Scientists decades-old theory origin faces comet test
Computer simulations challenge the idea that comets delivered life-starting molecules to early Earth. Researchers modeled the impacts 100,000 times and found…
Scientists decades-old theory origin is under new scrutiny after computer simulations found that comet impacts may have destroyed, rather than delivered, fragile molecules needed for life on early Earth. The study, published Wednesday in Scientific Reports, modeled a period of heavy bombardment more than 4 billion years ago.
The finding challenges the idea that comets helped supply the chemical ingredients for life. Researchers ran the models 100,000 times, and most simulations did not produce a successful impact that could leave those molecules intact on Earth.
What did the comet simulations find?
In 54% of the simulations, the comets did not hit Earth at all. The remaining 46% included at least one impact, while a tiny minority produced as many as three collisions, lead study author Richard Anslow said.
Even when a comet struck, its speed was a problem. The impact could generate high temperatures and pressures that destroy organic compounds, the study authors said. An ocean impact could also leave chemicals diluted quickly in water.
“Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts,” Anslow said. He conducted the research while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom.
That is a sharp challenge to a theory dating back to at least 1961. Comets consist of ice and dust but also contain organic material, earning them the nickname “dirty snowballs.” Scientists had proposed that these objects could have delivered water and life-related molecules to Earth.
Why hydrogen cyanide matters
The study focused on hydrogen cyanide, or HCN, a compound detected in dozens of comets, including Halley’s and Hale-Bopp. HCN contains hydrogen, carbon and nitrogen; when combined with water, it can contribute to the formation of amino acids, the molecules that make up proteins.
Life requires complex organic molecules, but scientists do not know whether early Earth had enough raw material for prebiotic chemistry: the reactions that form life’s building blocks. Comets appeared to offer one possible supply. The new simulations suggest their contents may not have arrived in a usable form.
“The typical velocities of these comets when they impact the Earth meant that actually very few, if any, would have been able to deliver intact organic molecules, so comets actually don’t seem to be as useful for prebiotic chemistry as was previously thought,” Anslow said.
The result does not rule out every successful collision. Anslow said a rare impact could have worked, but it would have needed to occur in a “perfect location” with liquid water and other chemicals already present. The study authors also argue that even a successful comet strike may not have supplied HCN at a useful concentration.
The broader proposal that ingredients for life arrived from space is known as panspermia and was scientifically postulated in 1908. The new work addresses the narrower claim about comets and early Earth. It also notes that prebiotic chemistry needs other precursor molecules, including compounds containing phosphorus and sulfur, which are not routinely found in comets.
“We can’t rule out confidently that there was never a successful impact — there could have been one of these fluke impacts,” Anslow said.
