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 does not test the RNA world hypothesis or the hydrothermal vent model, which deal with where and how life’s chemistry unfolded. It targets a narrower proposal: that comets supplied the raw materials. The biochemist Joan Oró set out that idea in a 1961 paper in Nature, around the time he showed that HCN can be converted into adenine, one of the bases of DNA and RNA.Since then, comets have often served as a convenient outside source whenever the early Earth’s own chemistry seemed too limited to make such compounds. It is a narrower cousin of panspermia, the older notion that life or its ingredients arrived from space, 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 space debris typically plunges at that speed or faster unless atmospheric friction slows it down, the safety buffer was razor-thin from the beginning. The recent study modeled the globe’s comet impact timeline on 100,000 separate occasions. Based on statistics provided by Anslow to CNN, 54 percent of those simulations yielded zero successful arrivals whatsoever, whereas the remaining 46 percent achieved at least one, with only a minor fraction reaching as many as three. As Science News documented, a fruitful impact demanded a comet exceeding 100 meters in width, traveling at a reduced velocity, striking at a slanted trajectory, and landing upon terrestrial ground. Striking an ocean would wash out any remaining HCN far too rapidly to sustain meaningful chemical reactions. Chronology likewise proved unfavorable for comets. Collisions decreased dramatically over roughly the initial 500 million years of the terrestrial timeline, restricting the window to merely a handful of chances.
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.