Modelling Biogeochemical Controls on Planetary Habitability Andrew J. Rushby August 2015 A thesis submitted to the School of Environmental Sciences of the University of East Anglia in partial fulfilment of the requirements for the degree of Doctor of Philosophy © This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that use of any information derived there from must be in accordance with current UK Copyright Law. In addition, any quotation or extract must include full attribution. The Last Stand of Gaia, original artwork by Thom Reynolds (2015). ii Abstract The length of a planet's `habitable period' is an important controlling factor on the evolution of life and of intelligent observers. This can be defined as the amount of time the surface temperature on the planet remains within defined `habitable' limits. Complex states of habitability derived from complex interactions between multiple factors may arise over the course of the evolution of an individual terrestrial planet with implications for long-term habitability and biosignature detection. The duration of these habitable conditions are controlled by multiple factors, including the orbital distance of the planet, its mass, the evolution of the host star, and the operation of any (bio)geochemical cycles that may serve to regulate planetary climate. A stellar evolution model was developed to investigate the control of increasing main-sequence stellar luminosity on the boundaries of the radiative habitable zone, which was then coupled with a zero-D biogeochemical carbon cycle model to investigate the operation of the carbonate-silicate cycle under conditions of varying incident stellar flux and planet size. The Earth will remain within habitable temperature limits for 6.34 Gyr (1.8 Gyr from present), but photosynthetic primary producers will experience carbon-starvation due to greatly increased terrestrial weathering from ∼5.38 Gyr (0.84 Gyr from present) onwards, with significant implications for planetary habitability. Planet mass was discovered to have a significant control on the length of the habitable period of Earth-like planets, but more data on the bulk density and atmospheric composition of newly-discovered exoplanets is required before definitive estimates of their long-term habitability can be made. Exoplanet case studies reveal habitable periods significantly longer than that of the Earth, possibly up to ∼80 Gyr in the case of planets in the orbit of M-dwarfs. Contemporary measures of habitability that rely strongly on surface temperatures are becoming obsolete, and a move towards the inclusion of integrated biogeochemical cycle models and the development of multiparameter habitability indices will strengthen contemporary understanding of the distribution and evolution of potentially habitable terrestrial worlds. iii Acknowledgements We are all here on Earth to help others; what on Earth the others are here for I don't know. John Foster Hall The Parson Addresses His Flock (1923) I'd like to extend my thanks to my original supervisors, Andrew Watson and Mark Claire, but especially to my adoptive supervisor Martin Johnson, for nearly 4 years of guidance and encouragement. As far as supervisory teams go, I could not have hoped for a more supportive and knowledgeable group. Thanks also to Ben Mills for help with MATLAB, for general advice and direction in navigating the murky quagmire of postgraduate research, and for enjoyable evenings of music and mirth. Thanks go to my colleagues and friends at the UEA for good times and lively conversation. Clare Ostle deserves a special mention for her encouragement and contagious positivity. To my non-scientist friends, Thom Reynolds and the Kitchen, Rosie Ruttley-Dornan, Johnny Shirley, as well as all the members of Scallywag's Cove; I thank you all for your continued and valued friendship and welcome distraction from academia. I'd also like to the thank the `abnormally large' renal calculus that lodged itself in my left kidney just as I began pulling my thesis together towards the end of 2014. In the throes of the debilitating six weeks of agony that followed, my desire to finish my research and get into better physical shape was a catalyst for wider perspective and productivity. Finally, the greatest thanks of all goes to my family - my mother Lynda and sister Allison - for the sacrifices they made, large and small, that allowed me to be able to be in the position to carry out this work, especially towards the end. Andrew Rushby, August 2015. University of East Anglia, Norwich. iv Dedicated to the memory of my father, William Sydney Rushby (1936 - 2012). A navigator of seas, a rescuer of men, and a paragon of humility. v vi What to expect: word cloud generated from thesis contents (http://timdream.org/wordcloud/) Contents 1 Introduction 1 1.1 Planetary Habitability . .2 1.1.1 Star and Planet Formation . .2 1.1.2 Planet Formation and Geological Evolution . .5 1.1.3 Exoplanets: Detection Techniques . .7 1.1.4 Theoretical Exoplanetary Geophysics . .8 1.1.5 Biogeochemical Feedbacks . 