, O. Botta, and D. Glavin (UCSD).

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Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques REFERENCES FOR PAPERS IN SESSION 4 1. J.W. Schopf, “Fossils and Pseudofossils: Lessons from the Hunt for Early Life on Earth,” in Size Limits of Very Small Micro-organisms: Proceedings of a Workshop, National Academy Press, Washington, D.C., 1999, pp. 88-93. 2. J.M. Schopf, “Modes of Fossil Preservation,” Review of Palaeobotany and Palynology 20:27-53, 1975. 3. J.D. Farmer and D.J. Des Marais, “Exploring for a Record of Ancient Martian Life,” Journal of Geophysical Research 104(E11):2697726995, 1999. 4. Space Studies Board, National Research Council, Size Limits of Very Small Microorganisms: Proceedings of a Workshop, National Academy Press, Washington, D.C., 1999, p. 85. 5. See, for example, R.E. Summons and M.R. Walter, “Molecular Fossils and Microfossils of Prokaryotes and Protists from Proterozoic Sediments,” American Journal of Science 290-A:212-244, 1990. 6. See, for example, M. 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Schopf, “Fossils and Pseudofossils: Lessons from the Hunt for Early Life on Earth,” in Size Limits of Very Small Microorganisms: Proceedings of a Workshop, National Academy Press, Washington, D.C., 1999, pp. 88-93. 11. See, for example, J.W. Schopf and M.R. Walter, “Archean Microfossils:New Evidence of Ancient Microbes,” in Earth’s Earliest Biosphere: Its Origin and Evolution, J.W. Schopf (ed.), Princeton University Press, Princeton, New Jersey, 1983, pp. 214-239. 12. See, for example, H.J. Hofmann and J.W. Schopf, “Early Proterozoic Microfossils,” in Earth’s Earliest Biosphere: Its Origin and Evolution, J.W. Schopf (ed.), Princeton University Press, Princeton, New Jersey, 1983, pp. 321-360. 13. R. Buick, “Microfossil Recognition in Archean Rocks: An Appraisal of Spheroids and Filaments from a 3500 M.Y. Old Chert-barite Unit at North Pole, Western Australia,” Palaios 5: 441-459, 1990. 14. A.H. 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