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 material are those of the authors and do not necessarily reflect the views of the sponsor.
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COVER: Europa image courtesy of the Jet Propulsion Laboratory. Design by Penny E. Margolskee.
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National Academy of Sciences
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National Research Council
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COMMITTEE ON PLANETARY AND LUNAR EXPLORATION
JEFFREY R. BARNES,*
Oregon State University
RICHARD P. BINZEL,*
Massachusetts Institute of Technology
WILLIAM V. BOYNTON,**
University of Arizona
W. ROGER BUCK,**
Lamont-Doherty Earth Observatory
WENDY CALVIN,
U.S. Geological Survey
RUSSELL DOOLITTLE,*
University of California, San Diego
HEIDI HAMMEL,*
Massachusetts Institute of Technology
LARRY HASKIN,*
Washington University
PETER B. JAHRLING,**
U.S. Army Medical Research Institute of Infectious Diseases
BRUCE JAKOSKY,*
University of Colorado
KENNETH C. JEZEK,
Ohio State University
GEORGE McGILL,*
University of Massachusetts
HARRY McSWEEN, JR.,*
University of Tennessee
KAREN J. MEECH,**
University of Hawaii
MICHAEL MENDILLO,
Boston University
JOHN F. MUSTARD,**
Brown University
KEITH S. NOLL,**
Space Telescope Science Institute
DAVID A. PAIGE,**
University of California, Los Angeles
J. WILLIAM SCHOPF,**
University of California, Los Angeles
GERALD SCHUBERT,*
University of California, Los Angeles
EVERETT SHOCK,
Washington University
EUGENE SHOEMAKER,***
Lowell Observatory
ANN L. SPRAGUE,**
University of Arizona
Staff
DAVID H. SMITH, Study Director
JACQUELINE ALLEN, Senior Program Assistant
SHARON SEAWARD, Senior Program Assistant
ERIN HATCH, Research Associate
SPACE STUDIES BOARD
CLAUDE R. CANIZARES,
Massachusetts Institute of Technology,
Chair
MARK R. ABBOTT,
Oregon State University
FRAN BAGENAL,
University of Colorado
DANIEL N. BAKER,
University of Colorado
LAWRENCE BOGORAD,*
Harvard University
DONALD E. BROWNLEE,*
University of Washington
ROBERT E. CLELAND,
University of Washington
GERALD ELVERUM,*
TRW Space and Technology Group
ANTHONY W. ENGLAND,*
University of Michigan
MARILYN L. FOGEL,
Carnegie Institution of Washington
MARTIN E. GLICKSMAN,*
Rensselaer Polytechnic Institute
RONALD GREELEY,*
Arizona State University
BILL GREEN, former member,
U.S. House of Representatives
JOHN H. HOPPS, JR.,
Morehouse College
CHRISTIAN J. JOHANNSEN,
Purdue University
ANDREW KNOLL,*
Harvard University
RICHARD G. KRON,
University of Chicago
JONATHAN I. LUNINE,
University of Arizona
ROBERTA BALSTAD MILLER,
Columbia University
BERRIEN MOORE III,*
University of New Hampshire
GARY J. OLSEN,
University of Illinois, Urbana-Champaign
MARY JANE OSBORN,
University of Connecticut Health Center
SIMON OSTRACH,*
Case Western Reserve University
MORTON B. PANISH,*
AT&T Bell Laboratories (retired)
GEORGE A. PAULIKAS,
The Aerospace Corporation
JOYCE E. PENNER,
University of Michigan
CARLÉ M. PIETERS,*
Brown University
THOMAS A. PRINCE,
California Institute of Technology
PEDRO L. RUSTAN,
Ellipso Inc.
