The Limits of Organic Life in Planetary Systems
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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.
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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
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
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
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:
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Terran life uses water as a solvent;
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It is built from cells and exploits a metabolism that focuses on the carbonyl group (C=O);
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It is thermodynamically dissipative, exploiting chemical-energy gradients; and
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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
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.
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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?
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What environments on Earth are so extreme that life with standard terran biochemistry has been unable to occupy it?
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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:
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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?
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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?
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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?
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Can a system capable of Darwinian evolution be demonstrated in the laboratory using nonstandard biopolymers or a compositional genome in water?
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What life forms are possible, still based on carbon, but not functioning in water? Issues to be explored include these:
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Can membranes be constructed in the laboratory that separate an organic solvent inside a cell from an organic solvent outside a cell?
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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?
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Can mineral systems be identified that interact in interesting ways with organic compounds in nonaqueous systems?
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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?
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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 purpose of this study is twofold:
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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
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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).
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.