Comets rarely brought intact hydrogen cyanide to Earth, study says By Luís Henrique Costa
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Hydrogen cyanide, a molecule linked to the chemistry that may have preceded life, would rarely have reached Earth intact aboard comets. That is the conclusion of simulations by researchers at the University of Cambridge, in the United Kingdom, published on Wednesday (28) in the journal Scientific Reports. Most of the scenarios tested did not produce concentrations high enough for the chemical reactions studied in laboratory.
“Our models show that comets, in general, hit Earth too fast and at very steep angles for these molecules to survive the impact,” said Richard Anslow, a researcher at the University of Cambridge and author of the study. For him, “this suggests that they probably did not play a major role in the origin of life.”
Why hydrogen cyanide matters for life’s chemistry
HCN, as the molecule is known, can take part in forming sugars, fats and components of DNA and RNA, which are central to living things. Comets carry this and other organic substances, and the idea that impacts billions of years ago supplied raw material for the chemistry before life has been debated for decades. The new work tested one specific mechanism of that hypothesis: the arrival of large quantities of intact HCN on the planet’s surface.
Speed and angle of impact threaten the molecule
In the simulations, comets usually hit Earth at high speed and at steep angles, conditions that favor the destruction of HCN. According to the researchers, the heat and pressure produced by the impact can break down part of the molecule before it reaches the ground. Even when it survives the shock, the authors note, water in the environment can dilute it quickly, shortening the time available for relevant reactions.
Five hurdles hydrogen cyanide faced before reaching the surface
- Passing through the atmosphere without breaking down.
- Reaching the planet at a workable speed.
- Coming down at a workable angle.
- Enduring the heat and pressure of the crash.
- Avoiding dilution by water once it lands.
Over half of the runs delivered no hydrogen cyanide at useful levels
Even under conditions chosen to favor the hypothesis, more than half of the simulations recorded no impact capable of delivering HCN at the concentration the authors analyzed. The result suggests that large-scale delivery would not be a common event but the product of a very specific combination of factors. According to the simulations, a comet would have to be larger than 100 meters across, move slowly and strike at an oblique angle, and on average only one such impact in more than a billion years would have delivered a meaningful amount. For the researchers, that rarity, rather than an absolute impossibility, is the core finding.
The study does not rule out every comet contribution
The results do not mean comets made no contribution to the origin of life. The study focuses mainly on direct, intact delivery of HCN in large concentrations, and the authors acknowledge that a particularly favorable impact cannot be completely excluded as a way of bringing the molecule to Earth. A specialized publication’s analysis adds that other organic molecules carried by comets may also have taken part in prebiotic chemistry.
Astrobiologist from Spain cautions against overreading the results
César Menor Salván, astrobiologist and professor at the University of Alcalá, in Spain, says the work does not show that comets had no part in the emergence of life. According to him, the study estimates that one specific mechanism, the direct delivery of large amounts of intact HCN, would be unlikely. “We should not confuse the low probability of a particular mechanism with the absence of a cometary contribution to the origin of life,” he said in remarks to a Spanish science outreach organization. He adds that comets carry several other organic substances and that impacts themselves can trigger chemical reactions and create favorable environments.
Read also: Stephen Hawking framed human limits as the spark for cosmic discovery
Planetary geologist says conclusions should not be stretched
Jesús Martínez Frías, planetary geologist and astrobiologist at the Geosciences Institute of Spain, made a similar assessment. For him, the simulations are rigorous, but their conclusions should not be extended to all possible participation of comets. He notes that the research centers on HCN and does not broadly assess other organic compounds or the role of water brought by these bodies.
Thinner air made Mars a friendlier target for comet impacts
The researchers also found differences between the two planets. In the simulations, Mars was more favorable than Earth for the arrival of concentrated HCN by comets, mainly because of its weaker gravity and thinner atmosphere, which let more comets reach the surface without losing their chemical cargo. The authors stress that this does not mean life arose on Mars, since the simulations assess only one specific step of the chemistry that may have preceded life.
Scientists still debate where life’s first molecules came from
The origin of life remains one of the big open questions in science. The molecules needed for the first chemical reactions may have formed on Earth or arrived from space on comets, asteroids and dust particles, and the weight of each source is still debated. For the authors, the arrival of large amounts of intact HCN by comets would have been extremely rare. This does not rule out other possibilities, but it suggests that life’s origin probably cannot be explained by one type of impact or one molecule alone.
Joan Oró’s 1960s work shaped the comet hypothesis
The hypothesis that comets carried molecules for the origin of life has been discussed since the last century. One milestone cited is the work of biochemist Joan Oró, who synthesized adenine from HCN in studies from the 1960s. Scientists long believed comets were important because they carried molecules containing carbon and nitrogen.
Where and when the Cambridge study was published
- Journal: Scientific Reports
- Institution: University of Cambridge, United Kingdom
- Author: Richard Anslow
- Molecule analyzed: hydrogen cyanide (HCN)
- Publication date: October 28, 2026
Team has announced no follow-up research
The Cambridge team has not said whether it plans new studies to deepen the topic. So far, the results are limited to the HCN simulations and to scenarios of direct delivery of the molecule by comets, with no measurements of other organic compounds or of other routes of arrival to Earth.
