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1 -- Plenary Sessions
TUESDAY, April 15, 2008: MORNING
8:00 – 9:30 am: Welcome and Morning Plenary: “Life in a Cosmic Context”
Organizer: Paul Davies, Arizona State University, USA
1-01-O. Emergent Complexity in a Cosmic Context
Lord Martin J Rees, Institute of Astronomy, Cambridge University, UK
The talk will first outline how cosmologists now envisage our dynamic universe, and identify the crucial steps that led from a ‘big bang’ to our emergence here on Earth. Several distinct questions then arise. (i) How ‘special’ did our Earth have to be, for creatures like us to emerge? (ii) Is there a much wider range of habitats in which other (quite different) embodiments of life could emerge, or in which our posthuman descendants could flourish in the far future? (iii) Does the emergence of anything as complex as life depend on special features of the fundamental laws? The fundamental laws of physics seem the same in all parts of the universe astronomers can observe. But this third question is interesting because we now realise that the domain that telescopes can probe—vast though it is—could be a tiny part of the aftermath of the big bang; moreover, ‘our’ big bang could be one of many. What we traditionally regard as fundamental laws may not be universal, but mere bylaws in our cosmic patch (a patch which, however, is far more than 10 billion light years in extent). This opens up a still grander perspective on the potential for emergent complexity.
TUESDAY, April 15, 2008: AFTERNOON
1:30 – 3:00 pm Afternoon Plenary: “The Formation of Habitable Solar Systems”
Organizers: Alan Boss, Carnegie Institution of Washington,USA, and Meenakshi Wadhwa, Arizona State University, USA
1-02-O. Making Water Worlds: The Role of 26Al
Steve Desch and Jeff Hester, Arizona State University, USA
The Earth is commonly perceived as a water-rich planet, but the fraction of the Earth's mass that is water is < 0.1%, much less than the cosmochemical rock/ice fraction. What little water the Earth has is believed to be due to accretion of planetesimals that formed beyond 2.5 AU in the present-day asteroid belt. Asteroids with semi-major axes < 2.5 AU are predominantly S type, associated with ordinary chondrites, which are < 0.1% water by mass; asteroids beyond 2.5 AU are predominantly C type asteroids, associated with carbonaceous chondrites which are ~ 10% water by mass. This dichotomy in the asteroid belt may be associated with a condensation front within the solar nebula (a snow line), but more reasonably is due to the relationship between accretion times and heliocentric distance (Grimm and McSween 1989). Asteroids inside 2.5 AU accreted to ~ 10 km quickly enough to be heated and desiccated by the 26Al they captured; because asteroids beyond 2.5 AU grew more slowly, they accreted little 26Al, experienced less radiogenic heating, and retained more water. The presence of 26Al in the solar system has been attributed to injection of radionuclides into the solar system by a nearby supernova within the Sun's birth cluster. The amount of 26Al accreted by the Solar System was happenstance which depended on the properties of the birth cluster of the Sun, the chance location of the young Sun within that cluster, and the timing and pattern of distribution of ejecta from the supernova. We would expect that the amount of 26Al in similar systems might vary appreciably. In particular, some systems probably form with little or no 26Al at all. Terrestrial planets formed in those systems easily could have at least 10 times as much water as does Earth. We will discuss the formation of Sun-like stars, the likelihood of acquiring 26Al, its role in desiccation of asteroids, and the terrestrial planets likely to form in various systems.
1-03-O. The Origin of the Elements
Anna Frebel, McDonald Observatory, University of Texas at Austin, USA
I will address one of the most fundamental questions of modern astrophysics, the origin of the elements. The National Academies have listed among their eleven big science questions in the new century “How Were the Elements from Iron to Uranium Made?” The answer is key to understanding the history of baryons, and thus ultimately linked to the origin of life. Following the Big Bang, the primordial nucleosynthesis created mostly hydrogen and helium. All other elements, including those most vital for life, were subsequently created in stars, during their life times and explosive deaths. The metal inventory of the Universe has thus been increasing ever since up to about the level found in the Sun. The origin and evolution of the elements over the age of the Universe will be discussed with particular emphasis on bio-essential nuclei such as carbon, nitrogen, oxygen, and iron. Understanding the production mechanisms and timescales of these elements may shed light on the history of the habitability of the Universe.
