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Executive Summary

The traditional view of life’s origin on Earth is that it began where atmosphere and oceans meet, where sunlight could drive life’s processes. But there is another, and astrobiologically exciting, possibility. Cracks along deep ocean floors spew out water heated to more than 350^oC by magma welling up beneath Earth’s mantle. That environment, primarily the product of bacteria, supports a thriving ecosystem that flourishes in timeless darkness. Life’s energy is derived chemically from dissolved nutrients. If scientists are right about this alternative theory of life’s origins, it means that the search for other planets in other star systems potentially capable of supporting life would no longer be limited to those with surface water.

The Carnegie Institution of Washington team is studying the physical, chemical, and biological evolution of hydrothermal systems, including vent complexes associated with ocean ridges, deep aquifers, and other subsurface aqueous environments, both on Earth and on other Solar System and extrasolar bodies. Such diverse systems are important environments for life on Earth and possibly elsewhere in the cosmos.

The traditional view of life’s origin on Earth has focused on processes near the photic zone at the ocean-atmosphere interface, where ionizing radiation provides energy for prebiotic organic synthesis. In the context of astrobiology, this origin paradigm restricts the initial "habitable zone" around stars to planets and moons that have surface water. According to this view, subsequent adaptations on Earth, and possibly elsewhere, led to expansion of the biosphere into subsurface habitats.

An alternative hypothesis is that life-forming processes may also occur in subsurface hydrothermal environments at the water-mineral interface. This hypothesis, that life on Earth originated from oxidation-reduction reactions in deep hydrothermal zones, perhaps at or near ocean ridge systems, opens exciting possibilities for astrobiological research. If a subsurface, high-pressure origin of life is possible, then the initial habitable zone around stars is greatly expanded to aqueous environments in which redox reactions can be driven along thermal and chemical gradients.

Several lines of evidence lend credibility to the hydrothermal-origins hypothesis. Numerous recent discoveries of high-pressure life, especially lithotrophic prokaryotes, suggest that hydrothermal environments support abundant life. Models of the Earth’s formation postulate large, surface-sterilizing impacts as recently as 3.8 billion years ago, but deep hydrothermal zones may have insulated life from these traumas. Studies of molecular phylogeny reveal that thermophilic microbes are perhaps the closest living relatives of the last universal common ancestor. Finally, hydrothermal organic synthesis experiments reveal unexpectedly facile synthetic pathways. Whether or not life originated in a subsurface hydrothermal zone, these lines of evidence, coupled with the assumed widespread distribution of such environments in our Solar System and elsewhere, point to the need and opportunity for an intense study of the characteristics of hydrothermal systems.

The Carnegie team’s research activities explore the physical, chemical, and biological evolution of hydrothermal systems from the following complementary fronts:

• model planetary formation and undertake the detection and characterization of extrasolar planets in an effort to understand the range of objects that develop hydrothermal systems, as well as the distribution of volatiles, especially water, within those objects;

• investigate the circumstances under which hydrothermal systems form on planets and other bodies and the expected physical and chemical characteristics of those systems as they evolve;

• study geochemical processes in hydrothermal systems, especially those that lead to abiotic organic synthesis. A particular focus is the role of mineral catalysis in these systems; and

• consider the origin and evolution of biological entities in hydrothermal systems through studies of the biochemistry of contemporary hydrothermal organisms.

A complete understanding of hydrothermal systems and their role in life’s origins requires dramatic advances on all of these fronts, as well as an extensive and challenging integration of these topics. During the past year we achieved significant progress in each of these research areas, as well as an increased attention to the interfaces among these theoretical, experimental, and field approaches.

Among the highlights from the past year’s research in the area of planetary formation and evolution, members of our team:

• discovered 20 new extrasolar planets, including the lowest-mass planet yet found and the first planet in a Jupiter-like orbit beyond 5 astronomical units (AU) from its parent star;

• carried out the first spatially resolved mid-infrared spectroscopy of a planet-forming disk, Beta Pictoris, which show how silicates are distributed at a young stellar age of 12 million years;

• proposed a new scenario for the formation of the four giant planets of our Solar System by gravitational instability in the protoplanetary disk, followed by partial disk removal and photoevaporation by extreme- and far-ultraviolet (EUV/FUV) radiation from a nearby massive star; and

• documented new evidence for the exchange of water between the surface and interior of Mars during the earliest Noachian epoch.

In regard to the evolution of organic matter and water in meteorites, we

• demonstrated that the hypothesis that shock waves in the solar nebula led to melting and chondrule formation can account for many aspects of chondrule petrology.

• showed through the first self-consistent chemical analyses of meteoritic macromolecular material that it consists predominantly of small aromatic molecules whose differences in bulk chemistry among three carbonaceous chondrites suggest different processing histories.

• showed that hydrogean and nitrogen are isotopically heterogeneous on a micron scale and that the lack of correlation between hydrogen and nitrogen indicates distinct presolar organic carriers of the isotope anomalies.

