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SignificanceUnderstanding the rise of atmospheric oxygen on Earth is important for assessing precursors to complex life and for evaluating potential future detections of oxygen on exoplanets as signs of extraterrestrial biospheres. However, it is unclear whether Earth’s initial rise of O2 was monotonic or oscillatory, and geologic evidence poorly constrains O2 afterward, during the mid-Proterozoic (1.8 billion to 0.8 billion years ago). Here, we used a time-dependent photochemical model to simulate oxygen’s rise and the stability of subsequent O2 levels to perturbations in supply and loss. Results show that large oxygen fluctuations are possible during the initial rise of O2 and that Mesoproterozoic O2 had to exceed 0.01% volume concentration for atmospheric stability. The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 102 and 105 y. Results also suggest that O2 between ~10−8 and ~10−4 mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O2 concentrations experience fractional variations in the surface CH4 flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic (~10−8) and oxic (~10−4) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O2, after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O2 exceeding 10−4 to 10−3 mixing ratio; otherwise, O2 would periodically fall below 10−7 mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

Chemical disequilibrium quantified using the available free energy has previously been proposed as a potential biosignature. However, researchers remotely sensing exoplanet biosignatures have not yet investigated how observational uncertainties impact the ability to infer a life-generated available free energy. We pair an atmospheric retrieval tool to a thermodynamics model to assess the detectability of chemical disequilibrium signatures of Earth-like exoplanets, focusing on the Proterozoic eon when the atmospheric abundances of oxygen–methane disequilibrium pairs may have been relatively high. Retrieval model studies applied across a range of gas abundances revealed that order-of-magnitude constraints on the disequilibrium energy are achieved with simulated reflected-light observations for the high-abundance scenario and high signal-to-noise ratios (50), whereas weak constraints are found for moderate signal-to-noise ratios (20–30) and medium- to low-abundance cases. Furthermore, the disequilibrium-energy constraints are improved by using the modest thermal information encoded in water vapour opacities at optical and near-infrared wavelengths. These results highlight how remotely detecting chemical disequilibrium biosignatures can be a useful and metabolism-agnostic approach to biosignature detection. Chemical disequilibrium is a known biosignature, and it is important to determine the conditions for its remote detection. A thermodynamical model coupled with atmospheric retrieval shows that a disequilibrium can be inferred for a Proterozoic Earth-like exoplanet in reflected light at a high O 2 /CH 4 abundance case and signal-to-noise ratio of 50.

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10 2 and 10 5 y. Results also suggest that O 2 between ~ 10 − 8 and ~ 10 − 4 mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O 2 concentrations experience fractional variations in the surface CH 4 flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic ( ~ 10 − 8 ) and oxic ( ~ 10 − 4 ) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O 2 , after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O 2 exceeding 10 − 4 to 10 − 3 mixing ratio; otherwise, O 2 would periodically fall below 10 − 7 mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10² and 10⁵ y. Results also suggest that O₂ between ∼10−8 and ∼10−4 mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O₂ concentrations experience fractional variations in the surface CH₄ flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic (∼10−8) and oxic (∼10−4) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O₂, after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid- Proterozoic O₂ exceeding 10−4 to 10−3 mixing ratio; otherwise, O₂ would periodically fall below 10−7 mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

The ancient Earth atmosphere is our only example for how a microbial biosphere impacts planetary atmospheres and is therefore a critical asset to the spectroscopic search for life on exoplanets. Additionally, for a subaerial origin of life, the nature of the earliest Earth atmosphere determines the environmental conditions under which life began. However, our understanding of the early Earth is shrouded by deep time; very few clues to its composition, climate and biosphere have been preserved over billions of years. To complement the sparse geologic record, this thesis uses thermodynamic, photochemical, and climate models to better understand the atmospheres of early Earth to inform the search for life on exoplanets and improve our understanding of the origin of life.In Part I of this dissertation, I investigate atmospheric chemical disequilibrium anti-biosignatures, as well as methane and oxygen biosignatures during the Archean (4.0 - 2.5 Ga) and Proterozoic (2.5 - 0.54 Ga) eons. By modeling the change in Earth's atmospheric composition when life first began, I argue that the disequilibrium coexistence of atmospheric H2 and CO2 or CO and water vapor is an anti-biosignature if observed on an exoplanet because these easily metabolized species should be consumed if life was present. Next, I estimate the likelihood of volcanism on an exoplanet mimicking the CH4+CO2 biosignature characteristic of the Archean Earth. I find that significant volcanic methane is unlikely, but, if possible, could be identified by observations of atmospheric CO because volcanoes that produce CH4 should also make CO. The final Chapter in Part I argues that atmospheric oxygen, Earth's most recognizable biosignature gas, was unstable during the Great Oxidation Event (~ 2.4 Ga). I also set a lower limit on O2 levels during the Proterozoic eon, which improves potential detectability of O2 on an exoplanet if it was like the ancient Earth.Part II explores how Earth's Hadean (4.5 - 4.0 Ga) atmosphere may have influenced the origin of life. Specifically, I use atmospheric models to estimate the HCN and HCCCN produced in the Hadean atmosphere in the wake of large asteroid impacts. Both HCN and HCCCN are critical ingredients in \"RNA world\" origin of life hypotheses. Simulations show that asteroid impacts make transient H2- and CH4-rich atmospheres that persist for millions of years, until hydrogen escapes to space. I find that impacts larger than between 5 x 1020 to 4 x 1021 kg (570 to 1330 km diameter) produce sufficient atmospheric CH4 to cause ample HCN and HCCCN photochemical production and rainout to the surface, while smaller impacts produce negligible amounts of origin-of-life molecules. The second chapter of Part II places these results in the context of Earth's impact history. I estimate when 5 x 1020 to 4 x 1021 kg impacts most likely occurred on the early Earth to shed light on when life began if it did so in an impact-driven scenario.

