Dirk Schulze-Makuch Louis N. Irwin Life in the Universe Expectations and Constraints ird Edition Life in the Universe Springer Praxis Books Astronomy and Planetary Sciences Series editors Martin A. Barstow Leicester, United Kingdom Ian Robson Edinburgh, United Kingdom Derek Ward-Thompson Preston, United Kingdom More information about this series at http://www.springer.com/series/4175 Dirk Schulze-Makuch • Louis N. Irwin Life in the Universe Expectations and Constraints Third Edition Dirk Schulze-Makuch Louis N. Irwin Center for Astronomy and Astrophysics University of Texas at El Paso Technical University Berlin El Paso, TX, USA Berlin, Germany Springer Praxis Books ISSN 2366-0082 ISSN 2366-0090 (electronic) Astronomy and Planetary Sciences ISBN 978-3-319-97657-0 ISBN 978-3-319-97658-7 (eBook) https://doi.org/10.1007/978-3-319-97658-7 Library of Congress Control Number: 2018955969 1st edition: © Springer-Verlag Berlin Heidelberg 2004 2nd edition: © Springer-Verlag Berlin Heidelberg 2008 © Springer Nature Switzerland AG 2018 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. 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This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland As humans in the early years of civilization sailed beyond the isles of the eastern Mediterranean to learn about a larger planet, so the human species, still in its technological youth, now searches for life on other worlds. To a species of life on a water-rich planet in an otherwise unremarkable solar system at an outlying part of the galaxy in an unexceptional part of the Universe, just beginning to explore the world beyond the home on which it began to evolve about 4 billion years ago. Preface While grounded in the hard sciences of physics, chemistry, geology, and astronomy, as well as well-accepted principles in the life sciences, astrobiology remains the most speculative of all the sciences because it assumes a reality that has not been demonstrated conclusively: the presence of life on other planetary bodies. Until the discovery of a living organism from another world is confirmed, this part of astrobiology will continue to be a science in search of a factual base. We wrote the first edition of this book in 2004 to bring scientific rigor to that search. However, astrobiology also includes investigations into the origin of life on Earth, life in extreme environments, and the future and fate of life on our planet, as a means of gaining insight into how life might emerge and play out wherever it exists. Those aspects of the field are backed up by robust (hence much less specu- lative) findings and increasingly insightful ideas. The second edition in 2008 was focused especially on those new developments. In both editions, we set forth reasons for expecting that extraterrestrial life, if it were similar to life as we know it, is highly likely to be carbon based, using water as a solvent, and sunlight as its primary source of energy. This assumption has broad general agreement within the scientific community—so much so, that the search for alien life has driven the field to focus its enthusiasm, perhaps excessively, on worlds that look like our own. There is little doubt and growing evidence that such worlds do exist, but in solar systems other than our own. Most exoplanets discovered to date, and all other planets and moons in our Solar System, are not at all like Earth. Therefore, the broader challenge for astrobiology is to look for life beyond where we are most likely to find—or at least recognize—it, and seek it wherever and in whatever form it might exist. This compels us to consider planetary environments where either (1) carbon-based organisms could exist in localized or global aqueous habitats considered extreme by terran standards or (2) exotic forms of life could be found in habitats unable to support life as we know it, but not incapable of supporting organisms with the generic properties of living systems. ix x Preface The goal of the first two editions has thus been retained in this one: To define as precisely as possible what constitutes a living entity, to define the limiting param- eters of its chemical composition, solvent requirements, and possible sources of energy, then systematically consider the possibilities for its distribution throughout the Universe in forms both known and unknown to us. In this third edition, we have retained the expanded consideration of both the origin and fate of living systems. We have also updated content and references extensively throughout to cover the vast number of experimental studies, discoveries, and conceptual advances that have occurred over the last decade. As before, our definition of life (Chap. 2) sidesteps the abstract concept of life through time, in favor of a utilitarian view of what constitutes a living organism at a given point in time. Recent advances in various forms of “artificial life,” from the reconstitution of living cells by insertion of genetic elements synthesized de novo (“synthetic life”), the tendency toward virtual and augmented reality (“virtual life”), and robotics (“mechanical life”), have compelled us to incorporate these develop- ments into our discussion of what it means to be alive. Our treatment of the origin of life (Chap. 3) has been updated with more recent research and references, including a more detailed discussion of the nature and implications of the genetic code. The core chapters of the prior editions on energy (Chap. 5), chemical building blocks (Chap. 6), solvents (Chap. 7), habitats (Chap. 8), and signatures (Chap. 9) have been retained but significantly updated and more extensively documented. The Viking Mission and alleged biomarkers in Martian meteorites are again described in detail (Chap. 10), as they still constitute the most serious claims to date of evidence for the existence of life on another world. The chapter now contains an updated repertoire of life detection instrumentation as well. Whereas the expectations of and constraints on the possible distribution of life on other worlds in our Solar System were scattered among different sections in the previous editions, we have now concentrated the discussion into one place (Chap. 11), with detailed examination of the prospects for life in the light of newly obtained data from Mars, Ceres, Europa, Enceladus, Titan, and the Pluto-Charon system. We are even urging in this edition a closer look at the astrobiological potential of our own Moon. The number of verified exoplanets has increased by an order of magnitude since the second edition, justifying an entirely new and complete discussion of exoplanets, exomoons, their means of discovery, and their taxonomy (Chap. 12). The fate and future of life—a subject of concern but largely ignored in the astrobiological literature until recently—is again treated in a fact-based way, draw- ing on established evolutionary principles from the history of life on Earth (Chap. 14). The search for extraterrestrial intelligence, which increasingly seems likely to include the search for extraterrestrial artifacts, has been described in a section of its own (Chap. 15). The controversy over the wisdom of trying to contact alien forms of life is described, and our thoughts on the so-called Fermi Paradox are offered in this chapter as well. Preface xi We conclude with a discussion of how the exploration of space can be optimized through the combination of both manned and robotic strategies (Chap. 16). We provide a detailed plan for establishing settlements on Mars, but consider the value of Venus, the Moon, Ceres, and the icy satellites of the outer Solar System as worthy targets for exploration also. Finally, we introduce the contentious topic of planetary protection which raises serious questions about missions that will land machines and/or humans on other worlds in the future. Dirk Schulze-Makuch thanks his family, particularly his wife, Joanna, and his children, for their patience and understanding of his preoccupation with science even more than usual. He also thanks all his students and colleagues for their willingness to bounce around ideas. Louis N. Irwin likewise appreciates the encouragement of his wife, Carol, and ongoing feedback from family members, friends, and former students who have commented kindly on the value of this work. Berlin, Germany Dirk Schulze-Makuch El Paso, TX, USA Louis N. Irwin Contents 1 Introduction ......................................... 1 2Definition of Life ..................................... 7 2.1 Problems with Common Assumptions About the Nature of Life . ....................................... 8 2.2 Historical Views on the Definition of Life . 12 2.3 Modern Definitions of Life .......................... 13 2.4 Thermodynamic Criteria ........................... 14 2.5 Reproducibility Criteria . 15 2.6 Evolutionary Criteria .