A Science Strategy for the Exploration of Europa

Committee on Planetary and Lunar Exploration

Space Studies Board

Commission on Physical Sciences, Mathematics, and Applications

National Research Council

Washington, D.C.

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A Science Strategy for the Exploration of Europa A Science Strategy for the Exploration of Europa Committee on Planetary and Lunar Exploration Space Studies Board Commission on Physical Sciences, Mathematics, and Applications National Research Council NATIONAL ACADEMY PRESS Washington, D.C.

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A Science Strategy for the Exploration of Europa 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. International Standard Book Number 0-309-06493-7 COVER: Europa image courtesy of the Jet Propulsion Laboratory. Design by Penny E. Margolskee. Copies of this report are available free of charge from: Space Studies Board National Research Council 2101 Constitution Avenue, NW Washington, DC 20418 Copyright 1999 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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A Science Strategy for the Exploration of Europa The National Academies National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council 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. Bruce M. Alberts 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. William A. Wulf 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. Kenneth I. Shine 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. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

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A Science Strategy for the Exploration of Europa COMMITTEE ON PLANETARY AND LUNAR EXPLORATION RONALD GREELEY,* Arizona State University, Chair JOHN A. WOOD,** Harvard-Smithsonian Center for Astrophysics, Successor Chair 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 *   Former member. **   Joined committee after June 1998. ***   Deceased.

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A Science Strategy for the Exploration of Europa 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 *   Former member.

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A Science Strategy for the Exploration of Europa 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)

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A Science Strategy for the Exploration of Europa 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

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A Science Strategy for the Exploration of Europa 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: Review and synthesize the current status of knowledge about Europa in view of the results from the Galileo mission. 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. Recommend a strategy for the further exploration of Europa, to include: Global mapping of the topography, geology, and composition of Europa's crust to understand its present state and the history of its evolution; 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; 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; The means of determining the extent of organic chemical evolution on or under the surface of Europa; and

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A Science Strategy for the Exploration of Europa 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. 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.