Computer Simulations Suggest Building Blocks of Life Didn't Arrive on Prehistoric Comets

CNN reports: The popular theory that comets were crucial in kick-starting life on Earth billions of years ago may not hold up, according to new computer simulations that counter decades of scientific thinking. The finding suggests the fragile organic molecules supposedly carried by the icy objects would have been destroyed on impact, upending an established origin of life theory that dates back to at least 1961 [with earlier versions postulated in 1908]... Early in its history — over 4 billion years ago — Earth may have experienced a period of heavy bombardment from more than 100,000 comets larger than 100 meters (328 feet), according to the new study, published Wednesday in the journal Scientific Reports. Those impacts, however, were mostly too heavy for life-starting molecules to withstand. The team used the latest models of comet formation and evolution to simulate this early planetary period 100,000 times. "Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts," said lead study author Richard Anslow, who conducted the research while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom... "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. More precisely, 54% of the simulations showed no impact at all, while the remaining 46% had at least one and a "tiny minority" returned up to three collisions, Anslow said. "We can't rule out confidently that there was never a successful impact — there could have been one of these fluke impacts." However, such flukes had to occur in a "perfect location" that already met several important conditions, including the presence of liquid water and other chemicals needed to support life. That combination makes the overall process unlikely, he added. The article notes his argument that even if a successful impact occurred, it's unlikely an important chemical like hydrogen cyanide (HCN) could survive the impact, and would've dissolved in an ocean impact. Plus, "many other precursor molecules are also needed, such as compounds that contain phosphorus and sulfur, but these are not routinely found in comets, Anslow said." An alternative cosmic source of organic material could have been asteroids, which typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings on samples from asteroids Bennu and Ryugu, which contain the majority of protein-building amino acids. The recent study also raises another intriguing possibility: While Earth may not have had the ideal conditions to foster HCN from comet impacts, ancient Mars might have been better suited. "Because Mars is much smaller than Earth and further away from the sun, the typical impact velocity of comets is much lower, so while these impacts on Earth were destroying most of the HCN, this wasn't the case so much for Mars," Anslow said. This idea could add another dimension to the long-standing question of whether the red planet once supported life. While the authors closely analyzed how HCN could have survived comet impact, the chemical may have undergone reactions before the icy bodies entered Earth's atmosphere, said Queenie Chan, an associate professor of Earth sciences at Royal Holloway University of London, who was not involved with the study. This process could have created biologically important compounds, such as those protein building blocks, that are less volatile and more likely to survive.

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