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Atmospheric Photochemistry, Surface Features, and Potential Biosignature Gases of Terrestrial Exoplanets by Renyu Hu M.S

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Atmospheric Photochemistry, Surface Features, and Potential Biosignature Gases of Terrestrial Exoplanets by Renyu Hu M.S. Astrophysics, Tsinghua University (2009) Dipl.-Ing., Ecole Centrale Paris (2009) B.S. Mathematics & Physics, Tsinghua University (2007) Submitted to the Department of Earth, Atmospheric and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Planetary Sciences at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2013 c Massachusetts Institute of Technology 2013. All rights reserved. Author.............................................................. Department of Earth, Atmospheric and Planetary Sciences May 18, 2013 Certified by. Sara Seager Class of 1941 Professor of Planetary Science and Physics Thesis Supervisor Accepted by . Robert van der Hilst Schlumberger Professor of Earth and Planetary Sciences Head, Department of Earth, Atmospheric and Planetary Sciences 2 Atmospheric Photochemistry, Surface Features, and Potential Biosignature Gases of Terrestrial Exoplanets by Renyu Hu Submitted to the Department of Earth, Atmospheric and Planetary Sciences on May 18, 2013, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Planetary Sciences Abstract The endeavor to characterize terrestrial exoplanets warrants the study of chemistry in their atmospheres. Here I present a comprehensive one-dimensional photochemistry- thermochemistry model developed from the ground up for terrestrial exoplanet at- mospheres. With modern numerical algorithms, the model has desirable features for exoplanet exploration, notably the capacity to treat both thin and thick atmo- spheres ranging from reducing to oxidizing, and to find steady-state solutions start- ing from any reasonable initial conditions. These features make the model the first photochemistry-thermochemistry model applicable for non-hydrogen-dominated thick atmospheres on terrestrial exoplanets. Using the photochemistry model, I explore the compositions of thin atmospheres on terrestrial exoplanets controlled by surface emission and deposition of gases. High- lights of my findings are: (1) oxygen and ozone may build up in 1-bar CO2 atmo- spheres to levels that have conventionally been accepted as unique signatures of life, if there is no surface emission of reducing gases; (2) volcanic carbon compounds (CH4 and CO2) are likely to be abundant in terrestrial exoplanet atmospheres; but volcanic sulfur compounds (H2S and SO2) are chemically short-lived and therefore cannot ac- cumulate in virtually any types of terrestrial exoplanet atmospheres. Also using the photochemistry model, I explore the ranges of molecular compo- sitions of thick atmospheres on terrestrial exoplanets. I find that carbon has to be in the form of CO2 in a H2-depleted water-dominated atmosphere, and that the pre- ferred loss of light elements from an oxygen-poor and carbon-rich atmosphere leads to formation of unsaturated hydrocarbons (C2H2 and C2H4). These results imply that chemical stability has to be taken into account when interpreting the spectrum of a super Earth/mini Neptune like GJ 1214b. Another intriguing category of terrestrial exoplanets is bare-rock exoplanets. I present the first theoretical framework to compute disk-integrated spectrum from a bare-rock exoplanet, taking into account the reflectivity and emissivity of solid min- erals on the surface. I find that silicate surfaces lead to prominent spectral features in the 8 - 13 µm range, detectable by mid-infrared spectroscopy using transit. There- 3 fore transit spectroscopy is an independent method to confirm the rocky nature of an exoplanet. Thesis Supervisor: Sara Seager Title: Class of 1941 Professor of Planetary Science and Physics 4 Acknowledgments I always feel fortunate to interact with great professors and peers who have made my education fruitful and pleasant at the Massachusetts Institute of Technology. I would like to express my sincere gratitude to those who helped me during the doctoral study. This thesis would not be possible without the mentorship and tireless guidance from my thesis advisor, Professor Sara Seager. It was Professor Seager who introduced me to the exciting field of exoplanets and suggested me to work on the photochemistry of exoplanet atmospheres. Over the four years, Professor Seager has been incredibly helpful for my research, education, and career. I especially appreciate her efforts to help me build research toolkits, think strategically, write effectively, and learn the American culture. I will always be inspired by Professor Seagers scientific intuition and focus on the big picture. I would like to especially thank Professor Maria Zuber who has advised me for another research project. Although the project with Professor Zuber is not included in this thesis, the education that I received working with Professor Zuber goes beyond the project and will go beyond my doctoral study. It was one of