A popular theory that comets helped start life on Earth billions of years ago faces a significant challenge from new computer simulations. The research suggests fragile organic molecules carried by the icy objects would usually have been destroyed during impact.

The findings challenge an origin of life theory dating to at least 1961. The broader idea that ingredients for life arrived from space, known as panspermia, is older and was scientifically proposed in 1908.

Life requires complex organic molecules, but scientists remain uncertain whether early Earth held enough raw materials for prebiotic chemistry, the reactions that create life's building blocks. Comets contain dust, ice and organic material, prompting suggestions that they delivered water and organic molecules to Earth's surface.

More than 4 billion years ago, Earth may have endured heavy bombardment from over 100,000 comets larger than 100 meters, or 328 feet. According to the study, published Wednesday in Scientific Reports, most of those collisions were too violent for molecules associated with life's beginnings to survive.

Researchers used recent models of comet formation and evolution to simulate that early period 100,000 times. Lead study author Richard Anslow conducted the work while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom.

“Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts,” Anslow said. Although the comets contained large concentrations of important molecules, the simulations indicated that they approached Earth too quickly to deposit them intact.

More precisely, 54% of the simulations showed no impact, while the remaining 46% produced at least one. A tiny minority returned as many as three collisions, and Anslow said the researchers could not confidently rule out a rare successful impact.

Such an impact would have needed to occur in a perfect location that already had liquid water and other chemicals necessary to support life, Anslow said. The requirement for those conditions to come together made the overall process unlikely.

The study centered on hydrogen cyanide, or HCN, a naturally occurring chemical and important precursor in prebiotic chemistry. Scientists have detected it in dozens of comets, including Halley’s and Hale-Bopp.

HCN consists of hydrogen, carbon and nitrogen. When combined with water, it can lead to amino acids, which form proteins, and previous research has identified the chemical as a potential key ingredient in the origin of life on Earth.

Comets striking Earth encounter high temperatures and pressures. Anslow said recent studies found that those conditions, along with subsequent impacts, can destroy many precursor molecules before they can contribute to prebiotic chemistry.

Even if an impact succeeded, the study authors argued that it probably would not have delivered HCN at a useful concentration. If the comet struck an ocean, the chemical would have dissolved quickly.

Prebiotic chemistry also requires other precursor molecules, including compounds containing phosphorus and sulfur, which are not routinely found in comets. Because comet impacts occur in random locations, Anslow said there was no guarantee that everything required for the chemistry would be present at one site.

Other possible sources remain under consideration. Earth’s primordial soup may have produced the necessary molecules independently, while asteroids could have supplied organic material because they generally struck Earth more slowly and in greater numbers.

Anslow pointed to findings from samples collected from the asteroids Bennu and Ryugu, which contain most of the amino acids used to build proteins. The new study also suggested ancient Mars may have offered better conditions for preserving HCN delivered by comets.

Because Mars is smaller than Earth and farther from the sun, typical comet impact velocities there are lower, Anslow said. Impacts that destroyed most HCN on Earth may have been less destructive on Mars, adding another perspective to questions about whether the planet once supported life.

Queenie Chan, an associate professor of Earth sciences at Royal Holloway University of London who was not involved in the study, said HCN may have reacted before comets entered Earth’s atmosphere. Those reactions could have created less volatile, biologically important compounds with a better chance of surviving.

Chan said inefficient delivery of intact HCN does not completely eliminate a possible role for comets in supplying other organic compounds. Nicolle Zellner, a physics professor at Albion College in Michigan, was not surprised by the findings but also remained reluctant to dismiss comets entirely, saying the authors had established useful parameters around the likelihood.