1.04-O. Astrochemistry and Cosmochemistry in Astrobiology
Edward Young, University of California Los Angeles, USA
Advances in studies of meteorites, the field of cosmochemistry, and the chemistry of protoplanetary disks, a new branch of astrochemistry, help us to gauge the likelihood that Earth-like planets are a typical, or at least a non-negligible, outcomes of planet formation. Reactions between water and rock in planetesimal settings are driven by decay of short-lived radionuclides (especially 26Al), the origins of which remain controversial. These reactions leave behind clues about the origin and amount of water harbored by planetary precursors. These clues are found in aqueously altered meteorites. Collateral effects of water-rock reactions include organosynthesis. Studies of altered meteorites can constrain the origin of Earth’s water; large amounts of water in planetesimals imply that terrestrial water could represent residuals left behind from coalescing water-rich planetesimals. Conclusions about the exogenous or endogenous origins of terrestrial water must include consideration of isotopic constraints. Isotopes of H, O, C and N are proving to be particularly diagnostic. Advances in a new brand of “disk astrochemistry” compliment meteoritical studies. Disk chemistry studies demonstrate that arguments about the origins of Earth’s water based on comparisons with cometary D/H are not unique. High-resolution spectroscopic observations of isotope ratios in circumstellar disks are now coming on line. Examples to be reported at this meeting establish a galactic context for terrestrial oxygen. With these new data it is possible to begin to trace the chemistry that leads to the formation of water and organic molecules in disks. Important reactions are triggered in part by photochemistry in disks.
WEDNESDAY, April 16, 2008: MORNING
8:00 – 9:30 am: Morning Plenary: “The Molecules of Life:
A Tribute to Stanley L. Miller and Leslie E. Orgel”
Organizer: Antonio Lazcano, Universidad Nacional Autónoma de México, MX
1-05-O. A Tribute to Stanley L. Miller
Jeffrey Bada, Scripps Institution of Oceanography, University of California, San Diego, USA
Stanley Miller is considered to be the father of prebiotic chemistry for the famous experiment he conducted in the early 1950s with his mentor Harold Urey on how the simple organic compounds considered necessary for the origin of life could have been synthesized on the early Earth. His landmark paper, published May 15, 1953, in Science, demonstrated how amino acids and other compounds were produced by the action of a spark discharge with hydrogen, water, methane and ammonia.
The first assistant professor recruited to the new Department of Chemistry at the University of California at San Diego in 1960, Stanley continued his research into the chemical origins of life for over four decades. Most of his research done at UCSD was focused on the prebiotic synthesis of small molecules, the conditions on the primitive Earth and how the transition from prebiotic molecules to the first self-replicating living entities may have taken place. In addition, he was a pioneer in the investigation of the natural occurrence of clathrate hydrates, both on Earth and in the solar system. Finally, Stanley also had an interest in the possibility of life beyond Earth, especially Mars. He worked on the development of a miniature amino acid analyzer and proposed the instrument for consideration on the 1976 NASA Viking missions, but his proposal was not accepted. Recently the idea of searching for amino acids on Mars has been revived and is the bases for the Urey Organic and Oxidant Detector selected for the instrument payload on the Pasteur rover of the European Space Agency’s 2013 ExoMars mission.
1-06-O. A Tribute to Leslie E. Orgel
Gerald Joyce, The Scripps Research Institute, La Jolla, CA, USA
Leslie Orgel played a leading role in setting the agenda for the experimental study of the origins of life. His own work addressed many aspects of this challenging problem, including the prebiotic synthesis of the building blocks of nucleic acids, the joining of nucleotides to form polynucleotides, and the non-enzymatic copying of RNA. He was the first to demonstrate information transfer from a pre-formed RNA template to a newly-synthesized RNA product in a purely chemical system. The concept of information transfer was critical for Orgel because his ultimate aim was to demonstrate a chemical system that is capable of undergoing Darwinian evolution, something he regarded as synonymous with the origin of life. My laboratory has similar goals, but employs techniques of in vitro evolution to develop RNA enzymes that catalyze reactions relevant to the origins of life. As in Orgel’s work, the key reaction is the RNA-templated joining of RNA, ultimately leading to RNA-catalyzed RNA replication. We and others have developed various RNA enzymes with RNA-joining activity. One such enzyme was configured so that it could produce additional copies of itself by joining two component oligonucleotides. It subsequently was converted to a cross-catalytic format whereby two RNA enzymes catalyze each other’s synthesis from a total of four oligonucleotides. Recently, we optimized the activity of the cross-replicating RNA enzymes so that they can undergo self-sustained exponential amplification in the absence of proteins. In one such experiment, the RNA enzymes underwent billion-fold amplification in 30 hours at a constant temperature of 42 °C.