the greatest times in the years at MIT when I discussed weather on Mars with Professor Zuber almost every Thursday in my second year. I very much appreciate Professor Zuber for her dedicated guidance and mentorship on my scientific research and academic career. I would also like to thank the members of my thesis committee for their advices on my study. I appreciate Professor Kerri Cahoy for her hands-on advisory, as well as her great efficiency in work that has set the norm for me. I appreciate Professor Bethany Ehlmann for her inspiration of new ideas on the surface characterization of terrestrial exoplanets. I appreciate Professor Cziczo for his perspective of cloud and aerosol microphysics that has made my knowledge more complete. I have had the chance to interact with great researchers during my doctoral study. My sincere thank you goes to Professor James Kasting, Professor Susan Solomon, Pro- fessor Linda Elkins-Tanton, Professor Kerry Emanuel, and Professor Richard Binzel. My study at MIT would not be as pleasant without the support and friendship 5 from the members of the Seager research group and other peers and staff at the Department of Earth, Atmospheric and Planetary Sciences. Special thank you to Dr. Brice-Oliver Demory for bringing in an observer's perspective, critical to this predominantly theoretical work. Also, I really appreciate the work of Kristen Barilaro, Roberta Allard, Derrick Duplessy, Mark Pendleton, Jacqueline Taylor, Carol Sprague, Vicki McKenna, Kerin Willis, and Margaret Lankow. I am grateful to Praecis Pharmaceuticals for the financial support through the MIT Presidential Fellowship. I am also grateful to the American Astronomical Society and the NASA Astrobiology Institute for the generous financial support for me to attend professional conferences and diseminate my results. This work was supported by NASA Headquarters under the NASA Earth and Space Fellowship Program - Grant \NNX11AP47H". My family has given me the greatest support for my doctoral study, for which I cannot thank enough. I met and married to my wife, Yuanyuan, in Boston. I very much appreciate her for supporting me pursuing a research career, and for such a wonderful company in the exploration of our life. Finally, I owe the most to my parents for bringing out the best of me. 6 Contents 1 Introduction 17 1.1 Terrestrial Exoplanets . 17 1.1.1 Discovery . 17 1.1.2 Population of Terrestrial Exoplanets in the Milky Way . 18 1.1.3 Observations of Exoplanet Atmospheres . 21 1.2 Physical Processes in Terrestrial Exoplanet Atmospheres . 22 1.2.1 Thin Atmospheres on Terrestrial Exoplanets . 27 1.2.2 Thick Atmospheres on Terrestrial Exoplanets . 28 1.3 Prospect of Terrestrial Exoplanet Characterization . 31 2 Photochemistry-Thermochemistry Model 35 2.1 Background . 35 2.1.1 Photochemistry-Thermochemistry Models for Planets and Moons in the Solar System . 35 2.1.2 Photochemistry-Thermochemistry Models for Exoplanets . 36 2.2 A Photochemistry-Thermochemistry Model for the Study of Exoplanet Atmospheres . 38 2.2.1 Fundamental Equations . 41 2.2.2 Species in the Model . 46 2.2.3 Chemical Kinetics . 48 2.2.4 Interaction with Radiation . 51 2.2.5 Treatment of Aerosols . 55 2.2.6 Boundary Conditions . 58 7 2.2.7 Coupling with Radiative Transfer Model . 63 2.3 Model Testing . 66 2.3.1 Transport-Only Model for an Earth-like Atmosphere . 67 2.3.2 Present-Day Earth . 70 2.3.3 Present-Day Mars . 79 2.3.4 Jupiter and Hot Jupiters . 83 2.4 Uncertainties and Limitations of the Model . 86 3 Thin Atmospheres on Terrestrial Exoplanets 91 3.1 Background . 91 3.2 Exoplanet Benchmark Scenarios . 92 3.2.1 General Results . 98 3.2.2 Chemistry of H2-, N2-, and CO2-Dominated Atmospheres . 99 3.2.3 Rationale of Model Parameters . 102 3.3 Redox-Controlling Effects of Surface Emission and Deposition . 105 3.3.1 Redox-Controlling Effects of CO2 in H2-Dominated Atmospheres105 3.3.2 Redox-Controlling Effects of H2 and Abiotic Formation of O2 in CO2-Dominated Atmospheres . 108 3.4 Sulfur Chemistry in Terrestrial Exoplanet Atmospheres . 112 3.4.1 Background . 112 3.4.2 Photochemistry Model Setup . 114 3.4.3 Sulfur Chemistry in Reducing and Oxidizing Atmospheres . 116 3.4.4 Results . 124 3.4.5 Can H2S be a Biosignature Gas? . 135 3.5 Summary . 137 4 Thick Atmospheres on Terrestrial Exoplanets 141 4.1 Background . 141 4.2 Model Atmospheres . 142 4.3 Results . 144 8 4.3.1 Chemical Classification of Thick Atmospheres on Terrestrial Exoplanets . 144 4.3.2 Ranges of Compositions of Water-Rich Atmospheres and Oxygen- Rich Atmospheres . 151 4.3.3 Ranges of Compositions of Hydrocarbon-Rich Atmospheres . 156 4.4 Application to Observations . 160 4.4.1 GJ 1214b . 160 4.4.2 55 Cnc e . 164 4.5 Discussion . 167 4.5.1 Finding Water-Dominated Thick Atmospheres May be Chal- lenging . 167 4.5.2 Chemical Stability of Atmospheric Gases . 168 4.5.3 Trace Gases as the Probe for Vertical Mixing and Internal Heating170 4.5.4 Atmosphere-Surface Exchange of Super Earths . 172 4.6 Summary . 173 5 Theoretical Spectra of Airless Terrestrial Exoplanets 175 5.1 Background .
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