The Chemistry of Life's Origins NATO ASI Series Advanced Science Institutes Series A Series presenting the results of activities sponsored by the NA TO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities. The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division A Life Sciences Plenum Publishing Corporation B Physics London and New York C Mathematical Kluwer Academic Publishers and Physical Sciences Dordrecht, Boston and London D Behavioural and Social Sciences E Applied Sciences F Computer and Systems Sciences Springer-Verlag G Ecological Sciences Berlin, Heidelberg, New York, London, H Cell Biology Paris and Tokyo I Global Environmental Change NATO-PCO-DATA BASE The electronic index to the NATO ASI Series provides full bibliographical references (with keywords and/or abstracts) to more than 30000 contributions from international scientists published in all sections of the NATO ASI Series. Access to the NATO-PCO-DATA BASE is possible in two ways: - via online FILE 128 (NATO-PCO-DATA BASE) hosted by ESRIN, Via Galileo Galilei, 1-00044 Frascati, Italy. - via CD-ROM "NATO-PCO-DATA BASE" with user-friendly retrieval software in English, French and German (© WTV GmbH and DATAWARE Technologies Inc. 1989). The CD-ROM can be ordered through any member of the Board of Publishers or through NATO-PCO, Overijse, Belgium. Series C: Mathematical and Physical Sciences -Vol. 416 The Chemistry of Life's Origins edited by J. M. Greenberg Laboratory Astrophysics, University of Leiden, Leiden, The Netherlands C. X. Mendoza-G6mez Laboratory Astrophysics, University of Leiden, Leiden, The Netherlands and V. Pirronello Istituto di Fisica, Universita di Catania, Catania, Italy Springer Science+Business Media, B.V. Proceedings of the NATO Advanced Study Institute and 2nd International School of Space Chemistry Erice, Sicily, Italy 20-30 October 1991 Library of Congress Cataloging-in-Publication Data The Chemistry of life's origins / edited by J.M. Greenberg. and C.X. Mendoza-Gomez. and V. Pirronel la. p. cm. -- (NATD ASI series. Ser ies C. Mathematical and physical sciences ; val. 416) Includes index. ISBN 978-94-010-4856-9 ISBN 978-94-011-1936-8 (eBook) DOI 10.1007/978-94-011-1936-8 1. Cosmochemistry--Congresses. 2. Cosmology--Congresses. 3. Life--Drigin--Congresses. I. Greenberg. J. Mayo (Jerome Mayo). 1922- II. Mendoza-Gomez. C. X. III. Pirronello. V. (Valerio) IV. Ser ies: NATD ASI ser ies. Series C. Mathematical and physical sciences ; no. 416. QB450.C546 1993 577--dc20 93-20895 ISBN 978-94-010-4856-9 Printed on acid-free paper AII Rights Reserved © 1993 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1993 Softcover reprint of the hardcover 1s t edition 1993 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photo copying, recording or by any information storage and retrieval system, without written permission from the copyright owner. CONTENTS Preface vii J.M. GREENBERG and C.X. MENDOZA-GOMEZ / Interstellar Dust Evolution: A Reservoir of Prebiotic Molecules V. PIRRONELLO / Laboratory Simulations of Grain Icy Mantles Processing by Cosmic Rays 33 W.J. DUSCHL / Physics and Chemistry of Protoplanetary Accretion Disks 55 B. FEGLEY, Jr. / Chemistry of the Solar Nebula 75 J.F. KASTING / Early Evolution of the Atmosphere and Ocean 149 L.M. MUKHIN / Origin and Evolution of Hartian Atmosphere and Climate and Possible Exobiological Experiments 177 L.H. MUKHIN and H.V. GERASIHOV / The Possible Pathways of the Synthesis of Precursors on the Early Earth 185 J.H. GREENBERG / Physical and Chemical Composition of Comets - From Interstellar Space to the Earth 195 J .R. CRONIN and S. CHANG / Organic 11atter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite 209 S. CHANG / Prebiotic Synthesis in Planetary Environments 259 J.P. FERRIS / Prebiotic Synthesis on Minerals: ~ RNA Oligomer Formation ~01 A.W. SCHWARTZ / Biology and Theory: RNA and the Origin of Life 323 A. BRACK / Chirality and the Origins of Life 345 A. BRACK / Early Proteins 357 M. SCHIDLOWSKI / The Beginnings of Life on Earth: Evidence from the Geological Record 389 Index 415 Index of Chemical Species 423 PREFACE This volume contains the lectures presented at the second course of the International School of Space Chemistry held in Erice (Sicily) from October 20 -30 1991 at the "E. Majorana Centre for Scientific Culture". The course was attended by 58 participants from 13 countries. The Chemistry of Life's Origins is well recognized as one of the most critical subjects of modem chemistry. Much progress has been made since the amazingly perceptive contributions by Oparin some 70 years ago when he first outlined a possible series of steps starting from simple molecules to basic building blocks and ultimate assembly into simple organisms capable of replicating, catalysis and evolution to higher organisms. The pioneering experiments of Stanley Miller demonstrated already forty years ago how easy it could have been to form the amino acids which are critical to living organisms. However we have since learned and