This isn’t the first time that lightning has been suggested as an essential part of what made life on Earth possible. Laboratory experiments have shown that organic material produced by lightning could have included precursor compounds like amino acids (which can combine to form proteins).
This new study approaches the role of lightning in a different way, however. A big question that scientists have always thought about is how early lives on Earth accessed phosphorus. Although there had been plenty of water and carbon dioxide available to work with billions of years ago, the phosphorus was enveloped in insoluble and unreactive rocks. In other words, the phosphorus has been locked away for good.
How did organizations gain access to this essential element? The dominant theory has been that meteorites delivered phosphorus to Earth in the form of a mineral called schreibersite – which can dissolve in water, making it readily available to life forms. The big problem with this idea is that when life began over 3.5 to 4.5 billion years ago, meteorite impacts were decreasing exponentially. The planet needed a lot of schreibersite containing phosphorus to survive. And the meteor impacts would also have been destructive enough to, well, either prematurely kill the nascent life (see: dinosaurs) or vaporize most of the schreibersite delivered.
Hess and his colleagues believe they have found the solution. Schreibersite is also found in glass materials called fulgurites, which form when light hits the Earth. When fulgurite forms, it incorporates phosphorus from terrestrial rocks. And it’s water soluble.
The authors of the new study collected fulgurite that had been produced by lighting hitting the ground in Illinois in 2016, initially just to study the effects of extreme flash heating as preserved in this type. samples. They found that the fulgurite sample was 0.4% schreibersite.
From there, it was simply a matter of calculating how much schreibersite might have been produced by lightning billions of years ago, around the time that first life emerged on Earth. There is an abundant literature estimating ancient levels of atmospheric carbon dioxide, a contributing factor to lightning strikes. Armed with an understanding of the correlation between carbon dioxide trends and lightning strikes, the team used this data to determine how much lightning would have been prevalent at the time.
Hess and his colleagues determined that billions of lightning strikes could have produced 110 to 11,000 kilograms of schreibersite each year. During this period, this activity should have made enough phosphorus available to encourage living organisms to grow and reproduce – and much more than would have been produced by meteorite impacts.
It’s interesting for understanding the history of Earth, but it also opens up a new perspective for thinking about life elsewhere. “It’s a mechanism that can work on planets where meteorite impacts have become rare,” Hess explains. This pattern of life through lightning is limited to environments with shallow water – lightning must produce fulgurite in areas where it can dissolve properly to release phosphorus, but where it will not be lost over a wide area. of water. But this limit is not necessarily a bad thing. In an age when astrobiology is obsessed with ocean worlds, the study focuses on places like Mars that have not been submerged in global waters.
To be clear, the study does not suggest that meteorite impacts play no role in making phosphorus accessible to life. And Hess points out that other mechanisms, like hydrothermal vents, can simply bypass the need for meteorites or lightning.
And finally, over 3.5 billion years ago, the Earth did not look the same as it does today. It is not entirely clear that there were enough rocks exposed to the air – where they could be struck by lightning and lead to the production of schreibersite – to make phosphorus available.
Hess will let other scientists deal with these questions, as the study is outside of his normal job. “But hopefully this will make people pay attention to fulgurites and further test the viability of these mechanisms,” he says. “I hope that our research will help us to think about the advisability of seeking life in shallow environments, as we are now on Mars.”