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Although mineral dehydration in hydrothermal and metamorphic processes is a commonly observed phenomenon, it is often stated that organic compounds will not dehydrate in the presence of an aqueous solution even at elevated temperatures and pressures. Both theoretical calculations and experimental measurements directly refute this paradigm. Results obtained in the present study indicate that increasing temperature tends to favor dehydration reactions among organic compounds in aqueous solution. Calculation of the thermodynamic properties for several aqueous organic dehydration reactions were conducted using equations, data, and parameters from Shock , and Helgeson (1990) and Shock (1992,1993a), together with additional data estimated in this study. Dehydration reactions which may proceed at elevated temperatures include amide formation from carboxylic acids and ammonia, ester formation from carboxylic acids and alcohols, and peptide formation from amino acids. Condensation of complex organic molecules may be energetically favored in hydrothermal solutions, which would greatly facilitate the ability of organisms to infiltrate high-temperature/pressure environments.

Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface. † To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming metastable equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (ƒ O 2(g) ) . ‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO2 (aCO2(aq)). The values of log ƒ O 2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log ƒ O 2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in metastable equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of aCO2(aq) in the waters, to calculate equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6–15, depending on the activity of aqueous CO2. However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous metastable equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca2+, H+, CO2, CH3COOH, CH3COO−, and other aqueous species in oil field waters may be in metastable equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs. § If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by metastable equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H2O at the oil-water interface to form lighter paraffins and CO2(aq), CH3COOH(aq), and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria. ∥ However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs.

The role of clay minerals in the abiotic synthesis of organic molecules near seafloor spreading centers was simulated experimentally. Clays are common hydrothermal alteration products of volcanic glass and due to their nano-scale crystal size, provide extensive and variably charged surfaces that interact with aqueous organic species. Volcanic gases H2and CO2have been shown to react on magnetite surfaces to form methanol, a primary organic molecule, under hydrothermal conditions. Therefore, our experiments simulated the temperature and pressure conditions (300°C, 100 MPa) that exist beneath hydrothermal vents, in stockwork fractures through which hydrothermal fluids interact with fresh basalt. We examined the products of reactions between aqueous methanol and three common clay minerals found in those environments (montmorillonite, saponite, illite).

Thermophilic and hyperthermophilic Archaea and Bacteria have been isolated from marine hydrothermal systems, heated sediments, continental solfataras, hot springs, water heaters, and industrial waste. They catalyze a tremendous array of widely varying metabolic processes. As determined in the laboratory, electron donors in thermophilic and hyperthermophilic microbial redox reactions include H2, Fe(2+), H2S, S, S2O3(2-), S4O6(2-), sulfide minerals, CH4, various mono-, di-, and hydroxy-carboxylic acids, alcohols, amino acids, and complex organic substrates; electron acceptors include O2, Fe(3+), CO2, CO, NO3(-), NO2(-), NO, N2O, SO4(2-), SO3(2-), S2O3(2-), and S. Although many assimilatory and dissimilatory metabolic reactions have been identified for these groups of microorganisms, little attention has been paid to the energetics of these reactions. In this review, standard molal Gibbs free energies (DeltaGr(0)) as a function of temperature to 200 degrees C are tabulated for 370 organic and inorganic redox, disproportionation, dissociation, hydrolysis, and solubility reactions directly or indirectly involved in microbial metabolism. To calculate values of DeltaGr(0) for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (DeltaG(0)) at temperatures to 200 degrees C are given for 307 solids, liquids, gases, and aqueous solutes. It is shown that values of DeltaGr(0) for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25 degrees C for systems at elevated temperatures introduces significant and unnecessary errors. The metabolic processes considered here involve compounds that belong to the following chemical systems: H-O, H-O-N, H-O-S, H-O-N-S, H-O-C(inorganic), H-O-C, H-O-N-C, H-O-S-C, H-O-N-S-C(amino acids), H-O-S-C-metals/minerals, and H-O-P. For four metabolic reactions of particular interest in thermophily and hyperthermophily (knallgas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis), values of the overall Gibbs free energy (DeltaGr) as a function of temperature are calculated for a wide range of chemical compositions likely to be present in near-surface and deep hydrothermal and geothermal systems.