• found new evidence that extraterrestrial water in exists in nakhlites, the least altered and shocked of meteorites thought to have come from Mars, indicating that shock metamorphism likely did not strongly influence the deuterium-to-hydrogen (D/H) ratios in melt inclusions in Martian meteorites.

• demonstrated that although iron isotopic compositions of terrestrial igneous rocks, chondrites, and iron meteorites are remarkably similar -demonstrating large-scale iron isotopic homogeneity in the inner Solar System- there are resolvable isotopic differences between iron in bacterial mats and in the rocks that surround them, suggesting that iron isotopes may serve as biomarkers.

Relative to experimental tests of proposed hydrothermal organic synthesis reactions, we

• identified a potential primordial carbon-fixation pathway by means of hydrothermal experiments involving mineral-catalyzed organic reactions;

• documented a broad range of catalytic activity in transition-metal sulfides, providing support for such sulfides as the most likely pre-enzymatic catalysts in the pre-biotic world;

• explored the stability of organic compounds at high pressure, raising the question of whether the ice-rock interface within the larger icy satellites (e.g., Ganymede) may be sterile; and

• completed new experimental measurements of amino acid oligomerization that point to a kinetics and thermochemistry framework governing prebiotic peptide formation that is considerably more favorable than current thinking would have predicted.

Regarding support of theoretical studies of hydrothermal synthesis reactions, we

• demonstrated by illustration that the combination of genomic data and geochemical modeling can lead to new ideas about the mechanisms of evolution and geochemical preservation of evolutionary history

• documented sources of geochemical energy that are available to support life in the vicinity of an ocean floor on Europa.

With respect to isotopic and molecular tracers of life, the following were accomplished.

• the distinctive carbon isotopic signature of lignin in fossil land-plant tissues hundreds of millions of years old

• demonstrated with C-XANES that fossil tracheid walls of a primitive 400-million-yeaf-old vascular plant preserve evidence that lignin and cellulose were structural polymers in this plant, suggesting that lignification may have provided a key structural evolution enabling plants to colonize the continents

• initiated measurements of oxygen isotopes in phosphate minerals, with the aim of determining if such measurements can track whether the phosphates experience biological processing

• established the Enspel formation in Germany, one of the premier sites for recovery of fossil bacterial biofilms associated with the preservation of soft tissues from leaves and animals, as a test bed for new biomarker techniques, including antibodies to hopanes and PAHs

• contributed to analyses of the Archean Apex chert (a flint-like rock) indicating that traces of biological activity in those samples are questionable and that the chert is part of an ancient hydrothermal system rather than from a shallow lagoon environment, as previously argued by others

• confirmed, through new measurements, the discovery of Farquhar and coworkers of mass-independent sulfur isotope anomalies in Archean and early Proterozoic rocks, as well as the absence of such anomalies in younger samples.

Pertaining to biological studies of hydrothermal systems, it was

• demonstrated that there is a high diversity of both bacteria and archaea in the subseafloor near deep-sea hydrothermal vent fields;

• observed that intact microbes exist throughout active sulfide chimney structures, including mineral zones thought to be at temperatures greater than 150°C;

• documented that lithotrophic Fe-oxidizing bacteria both increase the rate of Fe-oxidation by about 50% and decrease the rate of abiotic oxidation by 25%-30%;

• determined that calcite crystals, when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of D- and L-enantiomers on pairs of mirror-related crystal growth surfaces; and

• demonstrated that microbial life can survive at gigapascal pressures, levels found deep in the interiors of the terrestrial planets and icy satellites.

Under a new project initiated last year, members of our team are developing protein chip-based molecular-recognition technology as a method for detecting life on Earth and on other Solar System bodies. The Ciphergen Biosystems Protein Chip Reader utilizes specially tailored surface chemistry on small chips to capture, selectively, femtomole amounts of complex mixtures of organic molecules for molecular-weight determination by time-of-flight mass spectrometry. Together with collaborators from several other NAI teams, our group has begun to intercompare this new technology with different schemes for analysis of biomolecules. One recent application was to provide ground truth to protein assays performed by a microarray in high-, low-, and zero-gravity experiments involving antibody-antigen reactions. Antigen-antibody binding was found to be significantly greater in Martian gravity than during hypergravity (1.8 g). In another application, Shewanella was cultured at different temperatures to study the induction of heat shock proteins and the effect of temperature on the proteome. The protein chip technology was able to distinguish rapidly the changes in the microbial proteome as environmental conditions fluctuated.

In summary, this recent research, including discoveries of new planetary systems, exploration of possible hydrothermal regimes on other worlds, elucidation of robust hydrothermal synthetic pathways, documentation of novel microbial metabolic strategies, and the finding of unexpected high-pressure environments for life, inform the central questions of astrobiology. Taken together, these discoveries are changing our views about the origin of life and its distribution in the universe.