are still learning a great deal more about the primitive conditions on earth which has led us to a rethinking of where and how the condition for prebiotic chemical processes occurred. We have also learned a great deal more about the molecular basis for life. For instance, the existence of DNA was just discovered forty years ago. It is becoming clearer and clearer from fossil evidence and from carbon isotopic distributions that life was extensively distributed as far back as 3.8 billion years ago. Just as in Darwinian theory, the ancestry of life as we know it today in all its complex and varied forms must have started with the simplest microscopic organisms. We still see all around us many simple bacteria and by observing how they function we try to work backwards to what their primitive ancestors did. It turns out that by coming to understand how all bacteria 'digest' carbon, the most basic element of life, we can detect the presence of living organisms on the earth simply by comparing (with carbonate deposits) how much of the light and heavy forms of carbon is found in the sediments containing the carbonaceous remains of these life forms. It has been a great surprise to fmd that life was already extensively present on the earth almost immediately after it was first possible to survive the last great bombardment 3.8 billion years ago. This is confirmed as well by the evidence for fossil microorganisms. The difficulty is that the impact record on the moon and other solar system bodies suggests that massive bombardment of the earth created an enviromnent hostile to life for the first 700 million years of the earth's existence. With a total age of the earth of 4.5 billion years this severely limits the available time for life to have emerged. From molecules to living organisms must, in that case, have occurred not in hundreds of millions of years as earlier thought but perhaps only in thousands of years or even less. This raises some critical new questions about how life began. Was there enough time for building blocks to be formed on earth or were they imported from space? What are the building blocks? What are the minimum conditions or perhaps the ideal conditions for the emergence of life? Does the shortness of available time require a strongly localized favorable environment and set of prebiotic molecules rather than an extensively less favorable set of conditions? Is the emergence of life dependent on the preexisting chirality or handedness of the molecules which is a universal characteristic of living matter? Is there a minimum complexity threshold for the onset of autocatalytically closed sets of reactions to occur? Is racemization only inhibited with the advent of bounding semi-permeable membranes with chiral selectivity? We have attempted to introduce the subject of life's origins by considering all possible sources of prebiotic molecules, from interstellar space to the protosolar nebula, to meteorites and, of course, from the earth itself. Many different kinds of questions must be addressed of how, when and where life began. This is primarily a matter of chemistry. But what kind of chemistry and what environments? Could life have started or existed on other planets of our solar system? And, if not, why not? Was there ever life on Mars? In what ways do other bodies of the solar system resemble or differ from the earth both past and present? vii viii There is unquestionably a wide variety of earth and space environments in which prebiotic molecules could have been created as well as a wide variety of ways by which these molecules could have been considered as the initiators of life. The theory of the RNA world has received wide acceptance as providing the self-replicating requirement which is one of the keys to "living" beings. Is the term "world" an appropriate word or should we perhaps go back to the word "pond" as an initial state? Oligomerization certainly requires a high local concentration. The possibility that certain common clay minerals could have provided the catalytic reactions producing RNA oligomers is attractive because it is a highly localized process. If we consider the earth as the source as well as the bearer of life we have to recognize that the oxidation state of the prebiotic atmosphere is now generally believed to have been a poor source of the molecules hydrogen cyanide, formaldehyde and ammonia which are key chemical intermediates to prebiotic chemical evolution and which are readily produced in an atmosphere which was originally presumed to be reduced. However, instead of the atmosphere, could the early ocean, which was in an obviously reduced state, have been the prime source of the endogenous prebiotic molecules? Interestingly, we now believe that the oceans were to a major extent brought in by comets so, perhaps one way or another, exogenous molecular sources were involved in the origins of life. Once the ocean existed, marine hydrothermal systems may have provided a source of prebiotic molecules. Other possible sources offered