A Cambridge-led modeling study finds that comets rarely, if ever, delivered intact hydrogen cyanide to the young Earth, weakening a 1960s idea about where life’s raw materials came from.For more than six decades, scientists have entertained the idea that comets helped start life on Earth by delivering ready-made organic ingredients.
A new modeling study argues that this delivery route was, at best, a rare accident.The research, published October 28 in Scientific Reports, focused on hydrogen cyanide (HCN), a simple molecule that is common in comets and serves as a starting material in many laboratory recipes for the building blocks of RNA and proteins. The authors concluded that comets struck the young Earth too fast, and usually at too steep an angle, for HCN to survive and arrive in concentrations that chemistry could use.”It’s kind of a romantic idea, but our results suggest it’s highly unlikely,” lead author Richard Anslow, who carried out the work at the University of Cambridge’s Institute of Astronomy, said in a university statement.
Which theory is under scrutiny
The study bypasses both the RNA world hypothesis and the hydrothermal vent model, which instead explore the location and mechanism of prebiotic chemistry. Its focus is a more specific proposition: that comets delivered the initial ingredients. Biochemist Joan Oró initially proposed this concept in a 1961 Nature article, coinciding with his demonstration that hydrogen cyanide transforms into adenine, a fundamental component of DNA and RNA. Ever since, comets have frequently acted as a reliable extraterrestrial supplier whenever the primitive Earth’s native reactions appeared incapable of generating such molecules. This concept represents a more localized variant of panspermia, the longstanding hypothesis that living matter or its precursors were brought from the cosmos, as CNN noted in its coverage.
How the researchers reached their conclusion
The work involved no new laboratory experiments or telescope observations. Anslow and co-author Amy Bonsor-Matthews combined three lines of earlier research: simulations of what happens to HCN when a comet slams into a planet, studies of how comets break apart and slow down in an atmosphere, and dynamical models of how often short-period comets would have reached the inner planets.An earlier study by members of the same Cambridge group found that HCN rarely survives impacts faster than Earth’s escape velocity, about 11 kilometers per second, except in grazing strikes.
Since objects falling from space generally hit at least that fast unless the atmosphere slows them, the margin was slim from the outset.The new paper simulated the planet’s history of comet bombardment 100,000 times. According to figures Anslow gave CNN, 54% of those runs produced no successful delivery at all, while the remaining 46% produced at least one and only a small minority as many as three. As Science News reported, a successful delivery required a comet larger than 100 meters across, moving slowly, arriving at an oblique angle and hitting land. An ocean impact would dilute any surviving HCN too quickly for useful chemistry.Timing also worked against comets. Impacts declined sharply during roughly the first 500 million years of Earth’s history, leaving only a handful of opportunities.
What the study does and does not show
The authors do not rule comets out. Anslow acknowledged that a single fortunate impact cannot be excluded, though he described it as a one-in-a-million event. He also noted that even a successful strike would need to land where water and other ingredients, such as compounds containing phosphorus and sulfur, were already available.The study’s scope is limited. It models only HCN, and while Anslow has argued that more complex molecules would be even more fragile, that point was inferred rather than tested. The results also rest on assumptions about the early comet population and atmosphere, which remain uncertain.Researchers not involved in the work raised further caveats. In comments gathered by the Spanish Science Media Centre, astrobiologist César Menor Salván of the University of Alcalá said the paper shows that large HCN deposits from comets were extraordinarily rare, not that comets contributed nothing.
He noted that the model fails to capture the subsequent chemistry of cyanide or the warm aqueous environments that impacts themselves are capable of generating. Jesús Martínez Frías, a planetary geologist at Spain’s Institute of Geosciences (CSIC-UCM), described the modeling as meticulous yet cautioned against excessive extrapolation, drawing attention to water alongside the diverse organic molecules transported by comets.Queenie Chan, an Earth scientist at Royal Holloway, University of London, shared via email with CNN that hydrogen cyanide might undergo reactions prior to atmospheric entry, yielding more resilient substances—like protein precursors—capable of enduring the journey. Nicolle Zellner, a physicist at Albion College in Michigan, expressed hesitation in writing off comets entirely, though she acknowledged that the researchers have “put some good parameters around the likelihood.”Gordon Osinski, a planetary geologist at Western University in Canada, remarked to Science News that the study ought to prompt scientists to “think twice about comets being the originator” of the organic compounds essential for life, adding that he anticipates independent efforts to replicate the findings.
Where life’s ingredients may have come from instead
If comets played a minor role, other sources become more important. Asteroids struck the early Earth far more often and at lower speeds. Anslow pointed to samples returned from the asteroids Bennu and Ryugu, which contain most of the amino acids that build proteins. Chemistry on Earth itself, at volcanoes, hydrothermal vents or other settings, remains another candidate.The team also compared Earth with Mars. Because Mars is smaller and farther from the Sun, and has a thinner atmosphere, comets would have hit it more slowly, giving HCN better odds of survival.
The authors still consider cometary delivery there improbable. Anslow said that if comets had seeded life’s chemistry on Earth, they should have done so on Mars too, and that the more likely alternative is that they did so on neither planet.According to the Cambridge statement, the findings could also help future telescopes, such as NASA’s proposed Habitable Worlds Observatory, choose which planetary systems to study in the search for life elsewhere. The research was funded in part by the Royal Society, the Leverhulme Centre for Life in the Universe and the UK’s Science and Technology Facilities Council.
