In 2016, lightning struck the backyard of an Illinois family, vaporizing the clay-rich soil underground. It cooled to form what looked like a half-meter-tall, gnarled tree trunk with a glassy surface and a crust of charred soil.
“It’s pretty cool looking,” said Benjamin Hess, a Ph.D. student at Yale University in the Department of Earth and Planetary Sciences. At the time, Hess was an undergraduate studying geology at Wheaton College, to which the family donated the fulgurite (material formed when lightning strikes ground) from the strike.
The fulgurite contained an unusual mineral, schreibersite. Schreibersite is a phosphide, a form of reduced phosphorus containing phosphorus and a metal (in this case, iron-nickel). “It’s typically found in meteorites,” explained Hess. In a recent study published in Nature Communications, Hess and geologists from the University of Leeds wrote that if lightning strikes can create schreibersite, in an early Earth rich with electrical storms, lightning could have provided much of the phosphorus needed to create life.
A Shock to the System
Phosphates, phosphorus atoms bonded to four oxygen atoms, are key ingredients needed for life. “It’s in our DNA, our RNA, and it’s part of our cellular membranes. Without phosphate, life as we know it couldn’t work properly,” said Albert Fahrenbach, a lecturer in the School of Chemistry at the University of New South Wales. Fahrenbach was not involved in the new study.
Phosphates are plentiful in minerals on Earth, but their phosphorus is locked into those minerals and not a good source for prebiotic chemistry. Phosphides like schreibersite, however, are more reactive. “When water interacts with [schreibersite]…it plucks the phosphorus out of the schreibersite and dissolves it in the water,” said Hess. The phosphorus is then available for chemical reactions needed to make organic molecules.
Scientists think that meteorites carried most of the reactive phosphorus to Earth during the Hadean-Archean. Like meteorites, lightning was common on early Earth—the Hadean-Archean had an estimated 1–5 billion flashes per year, compared with the 560 million annual flashes today. If some of these lightning strikes reached the ground to produce schreibersite, they could have been a rich source of phosphorus, too.
Billions of Lightning Strikes
To see whether lightning strikes could be a prominent source of reduced phosphorus, Hess and his coauthors looked inside the fulgurite from the Illinois backyard. Using spectroscopy analyses, X-ray fluorescence and diffraction, and electron backscatter diffraction analysis, they saw that the 25-kilogram fulgurite structure was mostly silica glass but also contained between 0.06 and 0.17 kilogram of schreibersite.
Using models of early Earth conditions, the group then estimated how much reduced phosphorus could have formed from lightning strikes during the Hadean and early Archean about 4 billion years ago, and compared this with the amount of reduced phosphorus from meteorites. (Scientists are still uncertain about how much phosphide could have survived meteorite impacts, researchers acknowledged.) The group saw that cloud-to-ground lightning strikes could have provided a more consistent source of phosphides compared with meteorites, yielding between roughly 100 and 10,000 kilograms of reduced phosphorus annually.
“We found that lightning provided a similar amount of phosphorus [to what] was needed to help life form on Earth around the time that life formed, which is about 3.5 billion years ago,” said Hess. Although meteorites would have provided more reduced phosphorus initially, by 3.5 billion years ago, meteorite impacts were becoming less common. Lightning strikes may have taken the lead in providing a source of reduced phosphorus.
The Big Question Mark
“I think it’s reasonable, but I still think that meteorites would be a larger source,” said Matthew Pasek, a professor at the University of South Florida. Pasek wasn’t involved in this work but did conduct earlier studies showing that in water, schreibersite can release phosphorus that could be incorporated into organic molecules and that lightning strikes could be a source of reduced phosphorus.
Hess’s findings show how phosphorus could have become available for prebiotic chemistry, “but just because you know how to access phosphorus, for example, doesn’t mean you know how to get it into a functional cell,” said Fahrenbach.
“This abiotic-biotic transition is where the big question mark is,” said Betül Kaçar, an assistant professor in molecular and cellular biology and in astronomy at the University of Arizona. Kaçar was not involved in the new study.
Aside from uncovering another clue to how early Earth chemistry may have led to life, the study’s authors also proposed that lightning strikes could provide reduced phosphorus on other, Earth-like planets.
“Only by understanding early life, the early environment, and how the first life on our planet happened to be will we be able to find life elsewhere,” said Kaçar.
—Jackie Rocheleau (@JackieRocheleau), Science Writer