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The Role of Comets in Panspermia

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The Role of Comets in Panspermia THE ROLE OF COMETS IN PANSPERMIA By Janaki Tara Wickramasinghe BSc Thesis submitted in candidature for the degree of Doctor of Philosophy at Cardiff University UMI Number: U584951 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U584951 Published by ProQuest LLC 2013. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Declaration This work has not previously been accepted in substance for any degree and is not concurrently submitted in candidature for any degree. Signed. (candidate) Dat q .?:$/.$J .9.? STATEMENT 1 This thesis is being submitted in partial fulfillment of the requirements for the degree of PhD. Signed.. (candidate) Date. .*? STATEMENT 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. Signed . (candidate) Date. 2Sj/.$~./.Q 7 STATEMENT 3 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. STATEMENT 4 - BAR ON ACCESS APPROVED I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loans after expiry of a bar on access approved by the Graduate Development Committee. Signed . (candidate) Date. 2 . Contents Acknowledgements iii Summary iv Chapter 1: Introduction 1.1 General introduction 2 1.2 History of panspermia 2 1.3 The ultraviolet problem 4 1.4 Resilience of bacteria 4 1.5 Extremophiles 6 1.6 Discovery of organics in cosmic dust 7 1.7 Comets 8 1.8 The origin of life 10 1.9 Modem advances 12 1.10 Protoplanetary nebulae and extra-solar planetary systems 13 1.11 Habitable zone 16 Chapter 2: Interstellar Dust 2.1 Introduction 19 2.2 Modelling with spherical particles 22 2.3 Reflectivity of surfaces 24 2.4 Equation of transfer 24 2.5 Modelling the interstellar extinction curve 26 2.6 Beyond the Galaxy 29 2.7 Interpretation of a 2175 A absorption feature at z=0.83 30 2.8 Exploring models 32 2.9 Fluorescence 38 2.10 The origin of organic molecules in space 41 2.11 Molecules in meteorites 43 Chapter 3: Brief Overview of Comets 3.1 Introduction 45 3.2 The origin of comets and composition of comet dust 46 3.3 Theory of panspermia and comets 50 3.4 Basic dynamics of comets 50 3.5 Classes of comet 55 Chapter 4: Disturbing the Oort Cloud and Panspermia 4.1 Introduction 62 4.2 The effect of the vertical Galactic tide 64 4.3 Perturbation of the Oort cloud by a passing molecular cloud 66 4.4 Flux modulation due to the Sun’s z-motion in the Galactic disc 72 i Chapter 5: The Dark Hailey Population - a Link in the Chain to Panspermia 5.1 Introduction 81 5.2 Disintegration of Halley-type comets 82 5.3 Extremely dark comets 90 5.4 Current impact hazard 93 Chapter 6: Creation, survival and expulsion of microorganisms from the Solar System 6.1 Introduction 97 6.2 Expectations from impact cratering mechanisms 97 6.3 Mechanisms for ejection and fragmentation of boulders 99 6.4 13-Particles 99 6.5 Protective shielding in small p-particles 101 6.6 Carbonisation of the surface 101 6.7 Radiation pressure effects 102 6.8 Ratio of radiation pressure to gravity 103 6.9 Results and dynamical considerations 105 6.10 Surviving the hazards of galactic cosmic rays 106 6.11 Lithopanspermia vs Cometary Panspermia 110 Chapter 7: Liquid Water in Comets 7.1 Introduction 114 7.2 Primordial melting 115 7.3 Evidence of present-day melting 121 7.4 Results from Deep Impact 128 7.5 Concluding remarks 137 Appendix A 140 Bibliography 142 Acknowledgements I am very grateful to my supervisor Professor W.M. Napier for his careful guidance throughout the course of this work. His critical comments and suggestions have helped to develop the structure of the present thesis. I also wish to thank Max Wallis for all his help and advice. A special thank you goes to Brig Klyce and the Astrobiology Research Trust of the USA for generously sponsoring and supporting this work. And finally, thank you to my parents for their support and especially to my father, Chandra Wickramasinghe, who endlessly encouraged me throughout my academic pursuits and continues to inspire me. Janaki Tara Wickramasinghe Cardiff 25 May 2007 Summary The thesis presents a re-evaluation of the theory of panspermia. I show that collisions of comets with Earth and Earth-like planets play a role in transferring microbial life across the galaxy. After a brief overview of panspermia in Chapter 1, Chapter 2 develops the theory of cosmic dust. The extinction curve for the galaxy SBS0909+532 at red shift z=0.83, confirms the organic composition of dust upto distances that are a substantial fraction of the Hubble radius. In Chapter 3 a brief overview of comets is