Environmental Considerations in the Simulation of Early Earth Ultraviolet Conditions for Prebiotic Chemistry Experiments

This study examines the UV environments created in the laboratory in order to simulate prebiotic conditions and explore the chemistry of life’s origins. The majority of studies examining prebiotic chemistry do not provide details of the simulated ultraviolet environment beyond which UV lamp was used. By explicitly determining the ultraviolet spectra, we were able to compare experimental conditions with the spectra of both the modern Sun and a solar analog, \mathnorm{K}-Ceti, representing the Sun during the era of biogenesis. Specifically, this paper examines the UV environments implemented in two experiments simulating prebiotic conditions: one examining ribonucleotide synthesis (Powner et al., 2009) and the other examining the viability of the reduction of oxaloacetate to malate, a step in the Reductive Tricarboxylic Acid (rTCA) Cycle (Guzman and Martin, 2008). These studies demonstrated that reactions could be driven forward through photoelectrochemical pathways, which in the case of the Powner experiment was wavelength dependent. While neither study detailed the ultraviolet environment used, Guzman and Martin noted some literature values, stating that UV fluxes on the Earth 3.5-3.9Ga ago should be 40Wm^{-2} for 315-400nm, 5Wm^{-2} for 200-280nm (Guzman and Martin, 2008). For our study, we explicitly obtained the lamp spectra in order to determin the actual ultraviolet environments employed by these experiments. We were able to obtain spectra for a PenRay low-pressure Hg UV cold cathode lamp, an Ace Glass 450W medium-pressure mercury arc lamp with a quartz immersion well and water-cooling jacket, and a Newport 150W low-pressure Hg(Xe) lamp. Our results found that none of the lapms produced spectra that remotely resembled prebiotic conditions, as each failed to produce a continuum or replicate solar emission features. Furthermore, the flux ratios measured for the 150W and 450W lamps generally differed from those calculated from the literature values. Because the lamps inadequately simulate primordial conditions, better experimental designs are required to ensure realistic prebiotic chemical experiments. The accuracy of the simulation is important because several photochemical reactions involved in prebiotic synthesis are wavelength-dependent, and ultraviolet light has the ability to destroy bonds in key molecules, rendering a potential synthesis invalid (Calvert and Pitts, 1966)