[1] Recent data from the Cassini spacecraft suggest that an ocean exists inside Saturn's satellite Enceladus. Here, we show that the size of an ocean can be estimated if the NaCl concentration of the ocean is known. We find that concentrations of aqueous NaCl reported by Postberg et al. (2009) imply a relatively large ocean, comprising at least 17% by mass of volatiles on Enceladus. Structural calculations indicate that this ocean would underlie more of Enceladus' surface than just the geologically active south polar region, consistent with modeling of localized tidal dissipation. Conduction calculations suggest that a half-global ocean can be maintained by steady-state tidal heating, provided that clathrate hydrate dominates Enceladus' ice shell. We also find that rapid freezing of oceanic water could release large fluxes of stored tidal heat, perhaps partly explaining the anomalous energy output of Enceladus. An alternative, saltier ocean model is also examined.

Correlations between standard partial molal entropies ( S j 0 ) of aqueous mono-, di-, and trivalent ions at 25°C and 1 bar and effective electrostatic radii (re,j) calculated from the corresponding crystallographic radii (rx,j) have been constructed for coordination numbers from 4 to 12. These correlations permit estimates of S j 0 to include information from spectroscopic studies about ionic coordination in aqueous solution. As a consequence, values of S j 0 can be estimated for many more ions than previously possible. Close agreement between predicted and experimental data for tetravalent cations supports the validity and generality of this approach. Combined with the experimentally determined values of S j 0 used to generate the correlations, estimates of standard partial molal entropies at 25°C and 1 bar for over 130 mon-atomic aqueous ions of charges ranging from −3 to +6 provide a set of S j 0 values for 188 monatomic aqueous ions for use in geochemical calculations.

Estimated standard molal thermodynamic properties of aqueous dipeptides and their constituent amino acids indicate that temperature increases correspond to increased stability of peptide bonds relative to hydrolysis reactions. Pressure increases cause slight decreases in peptide bond stability, which are generally offset by greater stability caused by temperature increases along geothermal gradients. These calculations suggest that peptides, polypeptides, and proteins may survive hydrothermal alteration of organic matter depending on the rates of the hydrolysis reactions. Extremely thermophilic organisms may be able to take advantage of the decreased energy required to form peptide bonds in order to maintain structural proteins and enzymes at elevated temperatures and pressures. As the rates of hydrolysis reactions increase with increasing temperature, formation of peptide bonds may become a facile process in hydrothermal systems and deep in sedimentary basins.

Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

A coordinated study of water chemistry, sediment mineralogy, and sediment microbial community was conducted on four73 degrees C springs in the northwestern Great Basin. Despite generally similar chemistry and mineralogy, springs with short residence time (approximately 5-20 min) were rich in reduced chemistry, whereas springs with long residence time (1 day) accumulated oxygen and oxidized nitrogen species. The presence of oxygen suggested that aerobic metabolisms prevail in the water and surface sediment. However, Gibbs free energy calculations using empirical chemistry data suggested that several inorganic electron donors were similarly favorable. Analysis of 298 bacterial 16S rDNAs identified 36 species-level phylotypes, 14 of which failed to affiliate with cultivated phyla. Highly represented phylotypes included Thermus, Thermotoga, a member of candidate phylum OP1, and two deeply branching Chloroflexi. The 276 archaeal 16S rDNAs represented 28 phylotypes, most of which were Crenarchaeota unrelated to the Thermoprotei. The most abundant archaeal phylotype was closely related to "Candidatus Nitrosocaldus yellowstonii", suggesting a role for ammonia oxidation in primary production; however, few other phylotypes could be linked with energy calculations because phylotypes were either related to chemoorganotrophs or were unrelated to known organisms.

