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The UV Environment for Prebiotic Chemistry: Connecting Origin-Of-Life Scenarios to Planetary Environments

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The UV Environment for Prebiotic Chemistry: Connecting Origin-Of-Life Scenarios to Planetary Environments The UV Environment for Prebiotic Chemistry: Connecting Origin-of-Life Scenarios to Planetary Environments The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Ranjan, Sukrit. 2017. The UV Environment for Prebiotic Chemistry: Connecting Origin-of-Life Scenarios to Planetary Environments. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:41142052 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA The UV Environment for Prebiotic Chemistry: Connecting Origin-of-Life Scenarios to Planetary Environments A dissertation presented by Sukrit Ranjan to The Department of Astronomy in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Astronomy & Astrophysics Harvard University Cambridge, Massachusetts April 2017 c 2017 | Sukrit Ranjan All rights reserved. Dissertation Advisor: Professor Dimitar D. Sasselov Sukrit Ranjan The UV Environment for Prebiotic Chemistry: Connecting Origin-of-Life Scenarios to Planetary Environments Abstract Recent laboratory studies of prebiotic chemistry (chemistry relevant to the origin of life) are revolutionizing our understanding of the origin of life (abiogenesis) on Earth just as telescopes capable of searching for life elsewhere are coming online. This thesis sits at the intersection of these revolutions. I examine prebiotic chemical pathways postulated to be relevant to the origin of life and identify the environmental conditions they require to function. I compare these environmental requirements to what was available on Earth and other planets, and use the comparison to 1) improve studies of the origin of life on Earth and 2) explore the implications for the inhabitability of other worlds. Multiple lines of evidence suggest UV light may have played a critical role in the synthesis of molecules relevant to abiogenesis (prebiotic chemistry), such as RNA. I show that UV light interacts with prebiotic chemistry in ways that may be sensitive to the spectral shape and overall amplitude of irradiation. I use radiative transfer models to constrain the UV environment on early Earth (3.9 Ga). I find that the surface UV is insensitive to much of the considerable uncertainty in the atmospheric state, enabling me to constrain the UV environment for prebiotic chemistry on early Earth. Some authors have suggested Mars as a venue for prebiotic chemistry. Therefore, I explore plausible UV spectral fluences on Mars at 3.9 Ga. I find that the early Martian UV environment is comparable to Earth's under conventional assumptions about the iii atmosphere. However, if the atmosphere was dusty or SO2 levels were high, UV fluence would have been strongly suppressed. Intriguingly, despite overall attenuation of UV fluence, SO2 preferentially attenuates destructive FUV radiation over prebiotically-useful NUV radiation, meaning high-SO2 epochs may have been more clement for the origin of life. Better measurements of the spectral dependence of prebiotic photoprocesses are required to constrain this hypothesis. Finally, I calculate the UV fluence on planets orbiting M-dwarfs. I find that UV irradiation on such planets is low compared to Earth. Laboratory studies are required to understand whether prebiotic processes that worked on Earth can function on low-UV M-dwarf planets. In addition to UV light, the most promising pathways for the prebiotic synthesis of RNA require reduced sulfidic anions. I show that prebiotically-relevant levels of such anions derived from volcanically-outgassed SO2 should be robustly available on early Earth, and that episodes of high volcanism may be especially clement for these prebiotic pathways. However, H2S-derived anions are much less common, and prebiotic chemistry which invokes them must rely on alternate, localized sources. My work 1) provides initial conditions for laboratory studies of prebiotic chemistry, 2) constrains the inhabitability of Mars and planets orbiting M-dwarfs, and 3) demonstrates the need for laboratory studies to characterize the sensitivity of putative prebiotic chemistry to environmental conditions, e.g the spectral shape and amplitude of UV irradiation. All software associated with these studies, including models and data inputs, are publicly available for validation and extension. iv Contents Abstract iii Acknowledgments xi Dedication xiv 1 Introduction 1 1.1 Overview . .1 1.2 The Search for Life on Other Worlds . .2 1.2.1 The Search for Life Within Our Solar System . .2 1.2.2 An Abundance of Potentially Habitable Exoplanets . .5 1.2.3 Prospects for Exoplanet Biosignature Search . .6 1.2.4 Challenges For Exoplanet Biosignature Search . .7 1.2.5 Connection to Prebiotic Chemistry . 