The photochemical evolution of polycyclic aromatic hydrocarbons and nontronite clay on early Earth and Mars

The photochemical evolution of polycyclic aromatic hydrocarbons (PAHs), an abundant form of meteoritic organic carbon, is of great interest to early Earth and Mars origin-of-life studies and current organic molecule detection efforts on Mars. Fe-rich clay environments were abundant on early Earth and Mars, and may have played a role in prebiotic chemistry, catalyzing the breakdown of PAHs and freeing up carbon for subsequent chemical complexification. Current Mars is abundant in clay-rich environments, which are most promising for harboring organic molecules and have comprised the main studied features by the Curiosity rover in search of them. In this work we studied the photocatalytic effects of the Fe-rich clay nontronite on adsorbed PAHs. We tested the effect of ultraviolet radiation on pyrene, fluoranthene, perylene, triphenylene, and coronene adsorbed to nontronite using the spike technique, and in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy in a Mars simulation chamber. We studied the infrared vibrational PAH bands with first order reaction kinetics and observed an extensive decrease of bands of pyrene, fluoranthene, and perylene, accompanied by the formation of PAH cations, while triphenylene and coronene remained preserved. We further analyzed our irradiated samples with nuclear magnetic resonance (NMR). Our study showed certain PAHs to be degraded via the (photo)Fenton mechanism, even under a dry, hypoxic atmosphere. Using solar spectra representative of early Earth, early Mars, and current Mars surface illumination up to 400 nm, the processes occurring in our set up are indicative of the UV-induced photochemistry taking place in Fe-rich clay environments on early Earth and Mars.