Despite the ubiquity of ammonium in geothermal environments and the thermodynamic favorability of aerobic ammonia oxidation, thermophilic ammonia-oxidizing microorganisms belonging to the crenarchaeota kingdom have only recently been described. In this study, we analyzed microbial mats and surface sediments from 21 hot spring samples (pH 3.4 to 9.0; temperature, 41 to 86°C) from the United States, China, and Russia and obtained 846 putative archaeal ammonia monooxygenase large-subunit ( amoA ) gene and transcript sequences, representing a total of 41 amoA operational taxonomic units (OTUs) at 2% identity. The amoA gene sequences were highly diverse, yet they clustered within two major clades of archaeal amoA sequences known from water columns, sediments, and soils: clusters A and B. Eighty-four percent (711/846) of the sequences belonged to cluster A, which is typically found in water columns and sediments, whereas 16% (135/846) belonged to cluster B, which is typically found in soils and sediments. Although a few amoA OTUs were present in several geothermal regions, most were specific to a single region. In addition, cluster A amoA genes formed geographic groups, while cluster B sequences did not group geographically. With the exception of only one hot spring, principal-component analysis and UPGMA (unweighted-pair group method using average linkages) based on the UniFrac metric derived from cluster A grouped the springs by location, regardless of temperature or bulk water pH, suggesting that geography may play a role in structuring communities of putative ammonia-oxidizing archaea (AOA). The amoA genes were distinct from those of low-temperature environments; in particular, pair-wise comparisons between hot spring amoA genes and those from sympatric soils showed less than 85% sequence identity, underscoring the distinctness of hot spring archaeal communities from those of the surrounding soil system. Reverse transcription-PCR showed that amoA genes were transcribed in situ in one spring and the transcripts were closely related to the amoA genes amplified from the same spring. Our study demonstrates the global occurrence of putative archaeal amoA genes in a wide variety of terrestrial hot springs and suggests that geography may play an important role in selecting different assemblages of AOA.

Formate, a simple organic acid known to support chemotrophic hyperthermophiles, is found in hot springs of varying temperature and pH. However, it is not yet known how metabolic strategies that use formate could contribute to primary productivity in hydrothermal ecosystems. In an effort to provide a quantitative framework for assessing the role of formate metabolism, concentration data for dissolved formate and many other solutes in samples from Yellowstone hot springs were used, together with data for coexisting gas compositions, to evaluate the overall Gibbs energy for many reactions involving formate oxidation or reduction. The result is the first rigorous thermodynamic assessment of reactions involving formate oxidation to bicarbonate and reduction to methane coupled with various forms of iron, nitrogen, sulfur, hydrogen, and oxygen for hydrothermal ecosystems. We conclude that there are a limited number of reactions that can yield energy through formate reduction, in contrast to numerous formate oxidation reactions that can yield abundant energy for chemosynthetic microorganisms. Because the energy yields are so high, these results challenge the notion that hydrogen is the primary energy source of chemosynthetic microbes in hydrothermal ecosystems.

The common thread of energy release suggests that diverse microbial metabolic processes can be compared through thermodynamic analyses. The resulting energy and power requirements can provide quantitative constraints on habitability. Because previous thermodynamic analyses have focused on the minimum amount of energy needed for the growth of a microorganism or community, the focus of this study is to gain a fuller understanding of the microbial response to highly habitable conditions. This communication summarizes the results of a thermodynamic analysis of the energy and power consumed by microorganisms in experiments that were designed to optimize growth. Reports of microbial growth experiments taken from the literature were combined with speciation and standard state calculations to assess the overall Gibbs energy change during the experiments. Results show that similar numbers of cells (10(9) to 10(10) ) were produced in these experiments regardless of the duration of log phase growth (from2 to200 hours) or the total Gibbs energy change [from 1.3-29.6 kJ (mol electrons transferred)(1)]. As a result, optimal growth conditions appear to produce between 10(10) and 10(14) cells per watt of power consumed.

