© Copyright 2019 Steven F. Sholes Geomorphic and Atmospheric Investigations on the Habitability of Past and Present Mars Steven F. Sholes A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2019 Reading Committee: David C. Catling, Chair David R. Montgomery Jonathan D. Toner Program Authorized to Offer Degree: Earth and Space Sciences University of Washington Abstract Geomorphic and Atmospheric Investigations into the Habitability of Past and Present Mars Steven F. Sholes Chair of the Supervisory Committee: David C. Catling, Ph.D. Earth and Space Sciences While the current surface of Mars is viewed to be inhospitable to life as we know it, past Mars may have harbored habitable environments though the extent and duration of such environment is still unclear. There are several requirements to make an environment habitable, which include a liquid solvent (e.g. liquid water), a source of energy (e.g. redox gradients), bioimportant major and trace elements (e.g. CHNOPS), and sustained clement conditions for necessary biochemical reactions to take place (e.g. temperature, pH). This dissertation focuses on better constraining these requirements through atmospheric modeling and quantitative surficial geomorphological investigations. The first half of this dissertation explores the habitability of past and present Mars through the lens of atmospheric redox chemistry. The photochemically produced CO-O2 redox pair in the modern atmosphere produces the second largest atmospheric thermodynamic disequilibrium in the solar system (behind Earth’s atmosphere-ocean system), which represents an untapped source of free energy for potential life to exploit. A rigorous upper limit on the possible extant biomass that can be sustained from this free energy is presented. Volcanic outgassing of reducing gases, e.g. CO and H2, can shift the redox state of the atmosphere, changing the surface conditions towards being reducing and anoxic which are more favorable for the formation of prebiotic chemical compounds (e.g. amino acids). The required levels of volcanism needed to create reducing conditions and potential observables of such environments are also presented here. The latter half of this dissertation focuses on assessing the current state of coastal evidence for past liquid water oceans on Mars. While nearly all aspects of these hypothesized oceans are vigorously debated, availability of large sustained bodies of liquid water would be a boon for constraining the past surface habitability. Presented here is a toolkit developed for quantitatively identifying paleoshorelines using topographic, morphological, and spectroscopic investigations. This toolkit is then applied to 40 individual sites across Mars that have been proposed as ancient ocean shorelines and evaluated along with the general mapped contacts on their consistency with such an origin. None of the putative paleoshoreline sites provided compelling evidence nor consistency with a coastal origin and can all be explained through more conservative geological processes. Together, the chapters in this dissertation provide quantitative means of characterizing contributing aspects of potentially habitable environments on past and present Mars. TABLE OF CONTENTS List of Tables ............................................................................................................................... viii Chapter 1. Introduction ................................................................................................................. 12 1.1 Motivation ..................................................................................................................... 12 1.2 Dissertation Outline ...................................................................................................... 13 Chapter 2. Anoxic Atmospheres on Mars Driven by Volcanism ................................................. 16 2.1 Abstract ......................................................................................................................... 16 2.2 Introduction ................................................................................................................... 17 2.3 Methods......................................................................................................................... 19 2.4 Results ........................................................................................................................... 27 2.5 Discussion ..................................................................................................................... 32 2.5.1 Consequences of Volcanism ..................................................................................... 32 2.5.2 Implications for Habitability ..................................................................................... 34 2.5.3 Potentially Observable Consequences of Anoxia ..................................................... 35 2.6 Conclusions ................................................................................................................... 38 Chapter 3. A Maximum Subsurface Biomass on Mars From Untapped Free Energy .................. 40 3.1 Abstract ......................................................................................................................... 40 3.2 Introduction ................................................................................................................... 41 3.3 Methods......................................................................................................................... 44 3.3.1 Fixed Parameter Model ............................................................................................. 46 3.3.2 Optimized Parameter Model ..................................................................................... 48 3.3.3 Atmospheric Diffusion Model .................................................................................. 50 3.4 Results ........................................................................................................................... 53 3.5 Discussion ..................................................................................................................... 58 3.6 Conclusions ................................................................................................................... 64 Chapter 4. Quantitative High-Resolution Re-Examination of a Hypothesized Ocean Shoreline on Mars .............................................................................................................................................. 66 4.1 Abstract ......................................................................................................................... 66 4.2 Introduction ................................................................................................................... 66 4.2.1 Background Information ........................................................................................... 66 4.2.2 Site Introduction........................................................................................................ 70 4.3 Methods......................................................................................................................... 72 4.3.1 Photogeological Mapping ......................................................................................... 72 4.3.2 Topographic Analysis ............................................................................................... 74 4.3.3 Thermal Inertia.......................................................................................................... 79 i 4.3.4 Spectroscopy ............................................................................................................. 81 4.3.5 Classification............................................................................................................. 82 4.4 Results ........................................................................................................................... 83 4.4.1 Photogeology ............................................................................................................ 83 4.4.2 Topography ............................................................................................................... 87 4.4.3 Thermal Inertia and Spectroscopy ............................................................................ 91 4.5 Discussion ..................................................................................................................... 93 4.5.1 Are These Shorelines? .............................................................................................. 93 4.5.2 Alternative Hypotheses ............................................................................................. 95 4.6 Conclusions ................................................................................................................... 96 Chapter 5. Mars’ Ocean Shorelines: A High-Resolution Global Re-Examination ...................... 98 5.1 Abstract ......................................................................................................................... 98 5.2 Introduction ................................................................................................................... 98 5.3 Methods....................................................................................................................... 103 5.4 Observations ..............................................................................................................