Signs of Life
A Report Based on the April 2000 Workshop on Life Detection Techniques
THE NATIONAL ACADEMIES PRESS
Washington, D.C. www.nap.edu
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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 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for this project.
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COMMITTEE ON THE ORIGINS AND EVOLUTION OF LIFE
University of Washington,
JONATHAN I. LUNINE,
University of Arizona,
University of California, Santa Barbara
* SETI Institute
DAVID W. DEAMER,
University of California, Santa Cruz
MARILYN L. FOGEL,
Carnegie Institution of Washington
NORMAN R. PACE,
University of Colorado
DAVID A. STAHL,
University of Washington
JOAN ESNAYRA, Study Director
(Board on Life Sciences)
DAVID H. SMITH, Study Director
(Space Studies Board)
CARMELA CHAMBERLAIN, Senior Program Assistant
KIRSTEN ARMSTRONG, Research Associate
BRIAN DEWHURST, Research Associate
ACHEL BHATT, Research Assistant
CRAIG CORNELIUS, Research Assistant
Term ended in 2000.
SPACE STUDIES BOARD
JOHN H. McELROY,
University of Texas at Arlington (retired),
ROGER P. ANGEL,
University of Arizona
JAMES P. BAGIAN,
Veterans Health Administration's National Center for Patient Safety
JAMES L. BURCH,
Southwest Research Institute
RADFORD BYERLY, JR.,
University of Colorado
ROBERT E. CLELAND,
University of Washington
HOWARD M. EINSPAHR,
Bristol-Myers Squibb Pharmaceutical Research Institute
STEVEN H. FLAJSER,
Loral Space and Communications Ltd.
Oregon State University
DON P. GIDDENS,
Georgia Institute of Technology/Emory University
RALPH H. JACOBSON,
The Charles Stark Draper Laboratory
University of Washington
JONATHAN I. LUNINE,
University of Arizona
BRUCE D. MARCUS,
RICHARD A. McCRAY,
University of Colorado
HARRY Y. McSWEEN, JR.,
University of Tennessee
GARY J. OLSEN,
University of Illinois at Urbana-Champaign
GEORGE A. PAULIKAS,
The Aerospace Corporation (retired)
University of Chicago
ROBERT J. SERAFIN,
National Center for Atmospheric Research
EUGENE B. SKOLNIKOFF,
Massachusetts Institute of Technology
Marine Biological Laboratory
C. MEGAN URRY,
PETER W. VOORHEES,
JOSEPH K. ALEXANDER, Director
BOARD ON LIFE SCIENCES
COREY S. GOODMAN,
University of California, Berkeley,
R. ALTA CHARO,
University of Wisconsin, Madison
The Salk Institute for Biological Studies
DAVID J. GALAS,
Keck Graduate Institute of Applied Life Science
Texas A&M University
JAMES M. GENTILE,
LINDA E. GREER,
Natural Resources Defense Council
Harvard Medical School
ELLIOT M. MEYEROWITZ,
California Institute of Technology
ROBERT T. PAINE,
University of Washington, Seattle
GREGORY A. PETSKO,
STUART L. PIMM,
JOAN B. ROSE,
University of South Florida
GERALD M. RUBIN,
Howard Hughes Biomedical Research
BARBARA A. SCHAAL,
RAYMOND L. WHITE,
DNA Sciences, Inc.
FRANCES SHARPLES, Director
At the close of the 20th century the direct exploration of our cosmic neighborhood had left unanswered one of humanity's oldest questions: Does the universe play host to life on multiple worlds, or is Earth unique in this regard? Both scientific and popular interest in this question have peaked in the last 5 years in response to the discovery of planets beyond our solar system and the controversy over possible signs of biological activity in a meteorite that likely was once a piece of the crust of Mars. NASA has responded to this interest through a partial realignment of its science program in the form of a new interdisciplinary effort called astrobiology. Among astrobiology 's goals are the understanding of life's origin(s) and the detection of life, extant or extinct, beyond Earth. To search for life requires that samples of extraterrestrial material be acquired and then analyzed either in situ on a planetary body or in Earth-based laboratories. To fully address the question of earthly life's origin may require going beyond the terrestrial laboratory, where time and sample volumes are limited, to organic-rich environments elsewhere in the solar system where steps toward biology may also have occurred.
Attempts to detect life in material from beyond planet Earth extend back some four decades, including the study of meteorites and the in situ analysis of martian soil by the Viking landers. Although broad physical arguments have been made in support of the notion that life is a natural outcome of the process of cosmic evolution, earthly life remains the singular example. In recent years, the discovery of life in extreme terrestrial environments—namely, ecosystems at extremes of temperature, salinity, and acidity—has bolstered the expectation that extraterrestrial material may hold evidence of past or present life. It has also strengthened the concept, supported by computer calculations and the existence on Earth of meteorites bearing trapped gases of martian atmospheric composition, that life might hitch a ride on impact debris and hence travel in viable form from Mars to Earth or vice versa. This in turn imposes the additional challenge of determining whether life discovered in an extraterrestrial sample had a common or separate origin from that of Earth.