A study of literature reports of the concentrations of amino acids in extracts from the Murchison meteorite shows that many of the concentration ratios are constant. There are two possible interpretations of these ratios. One is that they are controlled by the pathways through which the amino acids formed, from which it follows that the amino acids are distributed in the same proportions throughout the meteorite. The other interpretation is that the ratios result from the analytical procedures used to extract the amino acids from the meteorite. These methods rely heavily on high-temperature (100 degrees C) aqueous extraction and subsequent high-temperature acid hydrolysis. A correlation was observed in the present study between the relative concentrations of several amino acids in the meteorite extracts and their relative aqueous solubilities at 100 degrees C (alanine, valine, leucine, isoleucine, norleucine, aspartic acid, glutamic acid and glycine). The extract solutions are dilute, and far from the saturation limits, but these correlations suggest that the sampling procedure affects directly the reported concentrations for these amino acids. Ratios of the concentration of serine to those of glycine are also constant but cannot be accounted for solely by relative solubilities, and, as suggested elsewhere, serine as well as phenylalanine and methionine may be terrestrial contaminants. Data for beta-alanine, alpha-aminobutyric acid, proline, sarcosine, alloisoleucine, beta-aminoisobutyric acid, beta-aminobutyric acid, and threonine also show constant abundances relative to glycine, but lack of solubility data at extraction conditions prohibits evaluating the extent of possible sampling bias for these amino acids. If the extraction process does not bias the results, and all extractable amino acids are removed from meteorite samples, then the properties of amino acids which control both their solubilities and their concentrations in the meteorite need to be established. The possibility of sampling bias needs to be tested experimentally before concluding that extraction is complete, and that the constant relative abundances indicate that the relative concentrations of amino acids are homogeneous in the meteorite.

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO2 and N2. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH2) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO2, H2O, and N2 are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.

Analysis of data reported by Miller and Bada (1988) shows that three aqueous amino acids approached metastable equilibrium concentrations toward the end of hydrous pyrolysis experiments conducted at 250°C and 265 atm. The experiments were terminated before the system had stabilized completely, but these results indicate that amino acids equilibrate in aqueous solution at hydrothermal conditions. Although unintentional, this may be the first experimental confirmation that metastable equilibria characterize the interaction of organic compounds in hydrothermal fluids.

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Analysis of the chemical disequilibrium provided by the mixing of hydrothermal fluids and seawater in present-day systems indicates that organic synthesis from CO2 or carbonic acid is thermodynamically favoured in the conditions in which hyperthermophilic microorganisms are known to live. These organisms lower the Gibbs free energy of the chemical mixture by synthesizing many of the components of their cells. Primary productivity is enormous in hydrothermal systems because it depends only on catalysis of thermodynamically favourable, exergonic reactions. It follows that hydrothermal systems may be the most favourable environments for life on Earth. This fact makes hydrothermal systems logical candidates for the location of the emergence of life, a speculation that is supported by genetic evidence that modern hyperthermophilic organisms are closer to a common ancestor than any other forms of life. The presence of hydrothermal systems on the early Earth would correspond to the presence of liquid water. Evidence that hydrothermal systems existed early in the history of Mars raises the possibility that life may have emerged on Mars as well. Redox reactions between water and rock establish the potential for organic synthesis in and around hydrothermal systems. Therefore, the single most important parameter for modelling the geochemical emergence of life on the early Earth or Mars is the composition of the rock which hosts the hydrothermal system.

The reactions that cause transformations in organic compounds in the Earth’s crust remain mysterious despite decades of research into how fossil fuel resources form. A major reason for this persistent mysteriousness is the failure of many researchers to realize the intimate involvement of water in those transformations. Our goal was to overcome this staggering ignorance by developing the means to calculate the consequences of reactions involving organic compounds and water. We pursued this research from 1989 through 2006, and this report focuses on progress between 2002 and 2006. There were two major obstacles that we overcame in the course of this research. On the one hand, we developed new theoretical equations that allow researchers to make these calculations. On the other hand, we critiqued available data and provided sound means to make estimates in the absence of experimental data for hundreds of organic compounds dissolved in water. Finally, we merged these two lines of research into an interactive web site that allows users to do the calculations with the equations and data. We call the web site ORCHYD for: “ORganic Compounds HYDration properties database,” but it is far more than a database since it allows users to make extremely accuratemore » predictions of data that may never have been measured. Our progress greatly exceeded our anticipations, and has permitted many new research investigations that were previously impossible. Despite the abrupt termination of funding for this project by the Department of Energy, we are maintaining the web site for the international scientific community. Major research results were published in eleven scientific papers, so they are all in the public domain. Benefits to the public include a new, rigorous, quantitative approach to testing ideas about the fate of organic compounds dissolved in water. These tests can be applied to geochemistry or to industrial processes. The increasing use of water as a solvent in Green Chemistry and Sustainable Industrial Practices, means that there is a pressing need for data and calculations of the type we have provided and enabled. Applications include: improved efficiency of industrial processes, improvements in health protections for workers who encounter organic solvents and other organic compounds, predictive tools for cleaning up landfills and other toxic waste, and the potential for a greater understanding of how petroleum forms.« less

