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The Limits of Organic Life in Planetary Systems The Limits of Organic Life in Planetary Systems Committee on the Limits of Organic Life in Planetary Systems Committee on the Origins and Evolution of Life Space Studies Board Division on Engineering and Physical Sciences Board on Life Sciences Division on Earth and Life Studies NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu
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The Limits of Organic Life in Planetary Systems THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. Support for this project was provided by Contract NASW 01001 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor. International Standard Book Number-13: 978-0-309-10484-5 International Standard Book Number-10: 0-309-10484-X Cover: Cover design by Penny E. Margolskee. The lower half of the cover is an image of a microbial mat community found at a depth of about 15 meters in Lake Vanda, Wright Valley, Antarctica; photo courtesy of Dale Andersen, SETI Institution. The upper half of the cover is a composite image of the Orion Nebula made by combining data from the Hubble Space Telescope and the Spitzer Space Telescope; image courtesy of NASA/Jet Propulsion Laboratory-California Institute of Technology, T. Megeath (University of Toledo), and M. Robberto (Space Telescope Science Institute). The crescent on the center right is an ultraviolet image of the planet Venus as seen by the Hubble Space Telescope; image courtesy of NASA/JPL/Space Telescope Science Institute. A limited number of copies of this report are available free of charge from: Space Studies Board National Research Council The Keck Center of the National Academies 500 Fifth Street, N.W. Washington, DC 20001 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Copyright 2007 by the National Academy of Sciences. All rights reserved. Printed in the United States of America
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The Limits of Organic Life in Planetary Systems THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. www.national-academies.org
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The Limits of Organic Life in Planetary Systems OTHER REPORTS OF THE SPACE STUDIES BOARD Building a Better NASA Workforce: Meeting the Workforce Needs for the National Vision for Space Exploration (2007) Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (2007) Exploring Organic Environments in the Solar System (SSB with the Board on Chemical Sciences and Technology, 2007) A Performance Assessment of NASA’s Astrophysics Program (SSB with the Board on Physics and Astronomy, 2007) An Assessment of Balance in NASA’s Science Programs (2006) Assessment of NASA’s Mars Architecture 2007-2016 (2006) Assessment of Planetary Protection Requirements for Venus Missions: Letter Report (2006) Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop (2006) Issues Affecting the Future of the U.S. Space Science and Engineering Workforce: Interim Report (SSB with the Aeronautics and Space Engineering Board [ASEB], 2006) Review of NASA’s 2006 Draft Science Plan: Letter Report (2006) The Scientific Context for Exploration of the Moon: Interim Report (2006) Space Radiation Hazards and the Vision for Space Exploration (2006) The Astrophysical Context of Life (SSB with the Board on Life Sciences, 2005) Earth Science and Applications from Space: Urgent Needs and Opportunities to Serve the Nation (2005) Extending the Effective Lifetimes of Earth Observing Research Missions (2005) Preventing the Forward Contamination of Mars (2005) Principal-Investigator-Led Missions in the Space Sciences (2005) Priorities in Space Science Enabled by Nuclear Power and Propulsion (SSB with ASEB, 2005) Review of Goals and Plans for NASA’s Space and Earth Sciences (2005) Review of NASA Plans for the International Space Station (2005) Science in NASA’s Vision for Space Exploration (2005) Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report (SSB with ASEB, 2004) Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report (2004) Issues and Opportunities Regarding the U.S. Space Program: A Summary Report of a Workshop on National Space Policy (SSB with ASEB, 2004) Plasma Physics of the Local Cosmos (2004) Review of Science Requirements for the Terrestrial Planet Finder: Letter Report (2004) Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics (2004) Utilization of Operational Environmental Satellite Data: Ensuring Readiness for 2010 and Beyond (SSB with ASEB and the Board on Atmospheric Sciences and Climate [BASC], 2004) Limited copies of these reports are available free of charge from: Space Studies Board National Research Council The Keck Center of the National Academies 500 Fifth Street, N.W., Washington, DC 20001 (202) email@example.com www.nationalacademies.org/ssb/ssb.html NOTE: Listed according to year of approval for release, which in some cases precedes the year of publication.