WEDNESDAY, April 16, 2008: AFTERNOON
1:30 – 3:00 pm Afternoon Plenary: “The Evolution of Life: New Approaches to an Ancient Problem”
Organizer: Dawn Sumner Moderator: Shuhai Xiao
1-07-O. 3.43 Billion Year-old Stromatolites and New Approaches to Deep Time Paleobiology
Abigail Allwood, Jet Propulsion Laboratory, California Institute of Technology, USA
The abundant, diverse and relatively well preserved stromatolites of the 3.43 Ga Strelley Pool Formation, Pilbara Craton, Western Australia, are potentially a rich cache of information about early life and environments. However, the amount of biological information captured in stromatolites, if any, is far from certain. Because stromatolites are essentially sedimentary structures, not true fossils, their interpretation requires a strong sedimentological perspective; integrating such factors as sedimentary fabrics and microfacies, facies assemblages, and depositional architecture of the entire host deposit. This contrasts with traditional paleontological and geochemical approaches, which center on stromatolite “specimens” or “localities” and have failed to yield conclusive insights to the origin of the structures. The integrated sedimentological approach has been applied to the Strelley Pool Formation and much insight gained from examining properties of the stromatolite assemblage as a whole, such as: co-variations in recrystallized fabric and stromatolite shape; correlated changes in lamina character and accretionary architecture; changes in stromatolite shape or distribution that coincide with changes in environment; variations in stromatolite shape or laminar fabric across unchanging environments. Now, as targeted, precise geochemical and organic geochemical data are obtained in the Strelley Pool Formation, their interpretation is greatly constrained by their relationship with the fabrics and facies they are found in. The approach has proven useful in revealing new types of evidence for the origin of the stromatolites, as well as generating principles that can be applied to other cases.
Acknowledgments: Geological Survey of Western Australia; NASA Postdoctoral Program
1-08-O. Sedimentary Organic Matter
Tanja Bosak, Massachusetts Institute of Technology, USA
Microbial behaviors and metabolisms may have been responsible for some of the most unusual events in the history of our planet: the oxygenation of the atmosphere and oceans, deposition of stromatolites, banded iron formations and possibly for early Snowball Earth events. Studies of modern microbes can therefore greatly inform our understanding of the deep past. In particular, the meaning of organic biomarkers can be reevaluated using molecular microbiology, genomic sequencing and laboratory studies of modern organisms. Most interpretations of the preserved fossil biomarkers have depended on studies that sought to identify modern precursors of common sedimentary organic biomarkers in a limited number of cultured microbes. Recent studies have already led to novel insights into the microbial sources and functions of organic biomarkers and the discovery of novel organic biomarkers. Microbiological work will also lead to improved models of stromatolites, from scales visible to the naked eye to the microscale. Finally, the coupling of geochemical biosignatures with preserved morphological information at the microscale may enable the detection and timing of the evolution of specific microbial metabolisms and processes in ancient sediments.
1-09-O. Sulfur Cycling in the Neoproterozoic and the Relationship between Earth Surface Redox and the Evolution of Animal Life
Matt Hurtgen, Northwestern University, USA
In recent years, advances in the ability to evaluate the sulfur isotope composition of ancient seawater sulfate (via carbonate-associated sulfate) and pyrite have improved our understanding of the Precambrian exogenic sulfur cycle, growth of the marine sulfate reservoir and the redox evolution of the coupled ocean-atmosphere system. Sedimentary pyrite formation proceeds under anoxic conditions via microbial sulfate reduction. During this process, a significant kinetic isotope effect occurs as microbes preferentially remove the lighter 32S in the production of sulfide, which then reacts with iron to form pyrite. The extent of the fractionation between seawater sulfate and co-occurring sedimentary pyrite is controlled in large part by sulfate availability, local redox conditions and the nature of microbial sulfur cycling. The overall effect of sulfate limitation is a smaller isotope fractionation between seawater sulfate and pyrite. Importantly, the amount of oxygen in the ocean-atmosphere system is believed to be a primary control on marine sulfate levels because the primary source of seawater sulfate is riverine delivery resulting in part from the oxidative weathering of pyrite. The sulfur isotope composition of carbonate-associated sulfate and pyrite will be used to reconstruct the evolution of the sulfur isotope difference between ancient seawater sulfate and pyrite through the Neoproterozoic (1000-542 Ma) in order to investigate the relationship between marine sulfate levels, oceanic redox and the evolution of macroscopic Metazoa.