[1] The discovery of sulfate-rich layered deposits with hematite spherules at the landing site of the Opportunity rover is consistent with mineral deposition in an aqueous environment. We evaluate conditions responsible for the formation of a jarosite-goethite-gypsum assemblage with speciation calculations. The results show that the assemblage could have precipitated from acidic solutions formed through near-surface aqueous oxidation of pyrite. Our hypothesis is that regional heating in the Meridiani Planum caused a release of sulfide-rich hydrothermal waters, leading to formation of pyrite-rich regional deposits in a depression. Aqueous oxidation of these deposits by atmospheric O2 created an acidic environment that allowed formation of sulfates and goethite. Partial neutralization of the solution caused further goethite precipitation and conversion of jarosite to goethite, leading to formation of goethite concretions. Subsequent dehydration of goethite to coarse-grained hematite would also have been facilitated by regional heating.

Jupiter’s three innermost moons experience tidal forcing due to the 4:2:1 orbital resonance they share. In Io, the closest, the energy dissipated by flexure of the moon’s mantle is enough to make it the most volcanically active body in the solar system. In Europa, dissipation may be vary from a tenth to only a hundredth that in Io, but in theory this is still sufficient to melt much of the 170 km of ice (Anderson et al 1997) covering its surface and provide a moderate heat source (Hussman 2003). In fact, surface geology and planetological properties measured by the Galileo probe support the idea that Europa has an ocean perhaps as deep as 160 km, with a primary salt composition of Mg and Na sulfates (Greenberg et al 2000). With the aim of including pressure effects in future simulations of composition and dynamics in the ocean, we find predictions of supcrt92 software consistent with experimentally obtained molar volumes around 50°C and up to 2000 atm, roughly the pressure at the base of Europa’s ocean. Understanding hydrothermal activity in Europa’s crust, in the past and possibly in the present, is crucial to understanding dynamics and chemistry of the ocean as a whole, and for evaluting any biological activity that may have occured during the moon’s history. In this paper we explore the limits of theoretical exploration of Europa’s ocean, highlighting areas for further research.

[1] Galileo spacecraft data obtained from Jupiter's ice-covered satellite Europa suggest the existence of a subsurface water ocean. The formation of this ocean is a direct consequence of early igneous and hydrothermal processes that may have facilitated oxidation through H2 and CH4 escape. These processes could have led to a moderately alkaline sulfate-carbonate ocean and a moderately oxidized magnetite-bearing silicate mantle. When the ocean formed, low-temperature chemical disequilibria involving oceanic sulfate, bicarbonate, and ferrous minerals in underlying rocks could have provided multiple energy sources for chemotrophic organisms. Potential metabolic processes include oxidation of ferrous iron, sulfides, native sulfur, methane, hydrogen, and organic compounds, as well as reduction of water, sulfate, bicarbonate and carbonate ions (methanogenesis and acetogenesis), native sulfur, and ferric iron. If they occurred, these reactions would have provided close coupling for biogeochemical cycles of S, C, and Fe in the ocean. Periodic supplies of fresh rocks and/or aqueous fluids at the oceanic floor could drive these cycles throughout the satellite's history, including the present epoch. Signs of chemical disequilibria in the ocean would indicate chemical sources of energy for metabolism but should not be considered as indicators of life. In turn, observational signs of low-temperature redox equilibration among sulfur and carbon species would be suggestive of ancient life.

