Earth Became Safe for Life's First Chemistry 4.33 Billion Years Ago, Impact Model Says
A 3D simulation of asteroid bombardment finds the planet stopped being sterilized around 4.4 billion years ago, and that impact-heated hot springs peaked just as the crust calmed down.
Earth may have become a safe place for the chemistry that led to life about 4.33 billion years ago, only a couple of hundred million years after the planet formed, according to a new study in Nature Communications.
The research was co-led by Oleg Abramov, a senior scientist at the Planetary Science Institute. His team built a three-dimensional computer model of how asteroids, comets and leftover planet-building debris pounded and heated Earth's crust between 4.5 billion and 3.5 billion years ago. The model was tied to real evidence, including the cratering record on the moon and the amounts of certain metals delivered by impacts to Earth's upper mantle.
The early planet was a hostile place. Frequent large impacts heated the crust so much that they repeatedly sterilized it, and the model shows those global sterilizing events continued until about 4.4 billion years ago. "Prebiotic chemistry needs continuously stable temperatures, not just a brief cool interval between impacts," Abramov said. "Before about 4.4 billion years ago, a region that had cooled enough for prebiotic chemistry could still be heated again by a later impact, so the clock kept resetting."
After that point, the simulation begins to show pockets of shallow crust that cooled and were never heated past the danger threshold again. The researchers call them "never-sterilized" regions. They grew as the bombardment eased and made up more than half of the modeled crust by 4.25 billion years ago.
The team was focused on the "RNA World," a proposed early stage of life in which RNA both stored genetic information and did chemical work, before DNA took over. For that to happen, RNA and similar molecules needed time to form and survive, and they break down if they get too hot. The model compared crust temperatures against those limits.
Impacts were not only destructive. The heat they left behind drove water through broken rock and created hydrothermal systems, hot-spring environments with water, heat and chemical energy that are a leading candidate for where life's chemistry began. The simulations show networks of these impact-made systems became especially common around 4.3 billion years ago.
That overlap is what points to 4.33 billion years. It was late enough that stable areas persisted, but early enough that impacts were still generating plenty of hydrothermal activity. "These criteria point to the Earth becoming suitable for an early stage of life between 4.4 billion and 4.3 billion years ago, with optimal conditions at approximately 4.33 billion years ago," Abramov said.
The finding does not say life began then, only that the conditions allowed it. The oldest widely accepted fossil evidence for life is about 3.5 billion years old, and some chemical traces suggest life may be older than that. If the model is right, Earth had a window of close to a billion years before those first signs in which life's chemistry could have been at work.