Chemical disequilibrium quantified via available free energy has previously been proposed as a potential biosignature. However, exoplanet biosignature remote sensing work has not yet investigated how observational uncertainties impact the ability to infer a life-generated available free energy. We pair an atmospheric retrieval tool to a thermodynamics model to assess the detectability of chemical disequilibrium signatures of Earth-like exoplanets, emphasizing the Proterozoic Eon where atmospheric abundances of oxygen-methane disequilibrium pairs may have been relatively high. Retrieval model studies applied across a range of gas abundances revealed that order-of-magnitude constraints on disequilibrium energy are achieved with simulated reflected-light observations at the high abundance scenario and signal-to-noise ratios (50) while weak constraints are found at moderate SNRs (20\\,--\\,30) for med\\,--\\,low abundance cases. Furthermore, the disequilibrium energy constraints are improved by modest thermal information encoded in water vapor opacities at optical and near-infrared wavelengths. These results highlight how remotely detecting chemical disequilibrium biosignatures can be a useful and metabolism-agnostic approach to biosignature detection.

A primary goal of characterizing exoplanet atmospheres is to constrain planetary bulk properties, such as their metallicity, C/O ratio, and intrinsic heat. However, there are significant uncertainties in many aspects of atmospheric physics, such as the strength of vertical mixing. Here we use PICASO and the photochem model to explore how atmospheric chemistry is influenced by planetary properties like metallicity, C/O ratio, \\(T_{\\rm int}\\), \\(T_{\\rm eq}\\), and \\(K_{\\rm zz}\\) in hydrogen-dominated atmospheres. We vary the \\(T_{\\rm eq}\\) of the planets between 400 K- 1600 K, across ``cold\",``warm,\" and `hot\" objects. We also explore an extensive range of \\(T_{\\rm int}\\) values between 30-500 K, representing sub-Neptunes to massive gas giants. We find that gases like CO and CO\\(_2\\) show a drastically different dependence on \\(K_{\\rm zz}\\) and C/O for planets with cold interiors (e.g., sub-Neptunes) compared to planets with hotter interiors (e.g., Jupiter mass planets), for the same \\(T_{\\rm eq}\\). We also find that gases like CS and CS\\(_2\\) can carry a significant portion of the S- inventory in the upper atmosphere near \\(T_{\\rm eq}\\) \\(\\le\\) 600 K, below which SO\\(_2\\) ceases to be abundant. For solar C/O, we show that the CO/CH\\(_4\\) ratio in the upper atmospheres of planets can become \\(\\le\\)1 for planets with low \\(T_{\\rm eq}\\), but only if their interiors are cold ($T_{\\rm int}$$\\le\\(100 K). We find that photochemical haze precursor molecules in the upper atmosphere show very complex dependence on C/O, \\)K_{\\rm zz}\\(, \\)T_{\\rm eq}\\(, and \\)T_{\\rm int}$ for planets with cold interiors (e.g., sub-Neptunes). We also briefly explore fully coupling PICASO and photochem to generate self-consistent radiative-convective-photochemical-equilibrium models.