It is often argued that substantially more carbon dioxide and water were degassed from the martian interior than can be found at present in the atmosphere, polar caps and regolith. Calculations have shown that atmospheric escape cannot account for all of the missing volatiles. Suggestions that carbon dioxide is stored as marine or lacustrine deposits, are challenged by Earth-based and spacecraft remote-sensing data. Moreover, recent modelling of the martian atmosphere suggests that rainfall or open bodies of water are in any case unlikely to have persisted for extended periods of time. Hydrothermal carbonates therefore provide a possible solution to this dilemma. Using an accessible terrestrial system (Iceland) as a guide to the underlying processes, and a host rock composition inferred from the least-altered martian meteorite, we present a geochemical model for the formation of carbonates in possible martian hydrothermal systems. Our results suggest that an extensive reservoir of carbonate minerals--equivalent to an atmospheric pressure of carbon dioxide of at least one bar--could have been sequestered beneath the surface by widespread hydrothermal activity in the martian past.

[1] Formation of a sulfate-bearing ocean on Jupiter's satellite Europa by quenched hydrothermal fluids provides a source of metabolic energy for low-temperature sulfate-reducing organisms that use dissolved H2 as an electron donor. Inhibition of thermodynamically favorable sulfate reduction in cooled hydrothermal fluids creates the potential for biologic reduction. Both high temperature and reduced conditions of ocean-forming hydrothermal solutions favor sulfate reduction in quenched fluids. The maximum amount of energy available to support autotrophic sulfate reduction is on the order of a few kilojoules per kilogram of water and is limited by the low abundances of either H2 or sulfate in ocean-forming fluids. Although this irreplaceable energy source might have supported early life on Europa, maintenance of biologic sulfate reduction throughout the ocean's history would require a supply of organic compounds from endogenic sources or from the satellite's surface.

Thermophilic microbial inhabitants of active seafloor and continental hot springs populate the deepest branches of the universal phylogenetic tree, making hydrothermal ecosystems the most ancient continuously inhabited ecosystems on Earth. Geochemical consequences of hot water-rock interactions render these environments habitable and supply a diverse array of energy sources. Clues to the strategies for how life thrives in these dynamic ecosystems are beginning to be elucidated through a confluence of biogeochemistry, microbiology, ecology, molecular biology, and genomics. These efforts have the potential to reveal how ecosystems originate, the extent of the subsurface biosphere, and the driving forces of evolution.

Densities of dilute aqueous solutions of isopropanol, 1,5-pentanediol, cyclohexanol, benzyl alcohol, diethyl ether, 1,2-dimethoxyethane, acetone, and 2,5-hexanedione were measured by means of a vibrating-tube flow densimeter at temperatures near T = (302, 373, 423, 473, and 521) K at a pressure of p = 28 MPa. At the lowest and highest temperatures, measurements were also made close to the saturation vapour pressure of water to investigate the effect of pressure on the volumes of solutes. Apparent molar volumes were calculated for each solute and extrapolated to give partial molar volumes at infinite dilution. The variation of the volume with temperature, pressure, and structure of solute is discussed qualitatively, and group contributions are determined at the temperatures of measurements and p = 28 MPa. Several equations proposed in the literature for correlating the partial molar volumes at infinite dilution as a function of state parameters are tested. Parameters of one selected equation are tabulated allowing calculation of the partial molar volumes at infinite dilution at temperatures and pressures up to T = 573 K and p = 40 MPa, respectively.

The potential biomass that could have existed on Mars is constrained by the total amount of energy available to construct it. From an inventory of the available geochemical sources of energy, we estimate that from the time of the onset of the visible geologic record 4 b.y. ago to the present, as much as 20 g cm−2 of biota could have been created. This is the same amount that could have been constructed on the early Earth in only 100 million years. This indicates that there likely was sufficient energy available to support an origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere. Similar calculations for Europa suggest that even less geochemical energy would have been available there.

