DIX Planetary Science Seminar

The nature of the earliest Earth atmosphere determines the environmental conditions under which life began. However, our understanding of the early Earth is shrouded in deep time; few clues to its atmospheric composition and climate have been preserved in the geologic record. In the same vein, our knowledge of distant exoplanet atmospheres depends on limited and noisy telescopic observations that are often challenging to decipher. In this presentation, we supplement these sparse data with atmospheric photochemical and climate models to constrain the atmospheres of early Earth and exoplanets and consequently improve our understanding of life's beginnings, and to search for distant habitable worlds. First, we use models to estimate the atmospheric production of key prebiotic molecules, like HCN, in the wake of massive asteroid impacts on Earth ~4 billion years ago. We find that impacts larger than ~600 km in diameter, in an optimistic case, results in ample prebiotic molecule production while smaller impacts make negligible amounts of origin-of-life molecules. Second, we use the same photochemical and climate models to interpret recent James Webb Space Telescope (JWST) observations of K2-18b, a habitable-zone sub-Neptune exoplanet. These observations revealed CH4 and CO2 in a H2-rich atmosphere, which some researchers have interpreted as a sign that the planet has a habitable liquid water ocean. Overall, our simulations find that a gas-rich mini-Neptune with no habitable surface is a better explanation of the data, because photochemistry prohibits simultaneous CO2 and CH4 in a habitable and lifeless scenario.