Theoretical predictions and observational data indicate a class of sub-Neptune exoplanets may have water-rich interiors covered by hydrogen-dominated atmospheres. Provided suitable climate conditions, such planets could host surface liquid oceans. Motivated by recent JWST observations of K2-18 b, we self-consistently model the photochemistry and potential detectability of biogenic sulfur gases in the atmospheres of temperate sub-Neptune waterworlds for the first time. On Earth today, organic sulfur compounds produced by marine biota are rapidly destroyed by photochemical processes before they can accumulate to significant levels. Domagal-Goldman et al. (2011) suggest that detectable biogenic sulfur signatures could emerge in Archean-like atmospheres with higher biological production or low UV flux. In this study, we explore biogenic sulfur across a wide range of biological fluxes and stellar UV environments. Critically, the main photochemical sinks are absent on the nightside of tidally locked planets. To address this, we further perform experiments with a 3D GCM and a 2D photochemical model (VULCAN 2D (Tsai et al. 2024)) to simulate the global distribution of biogenic gases to investigate their terminator concentrations as seen via transmission spectroscopy. Our models indicate that biogenic sulfur gases can rise to potentially detectable levels on hydrogen-rich waterworlds, but only for enhanced global biosulfur flux (\\(\\gtrsim\\)20 times modern Earth's flux). We find that it is challenging to identify DMS at 3.4 \\(\\mu m\\) where it strongly overlaps with CH\\(_4\\), whereas it is more plausible to detect DMS and companion byproducts, ethylene (C\\(_2\\)H\\(_4\\)) and ethane (C\\(_2\\)H\\(_6\\)), in the mid-infrared between 9 and 13 \\(\\mu m\\).

JWST recently measured the transmission spectrum of K2-18b, a habitable-zone sub-Neptune exoplanet, detecting CH\\(_4\\) and CO\\(_2\\) in its atmosphere. The discovery paper argued the data are best explained by a habitable \"Hycean\" world, consisting of a relatively thin H\\(_2\\)-dominated atmosphere overlying a liquid water ocean. Here, we use photochemical and climate models to simulate K2-18b as both a Hycean planet and a gas-rich mini-Neptune with no defined surface. We find that a lifeless Hycean world is hard to reconcile with the JWST observations because photochemistry only supports \\(< 1\\) part-per-million CH\\(_4\\) in such an atmosphere while the data suggest about \\(\\sim 1\\%\\) of the gas is present. Sustaining %-level CH\\(_4\\) on a Hycean K2-18b may require the presence of a methane-producing biosphere, similar to microbial life on Earth \\(\\sim 3\\) billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with \\(100 \\times\\) solar metallicity should have 4% CH\\(_4\\) and nearly 0.1% CO\\(_2\\), which are compatible with the JWST data. The CH\\(_4\\) and CO\\(_2\\) are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model predicts H\\(_2\\)O, NH\\(_3\\) and CO abundances broadly consistent with the non-detections. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H\\(_2\\) escape and potential supercriticality at depth, we favor the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data.

The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts that transiently reduced the entire atmosphere, allowing nitriles to form in subsequent photochemistry. Here, we investigate impact-generated reducing atmospheres using new time-dependent, coupled atmospheric chemistry and climate models, which account for gas-phase reactions and surface-catalysis. The resulting H\\(_2\\)-, CH\\(_4\\)- and NH\\(_3\\)-rich atmospheres persist for millions of years, until hydrogen escapes to space. HCN and HCCCN production and rainout to the surface can reach \\(10^9\\) molecules cm\\(^{-2}\\) s\\(^{-1}\\) in hazy atmospheres with a mole ratio of \\(\\mathrm{CH_4} / \\mathrm{CO_2} > 0.1\\). Smaller \\(\\mathrm{CH_4} / \\mathrm{CO_2}\\) ratios produce HCN rainout rates \\(< 10^5\\) molecules cm\\(^{-2}\\) s\\(^{-1}\\), and negligible HCCCN. The minimum impactor mass that creates atmospheric \\(\\mathrm{CH_4} / \\mathrm{CO_2} > 0.1\\) is \\(4 \\times 10^{20}\\) to \\(5 \\times 10^{21}\\) kg (570 to 1330 km diameter), depending on how efficiently iron reacts with a steam atmosphere, the extent of atmospheric equilibration with an impact-induced melt pond, and the surface area of nickel that catalyzes CH\\(_4\\) production. Alternatively, if steam permeates and deeply oxidizes crust, impactors \\(\\sim 10^{20}\\) kg could be effective. Atmospheres with copious nitriles have \\(> 360\\) K surface temperatures, perhaps posing a challenge for RNA longevity, although cloud albedo can produce cooler climates. Regardless, post-impact cyanide can be stockpiled and used in prebiotic schemes after hydrogen has escaped to space.