The origin of life on Earth is one of the most complex puzzles facing scientists. This is not only to identify the many chemical reactions that must take place to create a replicating organism, but also to find realistic sources for the ingredients necessary for each of the reactions.
A particular problem faced by longtime scientists studying the origin of life is the source of the elusive element, phosphorus. Phosphorus is an important element for basic cell structures and functions. For example, it forms the backbone of the double helix structure of DNA and associated molecular RNA.
Although the element is widespread, almost all of the phosphorus in early Earth – around 4 billion years ago – was trapped in minerals that were essentially insoluble and unreactive. This means that phosphorus, although present in principle, was not available to make the compounds necessary for life.
In a new article, we show that lightning strikes would have provided a widespread source of phosphorus. This means that lightning strikes may have helped start life on Earth and may continue to help start life on other Earth-like planets.
One potential source of phosphorus on early Earth is the unusual mineral schreibersite, which is found in small amounts in meteorites. Experiments have shown that schreibersite can dissolve in water, creating aqueous phosphorus which can react and form a variety of organic molecules important for life. Examples include nucleotides, the building blocks of DNA and RNA, and phosphocholine, a precursor of lipid molecules that make up the cell membrane.
But there is another potential source of schreibersite. By studying a glass structure created by a lightning strike called fulgurite, we found a substantial amount of the unusual mineral phosphorus inside the glass.
If lightning created a large amount of schreibersite and other reactive phosphorous minerals, lightning could be an alternate source of the reactive phosphorus necessary for life.
To determine if this was the case, we estimated the amount of phosphorus made available by lightning 4.5 billion years ago, when Earth formed, to 3.5 billion years ago, when we have the first fossil evidence of life.
To do this, we had to estimate three things: the number of fulgurites formed each year; how much phosphorus was in rocks on early Earth; and how much of this phosphorus is transformed into usable phosphorus, by lightning strikes.
Fulgurites form when lightning strikes the ground, so first we needed to know how much lightning there was. To determine the amount of lightning, we looked at estimates of the amount of CO₂ in the atmosphere on early Earth and estimates of how much lightning there would be on Earth for different amounts of CO₂. CO₂ in the atmosphere can be used to estimate global temperature, which is a key factor in controlling the frequency of thunderstorms.
We found that on early Earth there would have been between 100 million and 1 billion lightning strikes per year, each impact forming a fulgurite. In total, up to 1 quintillion (a follow-up of 18 zeros) of fulgurites are believed to have formed in the first billion years of Earth’s history.
For the second factor, we know that early Earth would likely have been dominated by rocks similar to the basalts that make up volcanic islands like Hawaii. We used the phosphorus content of some of these preserved rocks over 3.5 billion years old to determine an average phosphorus content.
Finally, we used our fulgurite studies and other published fulgurite studies to estimate how much schreibersite or similar forms of phosphorus would have been made available by lightning.
Combining all these factors, we calculated lightning strikes composed of more than 10,000 kg of phosphorus available for organic reactions each year.
Based on our best knowledge of early Earth, lightning likely provided as much reactive phosphorus as meteorites at the time of the origin of life, around 3.5 billion years ago. Therefore, lightning strikes, as well as meteorite impacts, most likely provided the phosphorus necessary for the emergence of life on Earth.
Life on exoplanets
Our research also highlights a new source of phosphorus necessary for the emergence of life on other Earth-like planets.
Lightning strikes are a more durable source of phosphorus than meteorite impacts. The abundance of large meteorites in a solar system decreases exponentially over time as material remaining in the system collides with planets.
So while meteorites provide a substantial amount of phosphorus that can be used for life early in a planet’s history, their abundance declines quite quickly. Lightning strikes, however, are relatively constant over time.
Our work is helping to expand the conditions under which life can form on other planets in our solar system and beyond. If a planet has an active, lightning-rich atmosphere, the phosphorus necessary for life will be available at all times.
This article by Benjamin Hess, PhD candidate, Earth and Planetary Sciences, Yale University; Jason Harvey, Associate Professor of Geochemistry, University of Leeds, and Sandra Piazolo, Professor of Structural and Tectonic Geology, University of Leedsis have republished from The Conversation under a Creative Commons license. Read the original article.