A new study led by the University of Cambridge challenges the long-standing idea that comets helped create the conditions for life on Earth. Computer simulations suggest that although comets contain hydrogen cyanide and other organic compounds, they would rarely deliver these chemicals to Earth’s surface in concentrations high enough to support prebiotic chemistry.
“Historically, scientists thought that comets were important for the origin of life on Earth because they carried molecules including carbon and nitrogen. These giant cosmic snowballs were therefore like ‘get-out-of-jail-free cards’ that brought the planet all the molecules it needed to generate life,” said lead author Dr Richard Anslow, a researcher at the Institute of Astronomy at the University of Cambridge.
“It’s a bit of a romantic idea, but our results show that it’s very unlikely.”
Simulating hydrogen cyanide delivery
Dr Anslow’s team used computer simulations to investigate whether comets striking the early Earth could have supplied hydrogen cyanide (HCN), a molecule involved in several proposed pathways for producing the building blocks of life.
Laboratory experiments have shown that hydrogen cyanide can help produce sugars, lipids and components of genetic material when present at sufficiently high concentrations. Comets contain HCN and other organic compounds that could serve as raw materials for life.
However, delivering those molecules intact to a planet’s surface is difficult. Small comets can break apart in the atmosphere, while the extreme temperatures produced by high-speed impacts can destroy organic molecules that survive the descent.
To assess how often comets might overcome these obstacles, the researchers modeled the evolution of short-period comets and their impacts on Earth. The simulations accounted for atmospheric entry, impact velocity and the amount of hydrogen cyanide that could survive an impact.
The results suggest that successful delivery is extremely rare. Across 100,000 simulated comet impacts on Earth, 54% did not record an impact that delivered hydrogen cyanide at concentrations considered sufficient for prebiotic chemistry.
“It really has to be a one-in-a-million event, because not only does it have to hit at the right speed and the right angle, but everything has to happen from that point forward in terms of concentration and speed at which the chemical origins of life proceed,” Dr Anslow said.
“We can’t rule out with certainty that something like that happened. But it seems like a very rare event. A one-off fluke at best.”
Mars may have been more favorable
The researchers also compared comet impacts on Earth, Mars and Venus. Their simulations showed that Mars would have been more favorable for delivering concentrated hydrogen cyanide.
The main reason is that Mars has weaker gravity and a thinner atmosphere, allowing more comets to reach the surface without losing their chemical cargo. Under the model’s assumptions, Mars experienced far more successful delivery events than Earth or Venus.
“Of course, we’re not saying this definitely happened on Mars, but it does suggest that it’s less likely that it happened on Earth than on Mars,” Dr Anslow said.
“Or, the other option, which is probably more likely, is that it didn’t happen on either planet.”
The study was published in Scientific Reports.
RJ Anslow et al. 2026. “Comets impact unexpected paths to the origins of life on Earth.” Scientific Reports 16, 31688. doi: 10.1038/s41598-026-74214-y
Source: www.sci.news
