For life to emerge on Earth 3.5 billion years ago, abundant phosphorus must have existed to generate key biomolecules, including DNA, RNA, and lipids. But most terrestrial phosphorus was locked away in insoluble minerals and unavailable to form organic compounds. Meteorites carrying a mineral called schreibersite likely provided soluble phosphorus. Now Benjamin Hess of Yale University and his colleagues have determined that signifcant additional amounts of schreibersite, and its available phosphorus, could have been created through lightning strikes on the clay-rich soils of early Earth. Their finding confirms a hypothesis about an alternative source of phosphorus.
In 1986 Eric Essene and Daniel Fisher (University of Michigan) discovered that schreibersite exists in glassy rocks, called fulgurites, that form when lightning strikes the ground. Now Hess and his colleagues, using a suite of spectroscopic techniques that include electron backscatter diffraction, have studied fulgurite samples from Illinois and confirmed the presence of phosphide minerals that match the crystal structure of schreibersite. The experiments prove that lightning strikes reduce minerals in certain clayey soils to produce soluble phosphorus, in a form that might have been expected on early Earth.
But how much lightning actually electrified ancient Earth, and how much schreibersite did it produce? To find out, Hess and his colleagues worked backwards from climate mode results to determine the atmospheric composition of Earth 3.5 billion years ago. In particular, they looked at its carbon dioxide content, which is a proxy for lightning frequency. Then they approximated the average phosphorus content in ancient rocks based on comparisons with modern volcanic rocks. Finally, they estimated how much schreibersite would form in a single lightning strike. Together the calculations suggest that trillions of lightning strikes could have produced more than 10 000 kg of available phosphorus each year at around the time of the origin of life.
The researchers concluded that combined with a similar amount of schreibersite from meteorite impacts, lightning strikes could have provided a widespread, available source of phosphorus on early Earth. The finding also highlights a possible source of prebiotic phosphorus on other Earth-like planets. (B. L. Hess, S. Piazolo, J. Harvey, Nat. Commun. 12, 1535, 2021.)