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The Limits of Organic Life in Planetary Systems COMMITTEE ON THE LIMITS OF ORGANIC LIFE IN PLANETARY SYSTEMS JOHN A. BAROSS, University of Washington, Chair STEVEN A. BENNER, Foundation for Applied Molecular Evolution GEORGE D. CODY, Carnegie Institution of Washington SHELLEY D. COPLEY, University of Colorado at Boulder NORMAN R. PACE, University of Colorado at Boulder JAMES H. SCOTT, Dartmouth College ROBERT SHAPIRO, New York University MITCHELL L. SOGIN, Marine Biological Laboratory JEFFREY L. STEIN, Sofinnova Ventures ROGER SUMMONS, Massachusetts Institute of Technology JACK W. SZOSTAK, Howard Hughes Medical Institute, Harvard University Staff DAVID H. SMITH, Study Director JOSEPH K. ALEXANDER, Senior Staff Officer ROBERT L. RIEMER, Senior Staff Officer (shared with the Board on Physics and Astronomy) CATHERINE A. GRUBER, Assistant Editor RODNEY N. HOWARD, Senior Project Assistant
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The Limits of Organic Life in Planetary Systems SPACE STUDIES BOARD LENNARD A. FISK, University of Michigan, Chair A. THOMAS YOUNG, Lockheed Martin Corporation (retired), Vice Chair SPIRO K. ANTIOCHOS, Naval Research Laboratory DANIEL N. BAKER, University of Colorado STEVEN J. BATTEL, Battel Engineering CHARLES L. BENNETT, Johns Hopkins University JUDITH A. CURRY, Georgia Institute of Technology JACK D. FARMER, Arizona State University JACK D. FELLOWS, University Corporation for Atmospheric Research JACQUELINE N. HEWITT, Massachusetts Institute of Technology TAMARA E. JERNIGAN, Lawrence Livermore National Laboratory KLAUS KEIL, University of Hawaii BERRIEN MOORE III, University of New Hampshire KENNETH H. NEALSON, University of Southern California NORMAN P. NEUREITER, American Association for the Advancement of Science SUZANNE OPARIL, University of Alabama, Birmingham JAMES PAWELCZYK, Pennsylvania State University RONALD F. PROBSTEIN, Massachusetts Institute of Technology HARVEY D. TANANBAUM, Harvard-Smithsonian Astrophysical Observatory RICHARD H. TRULY, National Renewable Energy Laboratory (retired) JOSEPH F. VEVERKA, Cornell University WARREN M. WASHINGTON, National Center for Atmospheric Research GARY P. ZANK, University of California, Riverside MARCIA S. SMITH, Director
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The Limits of Organic Life in Planetary Systems BOARD ON LIFE SCIENCES KEITH YAMAMOTO, University of California, San Francisco, Chair ANN M. ARVIN, Stanford University School of Medicine JEFFREY L. BENNETZEN, University of Georgia RUTH BERKELMAN, Emory University DEBORAH BLUM, University of Wisconsin R. ALTA CHARO, University of Wisconsin JEFFREY L. DANGL, University of North Carolina PAUL R. EHRLICH, Stanford University MARK D. FITZSIMMONS, John D. and Catherine T. MacArthur Foundation JO HANDELSMAN, University of Wisconsin, Madison ED HARLOW, Harvard Medical School KENNETH H. KELLER, University of Minnesota RANDALL MURCH, Virginia Polytechnic Institute and State University GREGORY A. PETSKO, Brandeis University MURIEL E. POSTON, Skidmore College JAMES REICHMAN, University of California, Santa Barbara MARC T. TESSIER-LAVIGNE, Genentech, Inc. JAMES TIEDJE, Michigan State University TERRY L. YATES, University of New Mexico FRANCES SHARPLES, Director
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The Limits of Organic Life in Planetary Systems Dedicated to Non-Human-Like Life Forms, Wherever They Are
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The Limits of Organic Life in Planetary Systems Preface As the search for life in the solar system expands, it is important to know what exactly to search for. Previous life-detection experiments have been criticized for being too geocentric. This study aims to inform research program managers, policymakers, and mission designers about the possibilities for life on other solar system bodies. Further, during planetary protection exercises at the National Aeronautics and Space Administration (NASA), questions concerning the possibility of nonterrana life recur repeatedly. Remarkably little knowledge is organized that might shed light on the plausibility of bizarre life as a concern for planetary protection. The search for signs of life, present or past, is an important goal of NASA’s robotic solar system exploration programs and, ultimately, for its astronomical programs designed to probe the gross characteristics of extrasolar planetary systems. To date, that search has been governed by a model of life that is based on the life that we know on Earth—terran