Even lightning has a fossil record. The brief flashes of electrical current in the sky are ephemeral events, here and gone in a flash, but sometimes lightning strikes the ground under such conditions that the discharge creates a root-like system of melted soil. The branching series of tubes is known to geologists as fulgurite, and this special form of mineraloid, a mineral-like object that doesn’t form crystals, might offer a critical clue about the history of an essential element for life.
All life needs phosphorus. This chemical element, notes Yale University geologist Benjamin Hess says, is used in biomolecules such as DNA, RNA and the fats that make up cell membranes. This ubiquitous need for phosphorus has led experts to hypothesize that an abundance of phosphorus must have been important to the emergence of Earth’s earliest organisms, but the question has been where life got its supply.
Up until now, it seemed that Earth might have been supplied with the necessary phosphorus by some meteorites, which carry a great deal of schreibersite—a mineral that produces a glut of phosphorus when wet. Lab experiments have shown how phosphorus from this source can form some of the basic organic molecules that life relies on. But now Hess and colleagues have suggested a different source for the schreibersite necessary for life to emerge. According to their research, published today in Nature Communications, the mineral might not have come from meteorites, but from lightning strikes.
“This project began with a lucky lightning strike,” Hess says. While an undergraduate at Wheaton College in Illinois, Hess recalls, lightning struck someone’s property in the nearby town of Glen Ellyn. By chance, this hit created a nest of fulgurite, which the property owners donated to Wheaton College for study. It was a great opportunity for Hess. Fulgurite samples aren’t exactly uncommon, but they also haven’t been well-studied. “They are still fairly mysterious,” Hess says.
Eventually, while at the University of Leeds in England, Hess set off to see what kind of mineraloid structure lightning creates when it turns soil to fulgurite. With the help of University of Leeds researchers Jason Harvey and Sandra Piazolo, Hess was able to get the fulgurite under a scanning electron microscope. Under high magnification, the scientists saw strange spherules within the fulgurite and wondered what they might be. The team found that these little dots were Fe3P, or schreibersite.
“Once I realized the role schreibersite is thought to play in the origins of life, the focus of our research completely shifted,” Hess says. Drawing from what’s known about early Earth and the requirements of life, the researchers came up with an adjustment to the meteorite hypothesis. “We show that lightning strikes could actually have formed a huge amount of schreibersite,’ Hess says, “comparable to the amount provided by meteorites on early Earth.”
Life on Earth, so far as researchers have been able to discern, originated more than 3.5 billion years ago. That’s a difficult time to study, with rocks that old being relatively rare on Earth’s surface. Nevertheless, from what experts estimated about the distant past from where lightning strikes today, Hess and colleagues propose that there were anywhere between one billion and five billion lightning flashes on the early Earth per year, resulting in a higher number of cloud-to-ground lightning strikes than scientists count on Earth today. Some of these strikes would have produced fulgurite, which would then be broken down to release phosphorus into the environment, providing a huge amount of the element for early life. “The source of bioavailable phosphorus on Earth and other Earth-like planets need not necessary be tied to meteorites,” Hess says.
“The authors provide a convincing case that lightning strikes are significant to the contribution of schreibersite on the early Earth,” says NASA Cosmic Ice Laboratory geologist Danna Qasim, who was not involved in the study. The discovery doesn’t mean that ancient meteorites weren’t important, however. Schreibersite from meteorites typically contains the metal nickel, Qasim notes, which seems to be important in making the schreibersite reactive and able to have its phosphorus start to form organic molecules.
Hess’s new hypothesis is only part of a greater picture. These lightning-produced rocks have been part of Earth’s history for billions of years, and almost certainly hold more clues about the origins of life and its early evolution, their branching shapes offering a flash of inspiration to scientists. “As a geologist, I’ve examined many rocks,” Hess says, “and I’ve never seen any minerals like those found in the fulgurite.”