In 2016, a family in Illinois thought that a meteorite hit their back yard. They called up the geology department at nearby Wheaton College to say that whatever struck their property had started a small fire and left a weird rock embedded in the scorched dirt.
"Meteorites, contrary to popular belief, are cold when they hit the ground," says Benjamin Hess, who was an undergraduate at the college but is now a graduate student at Yale University. "My professor readily figured out that that was probably a lightning strike."
When lighting strikes sand, soil or stone, it immediately melts the materials into a glassy clump known as a "fulgurite," or lightning rock. When geologists excavated the one in Illinois, they found something unexpected inside—an important ingredient for life that had long been thought to be delivered to the early Earth by meteorites.
A report on the find, in the journal Nature Communications, suggests that this could have been a way for lightning to play a key role in the emergence of life.
Most of the fulgurites that have been studied in the past were collected on beaches or deserts, says Hess, because "it's really easy to see a glass structure sticking out of the sand." One that is buried in the soil, and potentially hidden by random debris or vegetation, is harder to spot, although it might contain different minerals produced when the bolt hit something like clay.
When the researchers dug out the one in Illinois, they first saw glassy bits on the surface. Below that was a thick, tree-root-like structure, extending down about a foot-and-a-half. "It's just entirely made of glass and has, like, burnt soil on the outside of it," says Hess, adding that the object looked like a foggy gray mass with a lot of air holes.
Hess and two colleagues at the University of Leeds analyzed the minerals inside and found one called schreibersite. "Which was very strange," says Hess.
This reactive mineral contains phosphorus, an essential element for life. Phosphorus, "really plays a key role in a lot of the basic cell structures," says Hess. For example, it makes up the backbone of DNA.
Phosphorus was abundant in the early Earth, but geologists know that it was mostly inaccessible, because it was trapped inside non-reactive minerals that don't dissolve easily in water. That led to a mystery: where did the all the phosphorus needed to make biological molecules come from?
One possibility is meteorites, which can contain reactive minerals like schreibersite. When the Earth was forming, and for the first billion years or so afterwards, the planet was pelted with numerous meteorites.
"So people thought, aha! It could actually be an extraterrestrial phosphorous source that provided the reactive phosphorous needed for life to form," says Hess.
But it occurred to the researchers that lighting offered an alternative source of phosphorus for the young Earth, and one that had certain advantages.
After all, meteorite strikes declined in number over time as the solar system got cleared out, and meteorite impacts can also be hugely destructive. "Lightning doesn't destroy an entire 100-kilometer area when it strikes," Hess points out.
The team did a kind of back-of-the-envelope calculation to see if lightning strikes really might have contributed a significant amount of usable phosphorus.
"There are a lot of things to consider," says Hess, "like what was the dominant rock type that was being struck on early Earth? How much land might there have been? What was the atmosphere like? How much lightning would come from that atmosphere? How much phosphorus was in the rock type?"
Data from satellites and other monitors show that these days, there are currently over 500 million flashes of lightning a year, and about a quarter of them strike the ground.
Climate modeling suggested that when the Earth formed, about 4.5 billion years ago, to when life first emerged, about 3.5 billion years ago, there could have been 1 to 5 billion lightning flashes every year.
"Assuming there was a fair amount of land, it's upwards of a billion lightning strikes a year," says Hess.
He and his colleagues believe that around the time life formed, the amount of usable phosphorus created through lighting strikes would be about the same as that provided by meteorites. "There's a large uncertainty there, but basically we found that they are essentially similar," says Hess.
This new idea about lightning is "pretty cool," says Hilairy Hartnett, an astrobiologist at Arizona State University who thinks a lot about phosphorus and its role in the potential for life on other planets.
"All life on Earth requires phosphorus, from the tiniest virus to the largest organisms," she notes.
She thinks this team made a lot of reasonable guesses about whether or not lightning was important for making reactive phosphorus, but it's just hard to know, and it's clear that meteorites did deliver large amounts of the stuff.
Still, "the lightning strikes deliver more than they might have expected," says Hartnett.
So even if meteorite phosphorus was the big deal on early Earth, she says, this means lightning strikes are now a way that planets around other stars might get usable phosphorus, even if they aren't constantly smacked by meteorites.
"It's really nice," she says, "to be able to say there's more than one path to generating phosphorous that could be available to a planet that might be able to develop life."