Comet impacts unlikely source of concentrated hydrogen cyanide on early Earth, study finds

New modeling suggests comets were probably a poor way to deliver useful concentrations of hydrogen cyanide, a key prebiotic molecule, to early Earth, challenging a longstanding idea about how some of life’s chemical ingredients may have arrived.

The study, titled “Cometary impacts an unlikely pathway towards the origins of life on Earth,” was published online Oct. 28, 2026, in Scientific Reports. The authors — Richard J. Anslow, Catriona H. McDonald, Amy Bonsor, Paul B. Rimmer and Auriol S. P. Rae of the University of Cambridge and the University of Edinburgh — used simulations rather than new observations or samples. Their modeling combined Monte Carlo and N-body dynamical simulations with models of atmospheric entry and impact chemistry to test whether comets could deliver biologically useful amounts of hydrogen cyanide, or HCN, to Earth, Mars and Venus.

For Earth, the headline result was stark. Under the study’s best-case assumptions, the median number of “successful” comet impacts was zero, and 54% of simulation runs recorded no successful impact at all. When successful deliveries did occur, they were clustered very early in Earth’s history: The most probable timing was about 4.41 billion years ago, and fewer than about 5% of simulations showed delivery later than 4.03 billion years ago. As Anslow told CNN, “Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts.”

The researchers defined a successful delivery as one that produced at least 10 millimolar HCN, a concentration they said is relevant for downstream prebiotic chemistry in laboratory studies. They also described that threshold and several other assumptions as optimistic for cometary delivery. Even so, three problems repeatedly got in the way. High impact speeds and steep impact angles tended to destroy organics during collision. Many smaller or more oblique incoming comets were filtered out by the atmosphere before they could deliver material intact. And when HCN did survive, it was often deposited into oceans, where it would be heavily diluted and subject to hydrolysis, a reaction with water that breaks it down, rather than concentrated on exposed land. The paper estimated cumulative intact HCN delivery at about 2.65 × 10^13 kilograms — but diluted into a single Earth ocean, that would amount to only about 0.75 micromolar, far below concentrations used in many prebiotic experiments.

HCN matters because it is detected in comets, is relatively chemically robust and sits at the center of several influential lab studies of prebiotic chemistry, where it can help generate compounds related to nucleotides, sugars and lipids. The authors argue that if even HCN is this hard to deliver intact and in concentrated form, more fragile prebiotic molecules would likely be harder still. That makes the finding a direct challenge to the broad, long-discussed idea that comet impacts seeded early Earth with key feedstock for life. “Even though we see that comets contain all of these molecules in really high concentrations, it’s not straightforward to go from there to saying we’re going to have all of these available for prebiotic chemistry on the surface of the Earth,” Anslow told Science News.

The study does not say comets played no role at all, or that delivery from space was impossible. Its conclusion is narrower: Intact, concentrated delivery of HCN to early Earth by comet impacts appears to have been an “exceedingly rare event.” The paper points instead to other possible sources of prebiotic feedstock, including asteroid delivery and endogenous atmospheric or geochemical synthesis. In a press release quoted by Science Media Centre Spain, the authors called comet delivery “quite a romantic idea” but said their results suggest it is “highly improbable.”

Using the same modeling framework, the researchers found that Mars would have been a more favorable target than Earth or Venus for intact cometary HCN delivery, a result that could shape how scientists think about prebiotic chemistry beyond Earth.