The UV Environment for Prebiotic Chemistry: Connecting Origin-of-Life Scenarios to Planetary Environments

Abstract

Recent laboratory studies of prebiotic chemistry (chemistry relevant to the origin of life) are revolutionizing our understanding of the origin of life (abiogenesis) on Earth just as telescopes capable of searching for life elsewhere are coming online. This thesis sits at the intersection of these revolutions. I examine prebiotic chemical pathways postulated to be relevant to the origin of life and identify the environmental conditions they require to function. I compare these environmental requirements to what was available on Earth and other planets, and use the comparison to 1) improve studies of the origin of life on Earth and 2) explore the implications for the inhabitability of other worlds. Multiple lines of evidence suggest UV light may have played a critical role in the synthesis of molecules relevant to abiogenesis (prebiotic chemistry), such as RNA. I show that UV light interacts with prebiotic chemistry in ways that may be sensitive to the spectral shape and overall amplitude of irradiation. I use radiative transfer models to constrain the UV environment on early Earth (3.9 Ga). I find that the surface UV is insensitive to much of the considerable uncertainty in the atmospheric state, enabling me to constrain the UV environment for prebiotic chemistry on early Earth. Some authors have suggested Mars as a venue for prebiotic chemistry. Therefore, I explore plausible UV spectral fluences on Mars at 3.9 Ga. I find that the early Martian UV environment is comparable to Earth’s under conventional assumptions about the atmosphere. However, if the atmosphere was dusty or SO2 levels were high, UV fluence would have been strongly suppressed. Intriguingly, despite overall attenuation of UV fluence, SO2 preferentially attenuates destructive FUV radiation over prebiotically-useful NUV radiation, meaning high-SO2 epochs may have been more clement for the origin of life. Better measurements of the spectral dependence of prebiotic photoprocesses are required to constrain this hypothesis. Finally, I calculate the UV fluence on planets orbiting M-dwarfs. I find that UV irradiation on such planets is low compared to Earth. Laboratory studies are required to understand whether prebiotic processes that worked on Earth can function on low-UV M-dwarf planets. In addition to UV light, the most promising pathways for the prebiotic synthesis of RNA require reduced sulfidic anions. I show that prebiotically-relevant levels of such anions derived from volcanically-outgassed SO2 should be robustly available on early Earth, and that episodes of high volcanism may be especially clement for these prebiotic pathways. However, H2S-derived anions are much less common, and prebiotic chemistry which invokes them must rely on alternate, localized sources. My work 1) provides initial conditions for laboratory studies of prebiotic chemistry, 2) constrains the inhabitability of Mars and planets orbiting M-dwarfs, and 3) demonstrates the need for laboratory studies to characterize the sensitivity of putative prebiotic chemistry to environmental conditions, e.g the spectral shape and amplitude of UV irradiation. All software associated with these studies, including models and data inputs, are publicly available for validation and extension.