Supply of phosphate to early Earth by photogeochemistry after meteoritic weathering

Abstract

During terrestrial differentiation, the relatively small amount of phosphorus that migrated to the lithosphere was incorporated into igneous rock, predominantly in the form of basic calcium orthophosphate (Ca ₁₀(PO ₄) ₆(OH,F,Cl) ₂, apatite). Yet, the highly insoluble nature of calcium apatite presents a significant problem to those contemplating the origin of life given the foundational role of phosphate (PO ₄ ^3-) in extant biology and the apparent requirement for PO ₄ ^3- as a catalyst, buffer and reagent in prebiotic chemistry. Reduced meteorites such as enstatite chondrites are highly enriched in phosphide minerals, and upon reaction with water these minerals can release phosphorus species of various oxidation states. Here, we demonstrate how reduced phosphorus species can be fully oxidized to PO ₄ ^3- simply by the action of ultraviolet light on H ₂S/HS ^-. We used low pressure Hg lamps to simulate UV output from the young Sun and ³¹P NMR spectroscopy to monitor the progress of reactions. Our experimental findings provide a cosmochemically and geochemically plausible means for supply of PO ₄ ^3- that was widely available to prebiotic chemistry and nascent life on early Earth, and potentially on other planets.