The possibility for abiotic synthesis of condensed hydrocarbons in cooling/diluting terrestrial volcanic gases has been evaluated on the basis of the consideration of metastable chemical equilibria involving gaseous CO, CO2, H2 and H2O. The stabilities of n-alkanes and polycyclic aromatic hydrocarbons (PAHS) have been evaluated for several typical volcanic gas compositions under various conditions for cooling/diluting of quenched volcanic gas. The modeling shows that n-alkanes and PAHs have a thermodynamic potential to form metastably from H2 and CO below approximately 250 degrees C within the stability field of graphite. Despite the predominance of CO2 in volcanic gases, synthesis of hydrocarbons from CO2 and H2 is less favored energetically than from CO and H2. Both low temperature and a high H/C atomic ratio in volcanic gas generally favor stability of hydrocarbons with higher H/C ratios. PAHs are thermodynamically stable at temperatures approximately 10 degrees -50 degrees C higher than large n-alkanes; however, at lower temperatures, PAHs and n-alkanes have similar stabilities and are likely to form metastable mixtures. Both the energetic drive to form hydrocarbons and possible temperatures of formation increase as the oxidation state (fO2) of the volcanic gases decreases and as the cooling/dilution ratios of volcanic gases increase. Synthesis of hydrocarbons is energetically more likely in cooling trapped gases than in ashcloud eruptive columns. Mechanisms for hydrocarbon formation may include Fischer-Tropsch-type synthesis catalyzed by magnetite from solid volcanic products. On the early Earth, Mars, and Jupiter's satellite Europa, several factors would have provided more favorable conditions for hydrocarbon synthesis in volcanic gases than under current terrestrial conditions and might have contributed to the production of organic compounds required for the emergence of life.

Discoveries in biology and developments in geochemistry over the past two decades have lead to a radical revision of concepts relating to the upper temperature at which life thrives, the genetic relationships among all life on Earth, links between organic and inorganic compounds in geologic processes, and the geochemical supply of metabolic energy. It is now apparent that given a source of geochemical energy, in the form of a mixture of compounds that is far from thermodynamic equilibrium, microorganisms can take advantage of the energy and thrive without the need for photosynthesis as a means of primary productivity. This means that life can exist in the subsurface of a planet such as Mars without necessarily exhibiting a surface expression. Theoretical calculations quantify the geochemically provided metabolic energy available to hyperthermophilic organisms in submarine hydrothermal systems on the Earth, and help to explain the enormous biological productivity of these systems. Efforts to place these models in the context of the early Earth reveal that substantial geochemical energy would have been available and that organic synthesis would have been thermodynamically favored as hydrothermal fluids mix with seawater.

There are many readings of the universal phylogenetic tree constructed from small subunits of RNA, such as that shown in Fig. 1. As a geochemist, two messages seem particularly clear to me from deep in the tree. First, the emphasis is on geochemical energy sources, which take the form of naturally occurring disequilibrium states engendered by geologic processes. Second, deep autotrophy does not require sunlight. These two messages suggest that early life was more likely to have lived in subsurface rather than surface environments. A logical deduction from the 16s rRNA phylogenetic tree (Fig. 1) is that the setting for the emergence of life had little if anything to do with the atmosphere or surface of the Earth. According to some estimates the subsurface biosphere still rivals by mass the surface biosphere (Whitman et al., 1998). We probably find this surprising because we ascribe great importance to the surface biosphere owing to our banishment there by our need for O2. Nevertheless, deeply branching organisms in the universal phylogenetic tree show a proclivity for H2, or at least for reduced conditions where even trace concentrations of O2, sulfate, carbon dioxide and other oxidized compounds represent enormous energy sources. Open image in new window Fig. 1 (after Pace, 1997) Universal phylogenetic tree of life based on 16s rRNA data. Lengths of lines separating organisms represent the genetic difference between them. Deep, short branches of the tree are populated by thermophilic and hyperthermophilic organisms.

Thermodynamic calculations showed that the autotrophic synthesis of all 20 protein-forming amino acids was energetically favored in hot (100 degrees C), moderately reduced, submarine hydrothermal solutions relative to the synthesis in cold (18 degrees C), oxidized, surface seawater. The net synthesis reactions of 11 amino acids were exergonic in the hydrothermal solution, but all were endergonic in surface seawater. The synthesis of the requisite amino acids of nine thermophilic and hyperthermophilic proteins in a 100 degreesC hydrothermal solution yielded between 600 and 8000 kilojoules per mole of protein, which is energy that is available to drive the intracellular synthesis of enzymes and other biopolymers in hyperthermophiles thriving in these ecosystems.

If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.