life. Several features of terran life have attracted particular focus: Terran life uses water as a solvent; It is built from cells and exploits a metabolism that focuses on the carbonyl group (C=O); It is thermodynamically dissipative, exploiting chemical-energy gradients; and It exploits a two-biopolymer architecture that uses nucleic acids to perform most genetic functions and proteins to perform most catalytic functions. As a consequence, most of NASA’s mission planning is focused on locations where liquid water is possible, and it emphasizes searches for structures that resemble cells of terran organisms, small molecules that might be the products of carbonyl metabolism, particular kinds of chemical-energy gradients, and tests for amino acids and nucleotides similar to those found in terrestrial proteins and DNA. This approach is defensible given the absence of a general understanding of how life might appear if it had an origin independent of Earth. Experiments in the laboratory, however, are suggesting that life might be based on molecular structures substantially different from a The Committee on the Limits of Organic Life in Planetary Systems uses the term “terran” to denote a particular set of biological and chemical characteristics that are displayed by all life on Earth. Thus “Earth life” has the same meaning as “terran life” when the committee is discussing life on Earth, but if life were discovered on Mars or any other nonterrestrial body, it might be found to be terran or nonterran, depending on its characteristics.
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The Limits of Organic Life in Planetary Systems those known in contemporary terran life. These results suggest that if life originated independently, even within our own solar system, it might have nonterran characteristics and, thus, not be detectable by NASA’s in situ or remote-sensing missions designed explicitly to detect terran biomolecules or their products. Further, if life is possible in solvents other than liquid water, it might exist in planetary environments other than the few that are currently targeted as potential hosts of nonterran life. Other than on Earth, liquid water is now considered possible only on subsurface Mars and in sub-ice environments of the Galilean moons of Jupiter (Europa, Ganymede, and Callisto), and perhaps on Saturn’s moon, Enceladus. Nonaqueous solvents might, however, be present in other planetary environments. Because some of these spots (e.g., the surface of Titan) could be more accessible via spacecraft missions than either the deep subsurface of Mars or sub-ice Europa, evidence for life in solvents other than water might redirect missions to these other locales, and substantially improve the design of life-detection instrumentation generally. Similarly, nonterran life may change the gross characteristics of planetary environments in ways that differ from influences stemming from terran life, and these differences (e.g., the relative abundances of atmospheric species) may ultimately be observable over interstellar distances with astronomical facilities now on the drawing board. This report explores a limited set of hypothetical alternative chemistries of life by following a hierarchy of possibilities that have been ranked through experimental, exploratory, and theoretical work done in the past. The study briefly reviews current knowledge concerning the following questions or hypotheses and provides suggestions for future research. What environments on Earth that are extremes by terran standards harbor life? How must life-detection strategies be altered to discover this life on Earth? What extreme environments have not received attention? Are there synthetic environments that better represent conditions on alien worlds? What environments on Earth are so extreme that life with standard terran biochemistry has been unable to occupy it? What life forms are possible, still based on carbon and still functioning in water, but with a fundamental difference in the method of reproduction? Issues to be explored include the following: What types of polymeric structures, other than proteins built from the standard 20 amino acids, might support catalysis in water? For example, can 2-amino-2-methyl-carboxylic acids, which have been found to be enantiomerically enriched in meteorites, be the basis for a catalytic system? In the absence of biopolymers, would selected monomers provide catalysis sufficient to sustain life? What types of polymeric structures, other than nucleic acids built from the standard four