JOHN A. SIMPSON,*
Enrico Fermi Institute
GEORGE L. SISCOE,
Boston University
EUGENE B. SKOLNIKOFF,
Massachusetts Institute of Technology
MITCHELL SOGIN,
Marine Biological Laboratory
EDWARD STOLPER,*
California Institute of Technology
NORMAN THAGARD,
Florida A&M University/Florida State University
ALAN M. TITLE,
Lockheed Martin Advanced Technology Center
RAYMOND VISKANTA,
Purdue University
PETER W. VOORHEES,
Northwestern University
ROBERT E. WILLIAMS,*
Space Telescope Science Institute
JOHN A. WOOD,
Harvard-Smithsonian Center for Astrophysics
JOSEPH ALEXANDER, Director
COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS
PETER M. BANKS, ERIM
International, Inc.,
Co-chair
W. CARL LINEBERGER,
University of Colorado,
Co-chair
WILLIAM F. BALLHAUS, JR.,
Lockheed Martin Corp.
SHIRLEY CHIANG,
University of California, Davis
MARSHALL H. COHEN,
California Institute of Technology
RONALD G. DOUGLAS,
Texas A&M University
SAMUEL H. FULLER,
Analog Devices, Inc.
JERRY P. GOLLUB,
Haverford College
MICHAEL F. GOODCHILD,
University of California, Santa Barbara
MARTHA P. HAYNES,
Cornell University
WESLEY T. HUNTRESS, JR.,
Carnegie Institution
CAROL M. JANTZEN,
Westinghouse Savannah River Company
PAUL G. KAMINSKI,
Technovation, Inc.
KENNETH H. KELLER,
University of Minnesota
JOHN R. KREICK,
Sanders, a Lockheed Martin Company (retired)
MARSHA I. LESTER,
University of Pennsylvania
DUSA M. McDUFF,
State University of New York at Stony Brook
JANET L. NORWOOD, U.S. Commissioner of Labor Statistics (retired)
M. ELISABETH PATÉ-CORNELL,
Stanford University
NICHOLAS P. SAMIOS,
Brookhaven National Laboratory
ROBERT J. SPINRAD,
Xerox PARC (retired)
NORMAN METZGER, Executive Director (through July 1999)
MYRON F. UMAN, Acting Executive Director (as of August 1999)
Foreword
Terrestrial studies of life in extreme environments now show that Earth is teeming with microorganisms. Nearly every locale that contains two ingredients, liquid water and some form of energy, appears to host a variety of microbes living happily under conditions that just a few years ago would have seemed impossibly inhospitable. There is also increasing evidence that life emerged very early on Earth, almost as soon as the planet stopped being punished by the deadly rain of debris coursing through the young solar system.
These findings have greatly expanded the horizons of potential habitats for life in the solar system and beyond. Whereas the prior assumption of life as a fragile and extremely rare occurrence put the focus on Mars, the only other planet that might once have had earthlike conditions, the new view of life as relatively robust, if not unstoppable, brings several other bodies into contention.
Jupiter's moon Europa is foremost among the new candidates for harboring past or present life forms. Europa's smooth crust of fractured water ice suggests a subsurface ocean that might provide just the conditions that can host life on Earth. Discovered by Galileo and studied for the past few years by the spacecraft that bears his name, Europa is now considered "one of the places in our solar system with the greatest potential for the existence of life" (see p. 3 in the Executive Summary).
This study assesses our current knowledge of Europa and outlines a strategy for multiyear investigations that would lead to definitive understanding of this moon and its possible biota. COMPLEX concludes that Europa should have a priority for future investigation equal to that accorded to Mars. And as has already been stressed in the strategy for martian investigations, the report underlines the need for a systematic approach to obtaining a global view of Euorpa science, rather than attempting a rapid and possibly poorly conceived rush to detect life. Such a course will not be easy — the intense radiation environment around Jupiter's moon is just one of the many technical challenges. But the reward in understanding this remarkable object and in pursuing the possibility for discovery of extraterrestrial life will be substantial.
Claude R. Canizares, Chair
Space Studies Board
Preface
Over the last few decades the Space Studies Board and its standing discipline committees have devised a series of long-term scientific strategies for NASA's various space science programs. Priorities for the exploration of the solar system, for example, are contained in the report An Integrated Strategy for the Planetary Sciences: 1995-2010 (National Academy Press, Washington, D.C., 1994). One of the highest priorities identified in that report is the continued exploration of Jupiter and its system of satellites, rings, and complex plasma environment.
