Chemistry-Climate Dynamics of Warm Habitable Zone Extrasolar Planets

Chen, Howard

doi:https://doi.org/10.21985/n2-tt42-6f27

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Chemistry-Climate Dynamics of Warm Habitable Zone Extrasolar Planets

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Rocky exoplanets are indicated to be common in the galaxy. Future instruments including the {\it James Webb Space Telescope} (JWST), high resolution ground-based spectrographs, and direct imaging missions for under consideration by the 2020 Decadal Survey on Astronomy and Astrophysics are poised to unlock the atmospheres of habitable zone planets orbiting nearby and distant main-sequence stars. However, retrieval and interpretation of observational measurements will require understanding of possible atmospheric compositions, star-planet environmental context, and their consequences on the habitability and detectability of the planet. To inform future instruments, a range of model complexity, architectures, and frameworks have been used.Recently, emerging terrestrial general circulation model (GCM) results argue that planetary rotation can drive changes in heat redistribution, cloud formation, and circulation regimes, potentially influencing chemical transport and the spatial distribution of gaseous species such as atmospheric biosignatures. Observations of neighborhood stars show evidence for flaring events that deviates substantially from their time-averaged spectra over day-to-week time spans. These flares will likely accompany heightened UV radiation and energetic particle precipitation, effects that should be accounted for in single column and global climate models. These newfound importance of atmospheric dynamics and dayside photochemistry, particularly for slowly-rotating planets orbiting low-mass stars, has been hypothesized to have important ramifications for the cumulative and evolving atmospheric compositions, surface habitability, and remote detectability of molecular signals. In this dissertation, we employ an array of three-dimensional Earth-system models in a series of experiments that seek to self-consistently determine the atmospheric composition and dynamics of extrasolar planets. Specifically, this dissertation utilizes global chemistry-climate models in conjunction with observed data of stellar parameters to explore the habitability and observational prospects of rocky exoplanets orbiting G, K, and M-stars. In Chapter~1, we describe the motivation, background, and approach that serve as the foundation our work.In Chapter~2, we adapt a chemistry-climate model (CCM) to simulate slowly- and synchronously-rotating planets orbiting systems with masses less than the Sun. Using this newly adapted tool, we confined the degree of spatial heterogeneity of key biosignature compounds and find substantial chemical day-night side contrasts for planets with rotation periods of 60 Earth days. In Chapter~3, we employ a high-top version of the CCM to investigate hypothetical oxygen-rich exoplanets around a variety of M-dwarf spectral types. For planets at the inner edge of the habitable zone, we find that chemical-climate feedback driven by stellar forcing lead to thinning of the ozone layer as the model atmospheres move towards more active M-dwarfs or increasingly wetter climates. Further, the difference between these scenarios will likely manifest in observed atmospheric spectra and could be discriminated by instruments aboard the JWST. In Chapter~4, we apply the same CCM to discovered habitable zone planetary climates, but we use stellar spectra and lightcurves with the inclusion of flare activity as inputs. We find secular and order-of-magnitude variations in the global concentrations of habitability-associated gas-phase species (e.g., nitrogen and hydrogen oxides) over weekly to monthly timescales. In Chapter~5, we expand upon the results of Chapter~3 and explore the consequence of different global oxygenation levels from pO2 of 0.1% present atmospheric level (PAL) to 10% PAL on the habitability of dry and moist climates. The slow rise of molecular oxygen in Earth's history is a result of oxygenating photosynthesis on a planetary-scale, and our results from this chapter highlight the importance of 3D modeling in evaluating the effects of (exo)planetary evolution. Finally, in Chapter~6, we develop a volatile accretion model based on N-body planetary accretion simulations to trace the origins and sources of key elements (C, N, H) that make up Earth's hydrosphere and atmosphere

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