1-10-O. Using Biochemical Networks to Integrate Genomics with Biogeochemistry
Jason Raymond, University of California, Merced, USA
Our present knowledge of the history of life is the result of a) observing signatures, from fossils to isotope skews to biomarkers, that are preserved in the geological record and b) inferring characteristics of ancient organisms by comparing and contrasting their modern descendants. Occasionally these two approaches can be integrated, often with exciting (though sometimes controversial) results. A familiar example is the calibration of genetic distances using the fossil record, effectively attempting to add a timeline to the tree of life.
An alternate approach that we have recently been developing examines how biochemical networks--which can be reconstructed from genomic data--have adapted to perturbations in Earth's biogeochemical cycles, such as the first great oxidation event circa 2.5 billion years ago. By focusing on changes predicted to occur at the network level, we can begin to use biomarkers and other chemical signatures, such as the solubilization of trace metals in an oxic ocean, to gain insight not only into the appearance of individual reactions but also into their dependent reactions (both "downstream" and elsewhere in the network). Whereas calibrating species divergences using fossil ages works best when a rich, informative fossil record exists (i.e. recently), continuing improvements in resolving Precambrian chemical signatures suggests that this network-based approach may contribute novel insights into the development of early life.
1-11-O. New Approaches to an Ancient Problem of Sedimentary Organic Matter
Alex Sessions, California Institute of Technology, USA
Sedimentary organic matter is literally the remains of ancient life, and as such is arguably the most detailed record of that life. The extreme chemical complexity of organic matter is a paradox: it provides for very high information content, but also hinders efforts to measure and understand. I will argue that studies of ancient organic compounds are poised, over the next decade, to become one of the most important tools for deciphering the evolution of life. Three areas appear particularly appealing. First, the union of lipid (biomarker) analysis with genetics will provide a much more robust view of the distribution – in space and time – of particular molecular structures. Second, new instrumentation will allow for the analysis of organic materials at spatial scales relevant to microbes, and will have the same transformative impact on organic geochemistry that the electron microprobe had on petrology. Third, and somewhat farther in the future, the ability to examine isotopic abundance at the atomic rather than molecular scale will add a new dimension to the information content of organic structures. Ultimately, this capability may be the most convincing way to distinguish biotic versus abiotic materials.
THURSDAY, April 17, 2008: MORNING
Theme: The Search for Life
8:00 – 9:30 am: Morning Plenary: “Weird Life”
Organizers: John Baross, Jack Farmer and Bob Pappalardo
1-12-O. Weird or Impossible? The Limits of Environments for Biochemistry.
William Bains, Institute of Biotechnology, University of Cambridge, UK
What is the minimum description of life, and how does it constrain where we might look for it? Starting from a simple definition of “life”, I will discuss the basic physical and thermodynamic requirements for any biochemistry. Biology is unique in displaying replication that is not dependent on its substrate. So any biochemistry has to select a small number of reactions that form “metabolism” from an enormous reaction space. This requires catalysis (and hence modular catalysts such as polymers), and energy, both to drive reactions and for control. Polymeric catalysis requires a solvent, and the nature of the solvent and the level of metabolic energy needed are both dependent on temperature. However the stability and solubility of metabolites and the chemistry available to them are also dependent on temperature, and on the chemical nature of the solvent. Can we put these constraints together to rule out environments, such as Venerian sulphuric acid or Tritonian nitrogen lakes, in which a biochemistry could function? Is life in supercritical water or liquid hydrogen possible even in principle? I will describe how different temperatures can affect the chemical diversity available to construct a biochemistry, the thermodynamics of the resulting metabolism, and how this together with differences in solvent chemistry and in the solubility of metabolites together put limits on the environments in which the Œweirdest1 plausible chemical life could operate. Finally, I will discuss some examples of these limits, and types of chemical signatures by which life there could be recognised.