nucleotides, might be replicatable and might support Darwinian evolution in water? Can a functioning genetic system be established that is not based on a linear molecular structure? For example, can a compositional genome (a collection of monomers) sustain heredity? Can a system capable of Darwinian evolution be demonstrated in the laboratory using nonstandard biopolymers or a compositional genome in water? What life forms are possible, still based on carbon, but not functioning in water? Issues to be explored include these: Can membranes be constructed in the laboratory that separate an organic solvent inside a cell from an organic solvent outside a cell? What kinds of polymeric structures (or monomer collections) might support catalysis and genetics in nonaqueous environments, particularly in solvents found on solar system bodies other than Earth? Can mineral systems be identified that interact in interesting ways with organic compounds in nonaqueous systems? Can asymmetric induction, and spontaneous resolution that leads to the homochirality assumed to be necessary for life, be achieved in nonaqueous solvents, especially those found on solar system bodies other than Earth? Can a system capable of Darwinian evolution be demonstrated in the laboratory using nonstandard monomers and/or biopolymers in nonaqueous environments?
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The Limits of Organic Life in Planetary Systems The purpose of this study is twofold: To evaluate the possibility that nonstandard biochemistry (i.e., biochemistry different from what we find as the universal biochemistry on Earth) might support life in known solar system environments and conceivable extrasolar environments; and To define broad areas that might guide NASA and the National Science Foundation to fund efforts to expand knowledge in this area. The results of this study are meant to aid in the development of a new generation of life-detection experiments that can be conducted in situ on planetary surfaces or conducted on samples returned from other solar system bodies. Held on April 25, 2002, at the National Academies’ Georgetown facility in Washington, D.C., the “weird life” planning session was chaired by John Baross (University of Washington) and included presentations from Chris Chyba (SETI Institute and Stanford University), Steven Benner (Foundation for Applied Molecular Evolution), Jack Szostak (Harvard University), George Cody (Carnegie Institution of Washington), and Robert Shapiro (New York University). A discussion session was led by Mitch Sogin (Woods Hole Marine Biological Laboratory). A planning session for the Workshop on the Limits of Organic Life in Planetary Systems was held at the Constitution Avenue building of the National Academies in Washington, D.C., on March 2-3, 2004, and chaired by John Baross with input from NASA staff members Michael Meyer, Marc Allen, and John Rummel. The Workshop on the Limits of Organic Life in Planetary Systems was held on May 10-11, 2004, at the Constitution Avenue building of the National Academies, Washington, D.C. The co-chairs were Jack Szostak (Harvard University) and John Baross (University of Washington); panel moderators were Norman Pace (University of Colorado), James Kasting (Pennsylvania State University), Pascale Ehrenfreund (Leiden University), and Steven Benner (Foundation for Applied Molecular Evolution). Participants included Robert Blankenship (Arizona State University), Roger Summons (Massachusetts Institute of Technology), Ruth Blake (Yale University), Jonathan Eisen (Institute for Genomic Research), Eric Mathur (Diversa), Peter Ward (University of Washington), Christopher McKay (NASA Ames Research Center), David DesMarais (NASA Ames Research Center), James Ferry (Pennsylvania State University), Bruce Jakosky (University of Colorado), Robert Pappalardo (Jet Propulsion Laboratory), Jeffrey Kargel (U.S. Geological Survey), James Scott (Dartmouth College), Donald Button (University of Alaska at Fairbanks), Leslie Orgel (Salk Institute), Jonathan Lunine (University of Arizona), Dirk Schulze-Makuch (Washington State University), Douglas Clark (University of California, Berkeley), and George Cody (Carnegie Institution of Washington). A writing meeting was held on March 14-16, 2005, at the National Academies’ Arnold and Mabel Beckman Center, Irvine, California, and chaired by John Baross (University of Washington), with presentations from Steven Benner (Foundation for Applied Molecular Evolution), William Baines (Rufus Scientific), and Jonathan Lunine (University of Arizona).
