Planetary Systems and the Origins of Life
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This page intentionally left blank PLANETARY SYSTEMS AND THE ORIGINS OF LIFE Several major breakthroughs in the last decade have helped to contribute to the emerging field of astrobiology. These have ranged from the study of micro- organisms, which have adapted to living in extreme environments on Earth, to the discovery of over 200 planets orbiting around other stars, and the ambitious programs for the robotic exploration of Mars and other bodies in our Solar System. Focusing on these developments, this book explores some of the most exciting and important problems in this field. Beginning with how planetary systems are discovered, the text examines how these systems formed, and how water and the biomolecules necessary for life were produced. It then focuses on how life may have originated and evolved on Earth. Building on these two themes, the final section takes the reader on an exploration for life elsewhere in the Solar System. It presents the latest results of missions to Mars and Titan, and explores the possibilities for life in the ice- covered ocean of Europa. Colour versions of some of the figures are available at www.cambridge.org/9780521875486. This interdisciplinary book is a fascinating resource for students and researchers in subjects in astrophysics, planetary science, geosciences, biochemistry, and evolutionary biology. It will provide any scientifically literate reader with an enjoyable overview of this exciting field. Ralph Pudritz is Director of the Origins Institute and a Professor in the Department of Physics and Astronomy at McMaster University. Paul Higgs is Canada Research Chair in Biophysics and a Professor in the Department of Physics and Astronomy at McMaster University. Jonathon Stone is Associate Director of the Origins Institute and SHARCNet Chair in Computational Biology in the Department of Biology at McMaster University. Cambridge Astrobiology Series Editors Bruce Jakosky, Alan Boss, Frances Westall, Daniel Prieur, and Charles Cockell Books in the series: 1. Planet Formation: Theory, Observations, and Experiments Edited by Hubert Klahr and Wolfgang Brandner ISBN 978-0-521-86015-4 2. Fitness of the Cosmos for Life: Biochemistry and Fine-Tuning Edited by John D. Barrow, Simon Conway Morris, Stephen J. Freeland, and Charles L. Harper, Jr. ISBN 978-0-521-87102-0 3. Planetary Systems and the Origins of Life Edited by Ralph Pudritz, Paul Higgs, and Jonathon Stone ISBN 978-0-521-87548-6 PLANETARY SYSTEMS AND THE ORIGINS OF LIFE RALPH PUDRITZ, PAUL HIGGS, JONATHON STONE McMaster University, Canada CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521875486 © Cambridge University Press 2007 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2007 ISBN-13 978-0-511-36667-3 eBook (EBL) ISBN-10 0-511-36667-1 eBook (EBL) ISBN-13 978-0-521-87548-6 hardback ISBN-10 0-521-87548-X hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Contents List of contributors page xi Preface xv Part I Planetary systems and the origins of life 1 1 Observations of extrasolar planetary systems 3 Shay Zucker 1.1 Introduction 3 1.2 RV detections 4 1.3 Transit detections 7 1.4 Properties of the extrasolar planets 10 1.5 Other methods of detection 14 1.6 Future prospects for space missions 16 Acknowledgements 17 References 17 2 The atmospheres of extrasolar planets 21 L. Jeremy Richardson and Sara Seager 2.1 Introduction 21 2.2 The primary eclipse 21 2.3 The secondary eclipse 23 2.4 Characteristics of known transiting planets 25 2.5 Spectroscopy 27 2.6 Model atmospheres 30 2.7 Observations 32 2.8 Future missions 35 2.9 Summary 37 References 38 vi Contents 3 Terrestrial planet formation 41 Edward W. Thommes 3.1 Introduction 41 3.2 The formation of planetesimals 42 3.3 The growth of protoplanets 43 3.4 The growth of planets 47 3.5 The origin of the Earth–Moon system 52 3.6 Terrestrial planets and life 52 3.7 Summary 56 Acknowledgements 57 References 57 4 From protoplanetary disks to prebiotic amino acids and the origin of the genetic code 62 Paul G. Higgs and Ralph E. Pudritz 4.1 Introduction 62 4.2 Protoplanetary disks and the formation of planet systems 63 4.3 Protoplanetary disks and the formation of biomolecules 68 4.4 Measurements and experiments on amino acid synthesis 71 4.5 A role for thermodynamics 73 4.6 The RNA world and the origin of the genetic code 76 4.7 How was the genetic code optimized? 