Submarine hydrothermal systems on the early Earth may have been the sites from which life emerged. The potential for Strecker synthesis to produce biomolecules (amino and hydroxy acids) from starting compounds (ketones, aldehydes, HCN and ammonia) in such environments is evaluated quantitatively using thermodynamic data and parameters for the revised Helgeson-Kirkham-Flowers (HKF) equation of state. Although there is an overwhelming thermodynamic drive to form biomolecules by the Strecker synthesis at hydrothermal conditions, the availability and concentration of starting compounds limit the efficiency and productivity of Strecker reactions. Mechanisms for concentrating reactant compounds could help overcome this problem, but other mechanisms for production of biomolecules may have been required to produce the required compounds on the early Earth. Geochemical constraints imposed by hydrothermal systems provide important clues for determining the potential of these and other systems as sites for the emergence of life.

This chapter summarizes some of the insights gained about the behavior of organic acids in geochemical processes through thermodynamic calculations. Such calculations are possible because of the combination of numerous experimental studies on aqueous organic acids and theoretical equations of state, which allow accurate extrapolations of the measurements as well as predictions of thermodynamic properties of aqueous organic acids for which data have not been measured. Estimates of thermodynamic data for aqueous organic acids allow quantitative tests of several hypotheses concerning the role of organic acids in geochemical processes. For example, thermodynamic calculations described in this chapter indicate that decarboxylation of organic acids is unlikely to proceed to any significant extent under sedimentary basin conditions. At the partial pressures of CO2 and CH4 associated with oil-field brines, equilibrium constants for the decarboxylation of acetic acid require the acid concentrations to be many orders of magnitude lower than reported values.

Amino acids, a basic constituent of life, can form in dust grains that are similar to those found in the space between stars. But how much does this tell us about the origins of life on Earth?

Carbon-containing meteorites provide a natural sample of the extraterrestrial organic chemistry that occurred in the solar system ahead of life's origin on the Earth. Analyses of 40 years have shown the organic content of these meteorites to be materials as diverse as kerogen-like macromolecules and simpler soluble compounds such as amino acids and polyols. Many meteoritic molecules have identical counterpart in the biosphere and, in a primitive group of meteorites, represent the majority of their carbon. Most of the compounds in meteorites have isotopic compositions that date their formation to presolar environments and reveal a long and active cosmochemical evolution of the biogenic elements. Whether this evolution resumed on the Earth to foster biogenesis after exogenous delivery of meteoritic and cometary materials is not known, yet, the selective abundance of biomolecule precursors evident in some cosmic environments and the unique L-asymmetry of some meteoritic amino acids are suggestive of their possible contribution to terrestrial molecular evolution.

The serious scientific debate about spontaneous generation which raged for centuries reached a climax in the nineteenth century with the work of Spallanzani, Schwann, Tyndall, and Pasteur. These investigators demonstrated that spontaneous generation from dead organic matter does not occur (see reviews by Conant, 1953 and Kenyon and Steinman, 1969). Although no aspects of these experiments addressed the issue of whether organic compounds could be synthesized abiotically, the impact of the experiments was great enough to cause many investigators to assume that life and its organic compounds were somehow fundamentally different than inorganic compounds.

Perhaps because black-smoker chimneys make tremendous subjects for magazine covers, the proposal that submarine hydrothermal systems were involved in the origin of life has caused many investigators to focus on the eye-catching hydrothermal vents. In much the same way that tourists rush to watch the spectacular eruptions of Old Faithful geyser with little regard for the hydrology of the Yellowstone basin, attention is focused on the spectacular, high-temperature hydrothermal vents to the near exclusion of the enormous underlying hydrothermal systems. Nevertheless, the magnitude and complexity of geologic structures, heat flow, and hydrologic parameters whieh characterize the geyser basins at Yellowstone also characterize submarine hydrothermal systems. However, in the submarine systems the scale can be considerably more vast. Like Old Faithful, submarine hydrothermal vents have a spectacular quality, but they are only one fascinating aspect of enormous geologic systems operating at seafloor spreading centers throughout all of the ocean basins. A critical study of the possible role of hydrothermal processes in the origin of life should include the full spectrum of probable environments.