The detection of non-earthly life is a difficult and unsolved problem. Most techniques for detecting contamination of supposedly sterile surfaces are based on the properties of known organisms and on the remarkable uniformity of terrestrial biochemical processes. Yet even small variations in structure, metabolism, or information-encoding mechanisms could yield organisms capable of eluding such techniques. A combination of techniques is therefore required to maximize the chances of finding life, yet in situ packages must be severely limited in mass, power, size, and complexity given the present realities of planetary exploration. The first such package to be deployed on another planet, the Viking Mars landers, produced ambiguous results that failed to establish the presence of life, identified unusual oxidative soil chemistry not anticipated during the experiment design, and left
a lingering controversy over some of the results. Detection schemes in ground-based laboratories are not limited by weight, power, or size and can be heavily reliant on human intervention; yet even such systems may yield ambiguous results. The experience with the ALH84001 meteorite, where the claim of evidence for biological processes remains controversial and unresolved, is an important lesson in the fundamental complexity of identifying the faint traces of extant or extinct life. Perhaps even more difficult, if life or its remains is detected in a sample, will be the determination of whether it is a terrestrial contaminant from Earth, and if so, whether it was delivered by the spacecraft or in the natural process of cross-contamination via asteroidal or cometary impact.
Returned samples could pose a biohazard to life on Earth. The need for elaborate containment facilities, the prospects of sample destruction or alteration prior to release for general study, or even the deferral of sample return until extensive in situ analysis is complete will complicate the use of laboratory facilities to search for life. Conversely, the assurance that spacecraft launched from Earth will not contaminate astrobiologically interesting targets with viable organisms requires that sensitive techniques to assess the efficacy of sterilization processes be applied in the final stages of payload preparation prior to launch.
Given the active interest in the astrobiological exploration of Mars, Europa, Titan, and other targets, it is timely to assess the state of life detection techniques. The Space Studies Board (SSB) charged the Committee on the Origins and Evolution of Life (COEL)—which reports to both the SSB and the Board on Life Sciences—to organize a scientific workshop to explore advances in biology, biotechnology, medicine, and the environmental sciences likely to lead to new approaches to detecting life or its remnants. In response to the charge, the committee held a workshop at the Carnegie Institution's headquarters in Washington, D.C., on April 25-26, 2000. The workshop's goal was to address the following questions:
How does one determine if living organisms are on a spacecraft before launch?
How does one determine if there are living organisms in a returned sample?
How does one determine if living organisms have been present at some earlier epoch and have left fossil remnants behind in a returned sample?
How does one determine whether there are living organisms or fossils in samples examined robotically on another solar system body?
COEL was responsible for structuring the workshop, inviting speakers, summarizing the workshop in the form of this report, and drafting appropriate findings based on the presentations and discussions at the workshop and the committee's subsequent deliberations. Organization of the workshop followed from a series of discussions on topics related to life detection held by COEL at the National Academies' Beckman Center on January 27-29, 2000. The deliberations at this organizational meeting resulted in a guest list containing a balanced cross section of the community of scientists interested in life detection. The workshop was designed to review promising detection techniques that are currently available and areas for future research.
To facilitate a logical flow of papers and discussion, the workshop was organized in a fashion somewhat different from the list of questions given above. The workshop program is provided in Appendix B. The workshop opened with an introduction to the history of the search for life and the question of the generality of terrestrial biochemistry. The next session enumerated current understanding of solar system targets for sample return, including meteorites and interplanetary dust particles for which samples are available at present. The final two sessions dealt with techniques for detecting viable (including spore-forming) organisms and fossil remnants of life, respectively. Panel discussions followed the talks in the sessions on detection of extant and extinct life. After the workshop, COEL met on April 27 and again on August 21-23, 2000, to discuss the presentations, deliberate, and formulate its findings.
What is presented in this volume, a report based on a workshop, is but a preliminary step in the effort to develop a sound set of scientific strategies for the detection of life and its remains in samples on or from planetary bodies. To more fully capture the material presented at the workshop, speakers were asked to provide contributed papers derived from their talks. These papers are listed in Appendix C and are reproduced on the CD-ROM that contains the full report. No attempt was made to extensively rewrite or standardize the papers, although
grammatical and stylistic errors were corrected to improve clarity. The papers stand on their own as a detailed record of the workshop presentations.
The main body of the report was written by COEL and was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its 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 review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. The committee thanks the following individuals for their review of this report: Carl Agee (Johnson Space Center), Robert A. Frosch (Harvard University), David Galas (Keck Graduate Institute of Applied Life Sciences), Joseph L. Kirshvink (California Institute of Technology), and Eugene H. Levy (Rice 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 Mary J. Osborn, University of Connecticut Health Center. Appointed by the National Research Council, she 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.
The workshop papers are reproduced on the CD-ROM that contains the full report but are not included in the printed report owing to space limitations.