Zoe Todd successfully defends thesis "From Astronomy to Chemistry: Origins of the Building Blocks of Life."

April 15, 2020
Zoe Todd

Abstract: The circumstances surrounding the origins of life on Earth are still unknown, though substantial progress has been made recently. In general, the origins of life might have followed the path where first, simple feedstock molecules available in the planetary environment react to form the molecular building blocks of life, which then come together to make chemical polymers and eventually self-replicating systems that could constitute life. This thesis addresses the first part of this postulated process, attempting to answer the questions: 1) what planetary sources exist for various feedstock molecules and under what conditions do they provide relevant quantities for prebiotic chemistry, and 2) how can such molecules be used to make the building blocks of life, and in particular, what is the potential role of UV light in this process? 

 

We examine both impacts and the atmosphere as potential sources of feedstock molecules on a planet. Specifically, we look at the possibility of delivery of HCN from impacts of comets on the early Earth and assess which conditions and circumstances might allow for sufficient levels of this molecule to drive prebiotic chemistry. We find that on a global scale, HCN delivered from cometary impacts is not likely to be relevant for prebiotic chemistry; however, on a local scale, cometary delivered HCN could exist in relevant concentrations for up to millions of years. We then examine the atmosphere as a source of sulfur- and nitrogen-containing compounds in the form of sulfidic anions (HS^-, HSO_3^-, and SO_3^{2-}) and NO_x^- ions in order to constrain their possible concentrations in the planetary environment for origins of life chemistry. We find that SO_2 derived anions (HSO_3^-, and SO_3^{2-}) are more globally available at prebiotically-relevant concentrations than H_2S derived anions (HS^-), which suggests that SO_2 derived ions are more relevant feedstock molecules for prebiotic chemistry experiments. We also find that NO_x^- levels may have been lower than previously reported on the early Earth due to various degradation pathways that had not been considered. 

 

We then transition to asking how UV light can influence the prebiotic chemistry invoking these feedstock molecules, continuing to consider the overall planetary environment. UV light would have been more ubiquitous on the early Earth, given the lack of oxygen species in the atmosphere. Furthermore, UV photons provide enough energy to potentially make or break chemical bonds. In particular, we examine how prebiotic photochemical reactions proposed in the past under narrowband UV emission would behave under the UV-environment present on the early Earth to assess the plausibility of such reactions. We find that two specific photochemical reactions of potential prebiotic relevance - the generation of simple sugars from photoreduction of cyanocuprates and hydrogen cyanide and the photochemically-driven conversion of ribocytidine into ribouridine - should function under the UV-environment available on the early Earth. While UV light could potentially drive various prebiotic chemical reactions, certain molecules are also damaged by UV light. We assess the consistency of proposed prebiotic pathways by examining the UV-driven photodegradation of three key molecules in the 2-aminoazole family to place constraints on the environment under which such chemistry could function consistently. We find that the lifetimes of these molecules under the UV light expected on the early Earth range from 7-100 hours, setting limits on how fast these molecules must be used or produced. Finally, we look at the potential protection mechanisms for molecules that may be unstable to UV light, with the hope of constraining the environments and circumstances for a consistent prebiotic chemistry to develop and function. We find that the least stable 2-aminoazole from the previous set of experiments can have its lifetime extended by being present at higher concentrations (self-shielding) or being co-irradiated in the presence of other UV-absorbing molecules, specifically, nucleosides. 

Zoe was advised by Dimitar Sasselov.  She will be start a Sagan Post-doctoral Fellowship at University of Washington in the Fall

Abstract: In this work, we invoke an interdisciplinary approach integrating various aspects of astronomy and chemistry. The ultimate goal is to use the planetary environment to better inform both the astronomical and chemical conditions and circumstances surrounding the origins of life. 

See also: Fellowships, 2020, April