11 1.1.6 Defining Boundaries of Habitability . 11 1.1.7 Habitable for Whom? . 13 1.1.8 Habitability in Space and Time . 13 1.1.9 Photosystem Limits to Habitability . 18 1.2 Habitability Criteria Used in this Work . 21 1.3 Thesis Structure & Hypotheses . 21 2 Habitable Zone Lifetimes of Exoplanets around Main Se- quence Stars 22 2.1 Abstract . 22 2.2 Introduction . 23 2.2.1 The Habitable Zone . 23 2.2.2 On the Definition of the `Habitable Zone' . 26 2.3 Methods . 28 2.3.1 Stellar Evolution Models . 28 vii 2.3.2 Luminosity Evolution . 28 2.3.3 Determining the Habitable Zone . 34 2.3.4 Habitable Zone Boundary Transition Rates . 36 2.4 Results . 39 2.4.1 The Temporally Dynamic Habitable Zone . 39 2.4.2 Are These Planets Still in the Habitable Zone? . 49 2.4.3 Continuously Habitable Zones . 50 2.5 Discussion . 54 2.5.1 The Implications for the `Anthropic Model' . 57 2.6 Conclusions . 60 2.7 Addendum . 60 3 The Carbonate-Silicate Cycle on Earth-like Planets: Impli- cations for Long-Term Habitability 61 3.1 Abstract . 61 3.2 Introduction . 62 3.2.1 Earth Systems Models . 63 3.2.2 Defining Boundaries of Habitability . 63 3.3 Methods . 64 3.3.1 Biogeochemical Carbon Cycle Model . 64 3.3.2 Weathering Fluxes . 72 3.3.3 Atmospheric CO2 ..................... 75 3.3.4 Planetary Climate Parameterisations . 77 3.3.5 Albedo . 77 viii 3.3.6 Greenhouse Warming Factors for CO2, CH4 &H2O.. 80 3.3.7 CO2 Condensation & Cloud Formation . 85 3.3.8 Estimating Greenhouse Warming & Albedo from a 1-D Radiative-Convective Climate Model . 86 3.3.9 Methane Greenhouse . 89 3.3.10 Water Vapour Greenhouse . 90 3.4 Sensitivity Analysis . 92 3.4.1 Model Data Structure . 92 3.4.2 Effect of α on T and CO2 ................ 93 3.4.3 Effect of β on T and CO2 ................ 95 3.4.4 Effect of µ on T and CO2 ................ 96 3.4.5 Effect of roots & biology on weathering rates . 99 3.4.6 Comparisons between a radiative-convective climate model & temperature parameterisations . 99 3.4.7 Comparison to the COPSE model . 100 3.4.8 Comparison to other pCO2 proxies . 103 3.5 Results . 104 3.5.1 Default Model Setup . 104 3.5.2 The Cold Start Climate Catastrophe . 111 3.6 Discussion . 114 3.7 Conclusions . 118 4 Biogeochemical Cycling on Earth-Like Exoplanets 118 4.1 Abstract . 118 ix 4.2 Introduction . 118 4.3 Methods . 119 4.3.1 Planet Scaling Relationships . 119 4.3.2 The Effect of Ocean/Continental Fraction . 123 4.3.3 Latin Hypercube Sampling (LHS) . 125 4.4 Results . 128 4.4.1 Potentially Habitable Planets from the HEC . 128 4.4.2 Planet Mass and the Habitable Zone . 131 4.4.3 Biogeochemical Processes Affecting Habitability . 136 4.4.4 Re-examining the Effect of Star Mass . 139 4.4.5 Eccentricity & Habitability . 141 4.4.6 Latin Hypercube Sampling Analysis . 145 4.4.7 Exoplanet Case Studies . 152 4.5 Discussion . 157 4.5.1 Putting a Limit on the Lifespan of the Biosphere . 158 4.5.2 Limitations of this Work . 161 4.6 Conclusions . 163 5 Conclusions & Future Work 162 5.1 Thesis Conclusions . 162 5.2 Future Work . 165 5.2.1 Other areas of possible development in the future . 166 6 References 168 x 1 Introduction If we crave some cosmic purpose, then let us find ourselves a worthy goal. Carl Sagan Pale Blue Dot: A Vision of the Human Future in Space (1994) Placing humanity in some universal context, both temporally and spatially, is arguably one of the primary philosophical drivers behind the scientific revolution and enlightenment movement. Questions such as `are we alone?' have pervaded scientific and popular thought for centuries, but definitive answers remain difficult to obtain. In terms of planetary science, the relative commonality of the Sun, the Earth and the Solar System remains a key research topic, and evidence that our physical place in the Universe is in some way unique or privileged (the `Rare Earth' hypothesis) would provide some perspective on these larger issues. However, with limited evidence this hypothesis remains scientifically and philosophically problematic, with researchers pointing to an inherent `anthropic bias' from which we, by virtue of being intelligent observers, remain inextricably 1 bound. The relatively recent discovery of planets that orbit stars other than the Sun is casting some light on these questions, particularly on the relative normalcy of our planet relative to those in neighbouring star systems.