One of the key ingredients for life as we know it is phosphorus – and the multitude of lightning strikes that occurred when the Earth was young 4 billion years ago could have unlocked the amount of phosphorus needed to create the foundation of life.
Phosphorus is needed in molecules that form the basic cellular structures and cell membranes and even form the backbone of DNA and RNA phosphate, said Benjamin Hess, author of the study and a graduate student at Yale University in the Department of Earth and Planetary Sciences. .
But this element was evasive on early Earth, trapped in minerals.
“Most of the phosphorus on early Earth was trapped in minerals that are essentially insoluble and non-reactive, which means they could not be used to produce biomolecules needed for life,” Hess said. “Lightning provides a new mechanism for creating phosphorus in a form that can make compounds important to life.”
It has long been believed that meteorites provided the elements needed for life to appear on Earth. Meteorites are known to contain schreibersite, a phosphorus mineral that can be dissolved in water. If quite a few of them had collapsed on Earth, the schreibersite could have provided the right amount of phosphorus.
However, life began between 3.5 and 4.5 billion years ago, and then fewer meteorites had an impact on the Earth.
Schreibersite has another source in the glasses called lightning, also known as the glass that is formed when lightning strikes the ground.
Lightning was found to contain phosphorus released from surface rocks – and is soluble.
“Lightning is a mechanism that does not necessarily diminish over time, as meteorites usually do,” Hess added. “This mechanism can be very important for considering the formation of life on Earth-like planets after the impact of the meteorite has become rare.”
Lightning has also been a topic of interest to scientists when thinking about life on Earth early, as it leads to the creation of gases such as nitrogen oxides, which also played a role in the origin of life.
Hess and his research colleagues used this existing research to investigate and refine the rate of lightning on early Earth.
Today, on Earth, we experience about 560 million lightning strikes a year. On early Earth, this number was between 1 and 5 billion annually, with 100 to 1 billion of those hitting the earth.
Over a billion years, this could have led to 1 quintillion of lightning – and a lot of phosphorus.
Lightning was more widespread on early Earth because there was more carbon dioxide in the atmosphere. Carbon dioxide contributes to global temperature, and a higher global temperature causes more frequent and intense storms, Hess said.
Carbon dioxide levels were raised on Earth early after an object the size of Mars hit Earth to create the moon 4.5 billion years ago. This also released a lot of gases from inside the Earth, such as carbon dioxide – which then became trapped in the Earth’s atmosphere and led to more lightning, Hess said.
“Our proposal for lightning as a significant mechanism for creating reactive phosphorus is important for our understanding of the occurrence of life, because lightning is relatively constant over time,” Hess said.
Understanding the role of lightning as a way to create usable phosphorus has implications for the search for life beyond Earth.
“Our findings are probably applicable to any planet that has a lightning-generating atmosphere. As long as a planet has a significant amount of lightning, it would have a source of phosphorus necessary for life to occur,” Hess said.