Lightning strikes as a major facilitator of prebiotic phosphorus reduction on early Earth

The origin of life on Earth is a central question in science, with evidence suggesting life existed as early as 3.8-4.1 billion years ago. Phosphorus is crucial for life, a component of DNA, RNA, phospholipids, and ATP. While phosphate (PO43-), the oxidized form of phosphorus, is abundant on Earth, it's largely insoluble in apatite minerals. Reduced phosphorus, such as phosphide (P0) in schreibersite, is highly reactive when wetted, forming hydrous phosphate capable of creating essential organic molecules. Schreibersite is found in meteorites and fulgurites (glassy structures formed by lightning strikes). While meteorites have been considered the main source of prebiotic phosphorus, this study explores the potential contribution of lightning strikes.

Previous research has established schreibersite's role in prebiotic phosphorylation. Studies have shown its reactivity in forming key organic molecules and intermediate phosphorus species (hypophosphite and phosphite). Meteorites are a known source of schreibersite, and their flux to early Earth has been estimated. However, the contribution of other sources, such as fulgurites, has been considered insignificant. Pasek and colleagues have investigated the formation of reduced phosphorus species in fulgurites, highlighting the role of reducing agents like graphitic carbon and the temperatures achieved during lightning strikes. Ritson et al. (2020) focused on the supply of phosphate through photogeochemistry following meteoritic weathering.

This study analyzes a dm-scale type II fulgurite formed in clay-rich soil. The fulgurite was analyzed using various techniques: Raman spectroscopy (to identify mineral composition, specifically SiC and graphitic carbon), X-ray fluorescence (XRF) and diffraction (XRD) (for elemental abundances and mineral identification), electron dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) (for detailed mineralogical analysis of spherules). The researchers compared the phosphorus content of the fulgurite to that of the parent soil to estimate the amount of phosphorus reduced to phosphide. They also considered the formation conditions of the fulgurite, considering temperature and redox conditions to explain the formation of schreibersite and other reduced phosphorus species. The study then created a model to estimate the annual amount of phosphorus reduced by lightning strikes on early Earth, considering factors such as lightning frequency (influenced by pCO2 levels), the proportion of lightning strikes occurring over land, and the percentage of phosphorus reduced to phosphide and other species in fulgurites. Finally, the researchers compared the amount of reduced phosphorus from lightning strikes to the estimated annual meteoritic phosphorus flux.

The analysis of the fulgurite revealed abundant schreibersite spherules within the matrix. The fulgurite's formation involved temperatures exceeding 2000 K and highly reducing conditions due to the presence of graphitic carbon acting as a reducing agent. The difference in phosphorus content between the soil and fulgurite suggested significant phosphorus reduction during fulgurite formation. The model estimates indicate that lightning strikes could have produced between 10 and 1000 kg of phosphide and 100 to 10,000 kg of phosphite and hypophosphite annually during the Hadean and early Archean. These estimates consider various parameters such as pCO2 levels and lightning strike characteristics. The study also found that clay fulgurites are most likely to contain schreibersite due to their higher organic carbon content. Comparison with meteoritic phosphorus flux suggests that lightning-based phosphorus reduction likely surpassed meteorite influx after ~3.5 Ga. The reduced phosphorus from lightning strikes would likely have been heterogeneously distributed, concentrated on tropical landmasses.

The findings suggest that lightning strikes were a significant source of prebiotic reactive phosphorus on early Earth, potentially exceeding the contribution from meteorites after ~3.5 Ga. This continuous, relatively non-destructive source of reduced phosphorus could have played a crucial role in the emergence of life. The heterogeneous distribution of this phosphorus, concentrated on tropical landmasses, aligns with proposed early life environments. Unlike catastrophic meteorite impacts, lightning strikes provide a consistent and localized source of reactive phosphorus, potentially supporting the development of complex prebiotic molecules. The model's assumptions about early Earth conditions (significant exposed landmass and reactive hydrosphere) are consistent with existing early Earth models.

This study demonstrates that lightning strikes on early Earth could have been a major source of prebiotic reactive phosphorus, possibly surpassing the contribution from meteorites after ~3.5 billion years ago. This continuous and relatively localized supply of reduced phosphorus could have played a significant role in the emergence of life. Furthermore, the mechanism of lightning-induced phosphorus reduction offers a potential pathway for the emergence of life on other Earth-like planets with similar atmospheric and geological conditions.

The study's model relies on various assumptions about early Earth conditions, including the extent of exposed landmass, atmospheric composition, and the efficiency of phosphorus reduction in fulgurites. While the model attempts to account for uncertainties using ranges of values, the exact amounts of reduced phosphorus produced by lightning strikes remain an estimate. The study focuses on one fulgurite; a broader range of fulgurite samples would further strengthen the conclusions. Finally, the fate and transport of reduced phosphorus species after their formation in fulgurites require further investigation.