The origin of life on Earth is one of the most complex puzzles facing scientists. It involves not only identifying the numerous chemical reactions that must take place to create a replicating organism, but also finding realistic sources for the ingredients needed for each of the reactions. One particular problem that has long faced scientists who study 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 the related molecule RNA.
Though the element was widespread, almost all phosphorus on the early Earth, around four billion years ago, was trapped in minerals that were essentially insoluble and unreactive. This means the phosphorus, while present in principle, was not available to make the compounds needed for life. A glass structure created by a lightning strike called a fulgurite,
In a new paper, researchers show lightning strikes would have provided a widespread source of phosphorus. This means lightning strikes may have helped spark life on Earth, and may be continuing to help life start on other Earth-like planets.
To do this, they needed to estimate three things: the number of fulgurites formed each year; how much phosphorus was in the rocks on early Earth; and how much of that phosphorus is turned into usable phosphorus, by the lightning strikes.
Fulgurites form when lightning strikes the ground, so first they needed to know how much lightning there was. To determine the amount of lightning, they 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. The CO₂ in the atmosphere can be used to estimate global temperature, which is a key factor in controlling the frequency of thunderstorms.
On early Earth, there would have ranged from 100 million to 1 billion lightning strikes a year, with each strike forming one fulgurite. In total, up to 1 quintillion fulgurites would have formed in the first billion years of Earth’s history.
Life on Exoplanets
Research also highlights a new source of the phosphorus needed for life to emerge on other Earth-like planets.
Lightning strikes are a more sustainable source of phosphorus than meteorite impacts. The abundance of large meteorites in a solar system decreases exponentially over time as the leftover material in the system collides with planets.
So, while meteorites provide substantial usable phosphorus for life early in a planet’s history, they decrease fairly rapidly in abundance.