80 4.8 Protein evolution 82 4.9 Summary 84 Acknowledgements 84 References 84 5 Emergent phenomena in biology: the origin of cellular life 89 David Deamer 5.1 Introduction 89 5.2 Defining emergence 89 5.3 Emergence of life: a very brief history 90 5.4 The first emergent phenomena: self-assembly processes on the early Earth 91 5.5 Sources of amphiphilic molecules 92 5.6 The emergence of primitive cells 95 5.7 Self-assembly processes in prebiotic organic mixtures 100 5.8 Emergence of membrane functions 101 5.9 Emergence of growth processes in primitive cells 103 5.10 Environmental constraints on the first forms of life 105 Contents vii Acknowledgements 106 References 106 Part II Life on Earth 111 6 Extremophiles: defining the envelope for the search for life in the universe 113 Lynn Rothschild 6.1 Introduction 113 6.2 What is an extremophile? 114 6.3 Categories of extremophiles 115 6.4 Environmental extremes 115 6.5 How do they do it? 123 6.6 Examples of extreme ecosystems 125 6.7 Space: new categories of extreme environments 126 6.8 Life in the Solar System? 127 6.9 Conclusions 130 Acknowledgements 131 References 131 7 Hyperthermophilic life on Earth – and on Mars? 135 Karl O. Stetter 7.1 Introduction 135 7.2 Biotopes 136 7.3 Sampling and cultivation 138 7.4 Phylogenetic implications 139 7.5 Physiologic properties 141 7.6 Examples of recent HT organisms 143 References 147 8 Phylogenomics: how far back in the past can we go? 149 Henner Brinkmann, Denis Baurain, and Hervé Philippe 8.1 Introduction 149 8.2 The principles of phylogenetic inference 149 8.3 Artefacts affecting phylogenetic reconstruction 152 8.4 Strengths and limitations of phylogenomics 155 8.5 The importance of secondary simplification 160 8.6 The tree of life 164 8.7 Frequent strong claims made with weak evidence in their favour 167 viii Contents 8.8 Conclusions 171 Acknowledgements 171 References 172 9 Horizontal gene transfer, gene histories, and the root of the tree of life 178 Olga Zhaxybayeva and J. Peter Gogarten 9.1 Introduction 178 9.2 How to analyse multigene data? 179 9.3 What does the plurality consensus represent? Example of small marine cyanobacteria 182 9.4 Where is the root of the ‘tree of life’? 183 9.5 Use of higher order characters: example of ATPases 185 9.6 Conclusions 188 Acknowledgements 188 References 188 10 Evolutionary innovation versus ecological incumbency 193 Adolf Seilacher 10.1 The Ediacaran world 193 10.2 Preservational context 194 10.3 Vendobionts as giant protozoans 195 10.4 Kimberella as a stem-group mollusc 198 10.5 Worm burrows 202 10.6 Stability of ecosystems 203 10.7 The parasite connection 204 10.8 Conclusions 207 Acknowledgements 208 References 208 11 Gradual origin for the metazoans 210 Alexandra Pontefract and Jonathon Stone 11.1 Introduction 210 11.2 Collagen as a trait tying together metazoans 211 11.3 The critical oxygen concentration criterion 212 11.4 The Burgess Shale fauna: a radiation on rocky ground 213 11.5 Accumulating evidence about snowball Earth 215 11.6 North of 80◦ 216 11.7 Conclusion 219 Contents ix Acknowledgements 219 References 219 Part III Life in the Solar System? 223 12 The search for life on Mars 225 Chris P. McKay 12.1 Introduction 225 12.2 Mars today and the Viking search for life 227 12.3 Search for second genesis 229 12.4 Detecting a second genesis on Mars 235 12.5 Conclusions 238 References 238 13 Life in the dark dune spots of Mars: a testable hypothesis 241 Eörs Szathmáry, Tibor Gánti, Tamás Pócs, András Horváth, Ákos Kereszturi, Szaniszló Bérczi, and András Sik 13.1 Introduction 241 13.2 History 241 13.3 Basic facts and considerations about DDSs 243 13.4 Challenges and answers 250 13.5 Partial analogues on Earth 255 13.6 Discussion and outlook 257 Acknowledgements 258 References 258 14 Titan: a new astrobiological vision from the Cassini–Huygens data 263 François Raulin 14.1 Introduction 263 14.2 Analogies between Titan and the Earth 264 14.3 A complex prebiotic-like chemistry 271 14.4 Life on Titan? 278 14.5 Conclusions 280 Acknowledgements 281 References 282 15 Europa, the ocean moon: tides, permeable ice, and life 285 Richard Greenberg 15.1 Introduction: life beyond the habitable zone 285 15.2 The surface of Europa 286 x Contents 15.3 Tides 295 15.4 The permeable crust: conditions for a Europan biosphere 305 Acknowledgements 309 References 309 Index 313 Contributors Denis Baurain J.