UC Santa Cruz

Exoplanet science is now focusing on characterizing the physics and chemistry of exoplanet atmospheres, including those of terrestrial-class, potentially habitable planets. In this thesis, I use a combination of laboratory experiments and theoretical modeling to understand two themes related to these atmospheres: (1) their primordial outgassing compositions from an experimental cosmochemistry approach, and (2) the planetary context for observable biosignature gases using modeling tools.

There is no first-principles understanding of how to connect a planet’s bulk composition to its initial atmospheric properties. Since terrestrial exoplanets likely form their atmospheres through outgassing, an important step towards establishing this connection is to assay meteorites, remnants of planetary building blocks, by heating and measuring their outgassed volatiles. In the first theme, I use multiple experimental techniques to determine meteorites’ outgassing compositions over a range of temperatures and pressures. I describe the results of heating carbonaceous chondrite samples and measuring their abundances of released volatiles as a function of temperature in a high-vacuum environment. I find that these meteorites outgas significant amounts of H2O, CO, CO2 and smaller quantities of H2 and H2S. I also discuss a complementary bulk element analysis to monitor outgassing of heavier elements (e.g., sulfur, iron, zinc). I compare these experimental results to thermochemical equilibrium models of chondrite outgassing and determine how these experiments can improve atmospheric models and inform the connection between bulk composition and early atmospheres.

For the second theme, I perform a comprehensive analysis of the necessary planetary conditions for atmospheric methane to be a compelling biosignature gas. Methane is one of the only biosignatures that JWST can readily detect in terrestrial atmospheres. Therefore, it is essential to understand methane biosignatures to contextualize these imminent observations. Using a combination of multiphase thermodynamic and atmospheric chemistry models, I investigate abiotic sources of methane and determine the planetary conditions for which these sources could be enhanced on terrestrial planets so as to result in false positives. I determine that 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, providing the first tentative framework for assessing methane biosignatures.