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Exoplanet exploration has revealed that many—perhaps most—terrestrial exoplanets formed with substantial H 2 -rich envelopes, seemingly in contrast to solar system terrestrials, for which there is scant evidence of long-lived primary atmospheres. It is not known how a long-lived primary atmosphere might affect the subsequent habitability prospects of terrestrial exoplanets. Here, we present a new, self-consistent evolutionary model of the transition from primary to secondary atmospheres. The model incorporates all Fe-C-O-H-bearing species and simulates magma ocean solidification, radiative-convective climate, thermal escape, and mantle redox evolution. For our illustrative example TRAPPIST-1, our model strongly favors atmosphere retention for the habitable zone planet TRAPPIST-1e. In contrast, the same model predicts a comparatively thin atmosphere for the Venus-analog TRAPPIST-1b, which would be vulnerable to complete erosion via non-thermal escape and is consistent with JWST observations. More broadly, we conclude that the erosion of primary atmospheres typically does not preclude surface habitability, and frequently results in large surface water inventories due to the reduction of FeO by H 2 . Many rocky planets formed with large, H2-rich atmospheres. Here, the authors show that the loss of these primary atmospheres from temperate planets such as TRAPPIST-1e typically leaves behind secondary atmospheres and habitable surface conditions.

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH₄ and CO₂ with limited CO due to the strong redox disequilibrium between CH₄ and CO₂. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH₄ that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO₂ with comparatively little CO.

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.

Oxygen is a promising exoplanet biosignature due to the evolutionary advantage conferred by harnessing starlight for photosynthesis, and the apparent low likelihood of maintaining oxygen‐rich atmospheres without life. Hypothetical scenarios have been proposed for non‐biological oxygen accumulation on planets around late M‐dwarfs, where the extended pre‐main sequence may favor abiotic O2 accumulation. In contrast, abiotic oxygen accumulation on planets around F, G, and K‐type stars is seemingly less likely, provided they possess substantial non‐condensable gas inventories. The comparative robustness of oxygen biosignatures around larger stars has motivated plans for next‐generation telescopes capable of oxygen detection on planets around sun‐like stars. However, the general tendency of terrestrial planets to develop oxygen‐rich atmospheres across a broad range of initial conditions and evolutionary scenarios has not been explored. Here, we use a coupled thermal‐geochemical‐climate model of terrestrial planet evolution to illustrate three scenarios whereby significant abiotic oxygen can accumulate around sun‐like stars, even when significant noncondensable gas inventories are present. For Earth‐mass planets, we find abiotic oxygen can accumulate to modern levels if (1) the CO2:H2O ratio of the initial volatile inventory is high, (2) the initial water inventory exceeds ∼50 Earth oceans, or (3) the initial water inventory is very low (<0.3 Earth oceans). Fortunately, these three abiotic oxygen scenarios could be distinguished from biological oxygen with observations of other atmospheric constituents or characterizing the planetary surface. This highlights the need for broadly capable next‐generation telescopes that are equipped to constrain surface water inventories via time‐resolved photometry and search for temporal biosignatures or disequilibrium combination biosignatures to assess whether oxygen is biogenic. Plain Language Summary Next‐generation telescopes will search for life on exoplanets by looking for the spectral signatures of biogenic gases. Oxygen has been considered a reliable biosignature gas, especially for planets around sun‐like stars where non‐biological, photochemical production is unlikely. This motivates plans for future telescopes specifically designed for oxygen detection. Here, we develop a coupled model of the atmosphere‐interior evolution of terrestrial planets to show that lifeless planets in the habitable zone could develop oxygen‐rich atmospheres relatively easily. These false positives for biological oxygen could be distinguished from inhabited planets using other contextual clues, but their existence implies next‐generation telescopes need to be capable of characterizing planetary environments and searching for multiple lines of evidence for life, not merely oxygen. Key Points Terrestrial planets in the habitable zone of sun‐like stars may accumulate O2‐rich atmospheres without life This O2 accumulation requires initial volatile inventories that are either much larger or smaller than that of Earth, or a high C/H ratio Broadly capable next‐generation telescopes are required to discriminate biological oxygen from these false‐positive scenarios

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.

JWST collects vast amounts of information about exoplanets light years away from Earth. Back home, the measured optical constants of laboratory aerosols are critically input parameters in models to interpret the observational results.

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.