presented. Chapter 4 examines dynamical interactions of the Oort cloud of comets with molecular clouds in the galaxy. Using a numerical integration procedure, including the effect of the galactic tide, we show that such interactions lead to sporadic and recurrent injections of comets into the inner solar system. Chapter 5 examines the fate of comets deflected at the present epoch into the inner regions of the solar system. Estimates of the flux of such comets show that there should be -100 times more comets in Halley-type orbits than are actually observed. This discrepancy can be resolved if comets develop porous, organic crusts of very low albedos. Chapter 6 discusses the expulsion of microorganisms from the solar system and their survival prospects, leading to the conclusion that an adequate fraction survives for re­ incorporation into exosolar planetary systems. The final chapter considers the problem of liquid water in comets. Arguments for radiogenic melting of primordial comets are re-examined. It is shown that comets with radii > 8km can retain liquid water cores for timescales - lMy after their formation, thus providing sites for amplification of small initial compliments of bacteria. By modelling solar heat flow near perihelion onto a crusted, rotating comet it is shown that transient sub-surface pools can be generated, providing habitats for microbes. Data from Deep Impact for comet Tempel 1 is modelled to show evidence for clays, organics and liquid water, and potential microbial habitats. Chapter One: Introduction 1.1 General introduction In this thesis I propose to re-investigate the theory of panspermia in light of recent developments, focussing particularly on the role of comets in originating and distributing microbial life. Evidence pointing to the feasibility that terrestrial life may have come from elsewhere in the solar system has accumulated over the past decade. Complex organic molecules in interstellar dust and comets appear to be biologically derived, or at least closely related spectroscopically and structurally to such material or their degradation products. Developments in microbiology have led to the discovery of various extremophiles, bacteria that can survive in harsh conditions previously thought to be unfavourable to life. Such bacteria with properties ideally suited to space travel gives further credence to panspermia. An ever-increasing number of extra-solar planet discoveries suggest that planetary systems are quite common, renewing the notion that life could be widespread. The hundreds of billions of comets in the Oort cloud that surround the solar system could serve as a likely primordial reservoir of life in the solar system. Primitive microbial life contained and amplified within comets, could be dispersed as individual microbes or clusters of microbes to reach nascent planet forming nebulae. Additionally, evolved microbial ecologies on the surface of the Earth could be expelled by comet impacts into the zodiacal dust cloud which in turn would disperse them into molecular clouds where new stars and planets form. I intend to explore the role of comets in the spread of life on an interstellar scale. 1.2 History of panspermia The concept of panspermia, that life is ubiquitous within the Universe, dates back to ancient Greece. In the 19th century an important experimental basis for panspermia was discovered by Louis Pasteur. His classic experiments showed that microorganisms are always derived from pre-existing microorganisms, an idea which was expanded upon by Lord Kelvin: "Dead matter cannot become living without coming under the influence o f matter previously alive. This seems to me as sure a teaching o f science as the law o f gravitation..." The idea of interplanetary panspermia was discussed by Kelvin in 1871 and involved the transport of life-bearing rocks between objects within the solar system. It was his opinion that such transfers of life between the inner planets could occur sporadically as a consequence of asteroid and comet impacts. In 1874 the German physicist Hermann Von Helmholtz made the following statement: "It appears to me to be fully correct scientific procedure, if all our attempts fail to cause the production o f organisms from non-living matter, to raise the question whether life has ever arisen, whether it is not just as old as matter itself, and whether seeds have not been carried from one planet to another and have developed everywhere where they have fallen on fertile soil..." In the beginning of the twentieth century this idea was expanded upon by Arrenhius.
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