10 1.3 Prebiotic Chemistry: History and Recent Advances . 12 1.3.1 Prebiotic Chemistry: A Brief History . 12 1.3.2 Miller-Urey Syntheses . 14 1.3.3 Key Challenges Faced by Prebiotic Chemistry & The RNA World . 15 1.3.4 Recent Breakthroughs in Prebiotic Chemistry & Environmental Re- quirements . 17 1.3.5 Connection to Planetary Environment . 20 v CONTENTS 1.4 Thesis Summary . 21 2 Influence of the UV Environment on the Synthesis of Prebiotic Molecules 23 2.1 Introduction . 24 2.2 Background . 27 2.2.1 UV Light and Prebiotic Chemistry . 27 2.2.2 UV Light and the RNA World . 30 2.3 Results . 33 2.3.1 Solar UV Output in the Prebiotic Era . 34 2.3.2 Attenuation of UV Light By Aqueous Environments . 41 2.3.3 Attenuation of UV Light by the Terrestrial Atmosphere . 43 2.3.4 CO2 Shielding of Prebiotic Feedstock Gases From Photolysis . 49 2.4 Discussion . 55 2.4.1 Lessons for Laboratory Simulations . 55 2.4.2 Narrowband vs. Broadband Input . 57 2.5 Conclusions . 69 2.6 Appendix A: Constraints on the Era of Abiogenesis . 73 2.7 Appendix B: The 3.9 Ga Terrestrial Environment . 74 2.8 Appendix C: Derivation of Photolytic Constraints on Feedstock Gas Buildup 77 2− 2.9 Appendix D: Collection of Cu(CN)3 Absorption Spectrum . 79 2.10 Appendix E: Extinction Cross-Sections . 82 2.10.1 CO2 ................................... 82 2.10.2 SO2 ................................... 83 2.10.3 H2S................................... 83 3 Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry 85 3.1 Introduction . 87 vi CONTENTS 3.2 Background . 89 3.2.1 Previous Work . 89 3.2.2 Constraints on the Composition of the Atmosphere at 3.9 Ga . 93 3.3 Surface UV Radiation Environment Model . 95 3.4 Model Validation . 100 3.4.1 Tests of Model Physical Consistency: The Absorption and Scatter- ing Limits . 100 3.4.2 Reproduction of Results of Rugheimer et al. (2015) . 107 3.4.3 Comparison To Modern Earth Surficial Measurements . 110 3.5 Results and Discussion . 117 3.5.1 Action Spectra and UV Dose Rates . 117 3.5.2 Impact of Albedo and Zenith Angle On Surface Radiance & Prebi- otic Chemistry . 123 3.5.3 Impact of Varying Levels of CO2 On Surface Radiance & Prebiotic Chemistry . 128 3.5.4 Alternate shielding gases . 141 3.5.5 CH4 ................................... 143 3.5.6 H2O................................... 145 3.5.7 SO2 ................................... 149 3.5.8 H2S................................... 155 3.5.9 O2 .................................... 161 3.5.10 O3 .................................... 164 3.6 Conclusions . 166 3.7 Appendix A: Extinction And Rayleigh Scattering Cross-Sections . 170 3.7.1 N2 .................................... 170 3.7.2 CO2 ................................... 171 3.7.3 H2O................................... 173 3.7.4 CH4 ................................... 175 vii CONTENTS 3.7.5 SO2 ................................... 176 3.7.6 H2S................................... 178 3.7.7 O2 .................................... 180 3.7.8 O3 .................................... 181 3.8 Appendix B: Spectral Albedos . 183 3.8.1 Ocean . 184 3.8.2 Snow . 185 3.8.3 Desert . 185 3.8.4 Tundra . 186 3.9 Appendix C: Enhancement of Upwelling Intensity in the Discrete Ordinates & Two Stream Approximations . 187 4 Atmospheric Constraints on the Surface UV Environment of Mars at 3.9 Ga Relevant to Prebiotic Chemistry 191 4.1 Introduction . 193 4.2 Background . 196 4.3 Methods . 199 4.3.1 Radiative Transfer Model . 199 4.3.2 Atmospheric Profile . 202 4.3.3 Particulate Optical Parameters . 204 4.3.4 Action Spectra and Calculation of Dose Rates . 207 4.4 Results . 212 4.4.1 Clear-Sky H2O-CO2 Atmospheres . 212 4.4.2 Effect of CO2 and H2O Clouds . 217 4.4.3 Effect of Elevated Levels of Volcanogenic Gases . 221 4.4.4 Effect of Dust . 228 4.5 Discussion . 233 4.5.1 CO2-H2O Atmosphere . 233 viii CONTENTS 4.5.2 Highly Reducing Atmospheres . 242 4.5.3 Highly-Volcanic Mars (CO2-H2O-SO2/H2S Atmosphere) . 243 4.6 Conclusion . 254 4.7 Appendix A: Sample Atmospheric Profiles . 256 5 The Surface UV Environment on Planets Orbiting M-Dwarfs: Implica- tions for Prebiotic Chemistry & Need for Experimental Follow-Up 259 5.1 Introduction . 261 5.2 Background: Previous Studies of M-dwarf Planet UV . 264 5.3 Methods . 266 5.3.1 Radiative Transfer . 266 5.3.2 Atmospheric Model . 267 5.3.3 Stellar Fluxes . 269 5.3.4 Action Spectra and Calculation of Dose Rates . 271 5.4 Results . 276 5.4.1 Steady-State M-dwarf Emission . 276 5.4.2 M-dwarf Flares . 280 5.5 Discussion . 283 5.5.1 Low-UV on M-dwarfs: A Challenge for Abiogenesis? . 283 5.5.2 Possible Mechanisms to Compensate for Low M-dwarf UV . 287 5.5.3 Laboratory Follow-Up . 290 5.6 Conclusions . 291 5.7 Appendix A: Flare Frequency Distribution Calculation . 293 6 Cyanosulfidic Origins of Life Chemistry: Planetary Sources for Reduc- ing Sulfur Compounds 295 6.1 Introduction .
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