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The Limits of Organic Life in Planetary Systems Acknowledgments The committee thanks Space Studies Board (SSB) interns Stephanie Bednarek, Matthew Broughton, and Brendan McFarland and Board on Life Sciences program officer Evonne Tang for their work on compiling the glossary and researching references. The committee also thanks SSB research assistant Victoria Swisher for assistance with the report review process. This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s (NRC’s) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report: Robert H. Austin, Princeton University, Paul Davies, Macquarie University, Australia, Jack Farmer, Arizona State University, Katherine H. Freeman, Pennsylvania State University, James F. Kasting, Pennsylvania State University, Anthony Keefe, Archemix Corporation, Peter B. Moore, Yale University, Kenneth H. Nealson, University of Southern California, and Norman H. Sleep, Stanford University. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Leslie Orgel, Salk Institute for Biological Studies. Appointed by the National Research Council, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.
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The Limits of Organic Life in Planetary Systems Contents EXECUTIVE SUMMARY 1 1 INTRODUCTION 5 1.1 The Search for Life in the Cosmos, 5 1.2 Defining the Scope of the Problem, 6 1.3 Is Evolution an Essential Feature of Life?, 7 1.4 Brief Considerations of Possible Life Forms Outside the Scope of This Report, 8 1.5 Strategies to Mitigate Anthropocentricity, 9 1.6 References, 10 2 A SKETCH OF THE CHEMISTRY BEHIND KNOWN CARBON-BASED LIFE ON EARTH 11 2.1 Molecular Structure and Physical Properties, 11 2.1.1 Pairs of Electrons Form Bonds Between Atoms, 11 2.1.2 Distribution of Charge Is Key to the Physical Properties of Molecules, 11 2.1.3 Distribution of Charge Can Be Inferred from Molecular Structures, 12 2.2 Molecular Reactivity, 13 2.2.1 Reactive Centers in the Structure of a Molecule, 14 2.2.2 The Reactivity of Water, 15 2.3 Molecular Stability 2.3.1 Chemical Bonds Have Different Strengths, 16 2.3.2 Temperature Limits on Organic Molecular Stability, 17 2.4 Molecular Reactivity in Terran Life: Metabolism, 17 2.4.1 Heteroatoms Confer Reactivity to Hydrocarbons to Enable Metabolism, 17 2.4.2 The Energetic Requirements for Metabolism, 19 2.4.3 Terran Life Has a Common Set of Reactions That Form a Core Metabolism, 20 2.5 Catalysis, 21 2.6 Macromolecular Structure in Terran Life, 23
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The Limits of Organic Life in Planetary Systems 2.7 Supramolecular Structure in Terran Life, 24 2.7.1 Compartmentalization Arises from Supramolecular Structures, 24 22.214.171.124 Advantages of Compartmentalization, 24 126.96.36.199 Compartmentalization Exploits the Low Polarity of C—C and C—H Bonds, 24 188.8.131.52 Compartmentalization Assists in the Generation of High-energy Compounds, 26 2.7.2 Supramolecular Soluble Structures, 26 2.8 The Relationship Between Water and Biomolecules, 26 2.8.1 Adaptation of Terran Biomolecules to Water, 26 2.8.2 Disadvantages of Water for Terran Biomolecules, 27 2.9 References, 28 3 PUSHING THE BOUNDARIES OF LIFE 29 3.1 The Limits of Earth Life, 29 3.2 Extremophiles