A modeling study led by Cambridge indicates that comets seldom, if ever, brought pristine hydrogen cyanide to the early Earth, undermining a 1960s hypothesis regarding the origin of life’s essential building blocks.For over sixty years, researchers have embraced the concept that comets might have ignited life on our planet by supplying preassembled organic components.
A fresh modeling investigation suggests that this delivery path was, at best, an infrequent fluke. The study, released on October 28 within Scientific Reports, centered on hydrogen cyanide (HCN), a straightforward molecule frequently found in comets that acts as a precursor in numerous laboratory formulas for the fundamental components of RNA and proteins. The researchers deduced that comets impacted the primordial Earth at excessive speeds and typically at overly sharp angles, preventing HCN from enduring the trip and landing in quantities sufficient for chemical utilization.”It’s somewhat of a romantic notion, yet our findings indicate it is extremely improbable,” remarked lead researcher Richard Anslow, who conducted the investigation at the University of Cambridge’s Institute of Astronomy, 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 does not follow cyanide’s later chemistry or the warm, watery environments that impacts themselves can create. Jesús Martínez Frías, a planetary geologist at Spain’s Institute of Geosciences (CSIC-UCM), called the modeling rigorous but warned against extrapolating too broadly, pointing to water and the many other organic compounds that comets carry.Queenie Chan, an Earth scientist at Royal Holloway, University of London, told CNN by email that HCN may react before a comet reaches the atmosphere, forming sturdier compounds, such as protein building blocks, that could survive. Nicolle Zellner, a physicist at Albion College in Michigan, said she was reluctant to dismiss comets but that the authors had “put some good parameters around the likelihood.”Gordon Osinski, a planetary geologist at Western University in Canada, told Science News the work should make researchers “think twice about comets being the originator” of life’s organics, and said he hoped others would try to reproduce the results.
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 view cometary delivery as unlikely in that location. Anslow noted that if comets had actually planted the chemistry of life on Earth, they ought to have achieved the same on Mars, adding that the more plausible scenario is that neither world received them in this manner. As noted in the Cambridge statement, these discoveries might additionally assist upcoming telescopes—such as the Habitable Worlds Observatory proposed by NASA—in selecting which planetary systems to examine while hunting for extraterrestrial life. Financial support for this investigation came partly from the Royal Society, the Leverhulme Centre for Life in the Universe, alongside the Science and Technology Facilities Council of the UK.