Since that report was written, the ongoing Galileo mission has greatly expanded our knowledge of the jovian system and has, in particular, revealed much new information about the galilean satellite Europa. This new information, especially that relating to Europa's exobiological potential, has prompted NASA to identify Europa as a priority object in the future exploration of the outer solar system. As a result, the Space Studies Board charged its Committee on Planetary and Lunar Exploration (COMPLEX) to conduct a study to accomplish the following objectives:
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Review and synthesize the current status of knowledge about Europa in view of the results from the Galileo mission.
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Identify opportunities for Earth-based studies and technology development both to prepare for a program of exploration of Europa and to maximize the scientific connection in the Earth and life sciences of spaceflight missions to Europa.
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Recommend a strategy for the further exploration of Europa, to include:
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Global mapping of the topography, geology, and composition of Europa's crust to understand its present state and the history of its evolution;
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Measurements and/or tests that would allow the determination of the presence or absence of liquid water under or within the surface ice crust, and mapping of the thickness and internal structure of the crust;
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Determining its interior structure, including the size and composition of a core, the possible nature of geological processes at the water-or ice-rock interface, and whether any dynamic processes are continuing at the present;
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The means of determining the extent of organic chemical evolution on or under the surface of Europa; and
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Should liquid water be present, the means of determining the potential for or existence of organic chemical evolution and/or biological activity within that ocean.
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Coordination of planning and science-community participation with federal agencies, such as NASA, the National Science Foundation's Polar Programs and Ocean Sciences organizations, the National Oceanic and Atmospheric Administration, the Office of Naval Research, and others for Europa exploration and Earth-based preparations.
This project was formally initiated in December 1997. Presentations relating to it, however, began somewhat earlier and were conducted in the context of COMPLEX's standing oversight of NASA's planetary exploration programs and during the definition and development of the charge for this study. An initial outline of this report was completed at COMPLEX's February 1998 meeting, and a complete draft was assembled at COMPLEX's June 1998 summer-study meeting. The text was approved by the Space Studies Board in November 1998, sent to external reviewers in December 1998, and extensively updated in the spring and summer of 1999.
Although many COMPLEX members past and present worked on this report, the bulk of the task of assembling their many individual contributions was performed by Bruce Jakosky with the assistance of Wendy Calvin, Ronald Greeley, Larry Haskin, Kenneth Jezek, Michael Mendillo, and Gerald Schubert.
The work of the writing team was made easier thanks to the contributions made by Charles Barnes (Jet Propulsion Laboratory), Jay Bergstrahl (NASA Headquarters), Michael Brown (California Institute of Technology), Frank Carsey (Jet Propulsion Laboratory), Christopher Chyba (SETI Institute), James Cutts (Jet Propulsion Laboratory), Paul Geissler (University of Arizona), Robert Johnson (University of Virginia), Torrence Johnson (Jet Propulsion Laboratory), Krishan Khurana (University of California, Los Angeles), Arthur Lane (Jet Propulsion Laboratory), Christopher McKay (Ames Research Center), William O'Neil (Jet Propulsion Laboratory), Michael Purdy (National Science Foundation), Laurence Soderblom (U.S. Geological Survey), Richard Terrile (Jet Propulsion Laboratory), and Charles Yoder (Jet Propulsion Laboratory).
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 manuscripts remain confidential to protect the integrity of the deliberative process. COMPLEX thanks reviewers John A. Baross (University of Washington), Radford Byerly (Congressional Science Committee staff, retired), Marshall H. Cohen (California Institute of Technology), Stanton Peale (University of California, Santa Barbara), Jeffrey B. Plescia (U.S. Geological Survey), and Raymond J. Walker (University of California, Los Angeles) for many constructive comments and suggestions. Responsibility for the final content of this report rests solely with the authoring committee and the NRC.