and the Limits of Life, 31 3.3 Water, Desiccation, and Life in Nonaqueous Solvents, 33 3.4 Temperature, 35 3.5 Survival Strategies and Interplanetary Transfer, 37 3.6 The Plasticity of Human-like Biochemistry, 38 3.7 Limits of Anthropocentric Biochemistry, 38 3.8 Early Environments of Life on Earth, 39 3.9 Opportunities for Research, 39 3.10 References, 40 4 ALTERNATIVES TO TERRAN BIOCHEMISTRY IN WATER 43 4.1 Synthetic Biology as a Strategy for Understanding Alternatives to Terran Biomolecules, 44 4.1.1 Terran Nucleic Acids Are Not the Only Structures That Can Support Genetic-like Behavior, 44 4.1.2 Terran Amino Acids Are Not the Only Structures That Can Be Incorporated into Proteins, 46 4.1.3 Implications of Synthetic Biology for Our View of the Universality of Global Terran Proteins and Nucleic Acids, 46 4.2 What Features of Terran Genetic Molecules Might Be Universal in Genetic Molecules Acting in Water?, 47 4.2.1 A Repeating Charge May Be Universal in Genetic Polymers in Water, 47 4.2.2 A Repeating Dipole May Be Universal in Polymeric Catalytic Molecules in Water, 49 4.3 Is Water Uniquely Suited as a Biosolvent?, 49 4.4 Opportunities for Research, 50 4.5 References, 52 5 ORIGIN OF LIFE 53 5.1 Laboratory Synthesis of Organic Monomers, 54 5.2 Natural Availability of Biological-like Molecules, 55 5.2.1 Biological-like Molecules from the Cosmos, 55 5.2.2 Biological-like Molecules from Planetary Processes, 56 5.2.3 The Origin of Phosphorus, 56 5.2.4 The Origin of Metabolism, 57 5.3 Thermodynamic Equilibria, 57
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The Limits of Organic Life in Planetary Systems 5.4 Problems in Origins, 58 5.4.1 Nucleophilic and Electrophilic Reactions Can Destroy as Well as Create, 59 5.4.2 The Reactivity of Water Constrains Routes to Origins, 60 5.5 Minerals as a Possible Solution to the Instability of Ribose, 61 5.6 Minerals Involved in the Construction of Biomolecules, 63 5.7 Small-Molecule (“Metabolism First”) Theories of Life’s Origin, 63 5.7.1 Life Without a Replicator, 63 5.7.2 Coupling to an Energy Source as a Driver of Chemical Self-organization, 64 5.7.3 Significance and Implications for Astrobiology, 65 5.8 Opportunities for Research, 65 5.8.1 Research on Earth, 65 5.8.2 Research in Space, 66 5.9 References, 66 6 WHY WATER? TOWARD MORE EXOTIC HABITATS 69 6.1 Is Water Uniquely Suited for Life?, 69 6.2 If Not Water, Then What Solvent?, 71 6.2.1 Polar Solvents That Are Not Water, 72 184.108.40.206 Ammonia, 72 220.127.116.11 Sulfuric Acid as a Possible Solvent, 73 18.104.22.168 Formamide as a Possible Solvent, 74 6.2.2 Nonpolar Solvents, 74 6.2.3 Cryosolvents, 75 22.214.171.124 Dihydrogen, 75 126.96.36.199 Dinitrogen, 75 188.8.131.52 Other Supercritical Cryosolvents, 76 6.3 Still More Exotic Habitats, 77 6.3.1 Life in the Gas Phase, 77 6.3.2 Life in the Solid Phase, 77 6.4 Opportunities for Research, 77 6.5 References, 78 7 LIFE DETECTION AND BIOMARKERS 80 7.1 Chirality as a Biomarker, 80 7.2 Thermodynamic Relation of Metabolic Intermediates as a Biosignature, 82 7.3 Reference, 83 8 CONCLUSIONS AND RECOMMENDATIONS. 84 8.1 Laboratory Studies, 85 8.2 Field Studies, 85 8.3 Space Studies, 86 APPENDIXES A Glossary 91 B Biographies of Committee Members and Staff 97
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