1-13-O. Four Experimental Paths Towards a Theory of Life
Steven A. Benner, Foundation for Applied Molecular Evolution, Gainesville, Florida, USA
While models for the living state (mathematical, cellular, and chemical, for example) are well advanced for contemporary terran life, any effort to constrain how life might appear if it had no relation to known terran life has difficulties breaking free of “terracentricity.” This issue was discussed at length in the recent report (Baross et al. (2007), The Limits of Organic Life in Planetary Systems, National Academies Press). According to Cleland and others, these difficulties arise in part from the absence of a "theory" for life, which would identify some essence of the living state. We expect that experimentation will complement philosophy in any effort to construct such a theory, and multiple lines of experimentation are available for this purpose. This talk will focus on just four of these: (a) paleogenetics, where ancient genes and proteins from now-extinct organisms are resurrected for study in the laboratory, (b) origins, where conditions are sought where organic species known in the cosmos are converted to biomolecules without the generation of tar, (c) planetary exploration, where the inventory of planetary systems able to support life is increased through a combination of planetary science and biology, and (d) synthetic biology, where artificial chemical systems are generated without experimentation being constrained by theories of origin or contemporary biology. The talk will discuss recent work in areas where these approaches join.
THURSDAY, April 16, 2008: AFTERNOON
1:40 – 3:00 pm Afternoon Plenary: “Life on Exoplanets: What Will Space Missions Tell Us?”
Organizer: Lisa Kaltenegger
1-14-O. Life on Exoplanets: What Will Space Missions Tell Us? An Introduction
Lisa Kaltenegger, Harvard Smithsonian Center for Astrophysics, USA
There are many puzzle-pieces to assemble if we want to detect life, signs of life, or even conditions for life, on exoplanets. Life is believed to require four conditions: thermodynamic disequilibrium, carbon chemistry, a liquid environment, and a molecular system that can support Darwinian evolution. What detectable signatures does such life leave in a planetary atmosphere? Future space missions will provide the pieces to solve this puzzle. In this session we will look at the evidence that we are searching for, as well as what we are likely to get, to define these pieces. This panel (J. Baross, S. Seager and W. Traub) will ask and address to what extent these conditions can be detected on exoplanets, using the suite of space missions that is presently being built or proposed.
1-15-O. Can the Limits of Carbon-Based Life Help Constrain Habitability Conditions of Extra-Solar Planets?
John A. Baross, University of Washington, USA
Since Earth is the only planet that unequivocally supports ecosystems, it is logical as a first-order priority to search for extra-solar planets that have measurable characteristics that resemble Earth. The very basic requirements of Earth-life are liquid water, sources of carbon, nitrogen, light or chemical energy, and other nutrients. Their identification in the atmosphere of an extra-solar planet would be exciting and profound. An argument can also be made that obtaining evidence for active tectonics and hydrothermal activity, mechanisms that are vital for extracting life-supporting volatiles and elements from rocks and creating diverse environmental settings, would also increase the probability for a life-supporting planet. However, inasmuch as we can use Earth-life as a point of comparison, the search for extra-solar Earth-like planets is limited by our incomplete understanding of the more than four billion years of environmental changes associated with evolving physiological diversity of life and changing ecosystems and the elucidation of contingent factors directing the evolution of life. An Earth-like extra-solar planet may evolve successful ecosystems and even highly complex organisms that bare no resemblance to Earth life either at the biochemical level or in the way the biosphere modulates atmospheric conditions. The following questions will be addressed in the context of our search for extra-solar planets. How versatile and adequate is the carbon-based model to environmental conditions that extend beyond the bounds found on Earth? Are there alternate carbon-based biochemistries that would allow organisms to exist under very different environmental conditions than can Earth-life? What are the limitations to evolutionary innovations in carbon-based life?
1-16-O. Biosignatures from Habitable Exoplanets
Sara Seager, Massachusetts Institute of Technology, USA
Our search for signs of life on exoplanets in the solar neighborhood requires first that we find suitable planets. Earth is the only suitable planet for life that we know of, and its large-scale biosignatures in the present and past have consequently been well documented. When discovered in large numbers, terrestrial exoplanets should have a wide range of masses and semi-major axes, just as the giant exoplanets have, caused by the stochastic nature of planet formation. For the same reason, terrestrial exoplanets should be very diverse in their interior and atmospheric compositions. I will discuss Earth's biosignatures, followed by the range of terrestrial exoplanet conditions that may exist. Given freedom from conventional requirements for life (presented in the first talk), I will venture ideas about potential biosignatures on planets very different from Earth.
1-17-O. How Will Space Missions Detect Signs of Life on Exoplanets?
Wesley A. Traub, Jet Propulsion Laboratory, California Institute of Technology, USA
We will detect signs of life on exoplanets in the solar neighborhood by measuring the atmospheric composition using photometry and spectroscopy. Assuming that we know (from the two previous talks) what conditions are needed for life, and what potentially detectable signs of life we might expect, I will discuss how we can use space missions to search for these conditions and signs.