by the ocean could have been in the physical and chemical processing which can occur in its interface with the atmosphere where waves cause continuous mixing in sprays and bubbles. But the advantages offered by the ocean for prebiotic evolution are balanced by the disadvatages of dilution of the products. Does one, even with the ocean, require a specific local concentration or site of interaction for the prebiotic molecular pathway to succeed beyond the simplest form? In recent years there have been many suggestions that the basic building blocks of life may have been created not on earth but rather in the vast regions of the space between the stars in our Milky Way and delivered to earth in sufficient quantities and in the required time frame to trigger rapid developments. The very small particles that make up the clouds of dust in space are very active chemical factories driven by the ultraviolet light of distant stars leading to complex organic molecules. This conjecture has been tested in the laboratory and has actually been confirmed by astronomical observation. It turns out that about 20% of all solid matter in our galaxy consists of largely prebiotic molecules and that comets are the repositor of such molecules in our solar system. Of course there are many, many unanswered questions about how these interstellar prebiotic molecules could have survived the impact of the comets and asteroids on earth. But at least as far as comets are concerned, the fact that they have been shown to be made up of very loosely held together small particles leads to the possibility that they would, upon impacting with the earth's atmosphere, break up into many fragments, many of which could then relatively gently floated to the earth. Of significance here is the requirement of a dense early atmosphere. The subject of life or of prebiotic type conditions in the solar system other than earth has also been discussed. Prebiotic synthesis in planetary environments has been reviewed with the introduction of many recent ideas which circumvent some of the problems created by the earth's atmosphere possibly not providing the simple moleculear ingredients thought to be needed for creating the amino acids. These take into account the evolution of the earth's atmosphere and ocean. The exogenous point of view of the origin of the prebiotic molecules is covered in part by a very. detailed description of the analysis of the prebiotic products contained in meteorites. Whether the initial chemical composition of these meteorites was created in the protosolar nebula or in inter stellar space can be examined by comparing the discussions in the chapter on protosolar chemistry and on the chemical evolution of interstellar dust with the analysis of the meteorites and the current knowledge of comet ingredients. The scientists at the Ettore Majorana school for Space Chemistry on the Chemistry of Life's Origins addressed all these problems. Some considered the early atmospheres of the Earth and Mars. Others spoke of the present atmosphere of Titan as a useful global laboratory in which we can study some of the basic chemical processes in a primitive prebiotic atmosphere. Some discussed the amino acids found in meteorites. It is important to realize that the methods used in the study of planetary atmospheres apply ix also to our own current problems of contamination of the atmosphere by carbon dioxide and its possible deleterious effects on the earth's climate. There are messages being deciphered that tell of the existence of living material in places as inhospitable as the Arctic and Antarctic as well as in geothennal vents. Were the prebiotic conditions for life to have evolved much more exotic than we had pictured? Planetary scientists, geo-chemists, astronomers and astrophysicists are searching for new answers. Space probes such as the Halley, Giotto and Vega missions have provided us with proof of the abundance of organic molecules and interstellar dust in comets. Further space probes of the outer solar system, of Mars and of comets are aimed at understanding the origin both of life and of our solar system in geneml. Like all the other great problems that have fascinated and stimulated the minds of men through the ages, we believe the answers to the origin of life are there to be uncovered and it is a pleasure to acknowledge the opportunity provided by the support of NATO and of the "Ettore Majomna Centre for Scientific Culture" in Erice to bring together outstanding scientists from so many and varied disciplines to study the methodology and the hypotheses which are being applied so diligently to discover our origins. A special debt of gmtitude is owed to the staff of the Centre for their hospitality and constant support during the meeting.! would also like to express my sincere thanks to Dr. Celia Mendoza Gomez and Professor Valerio Pirronello for their help in editing this volume. J. Mayo Greenberg University of Leiden Labomtory Astrophysics Niels Bohrweg 2 2300 RA Leiden The Netherlands