Cambridge-led simulations: comets rarely delivered prebiotic molecules to early Earth
Models of comet orbits, impacts and atmospheric entry indicate that fragile organic molecules were mostly destroyed, so successful delivery was an exceedingly rare event.
Researchers led by the University of Cambridge used computer simulations to test whether comets could have delivered the molecular building blocks of life to early Earth. The results were published in Scientific Reports. The authors conclude that successful delivery of prebiotic feedstock molecules was an exceedingly rare event nature.com
Scientific Reportsnature.comOpen source ↗.
For decades, an early period of cometary bombardment has been proposed as a source of the molecules needed for prebiotic chemistry. Comets contain a rich variety of volatile organic molecules, including carbon-rich and nitrogen-rich compounds, and many of them are thought to be limiting for prebiotic chemistry. Earlier modelling had already suggested that a significant proportion of these molecules would be destroyed in hypervelocity impacts.
Simulations of impact and survival
The team combined recent advances in cometary impact simulations, comet-atmosphere interactions and dynamical modelling of the origin and evolution of short-period comets. They focused on hydrogen cyanide, a simple compound found in comets that can take part in reactions producing basic building blocks of life.
According to the simulations, comets generally approached Earth too fast or at unsuitable angles. Impact heating would have destroyed much of the fragile organic material they carried nature.com
Scientific Reportsnature.comOpen source ↗. Even where some hydrogen cyanide survived, delivering a useful amount to one location would have been extraordinarily rare, and the compound could disperse before further reactions took place.
Fewer impacts, fewer chances
The modelling also points to a rapid decline in Earth's cometary bombardment. According to a summary of the study, impacts became much less common during the first 500 million years of Earth's history, which left only a small number of possible opportunities for a successful delivery.
The authors do not say that no comet could ever have contributed useful chemicals. They argue instead that a reliable supply of life's ingredients from comets is an unlikely explanation, which challenges the long-held assumption that cometary impacts played an important role in the origins of life on Earth.
The summary adds that Mars, being smaller and having a thinner atmosphere, may have offered somewhat better conditions for such deliveries, although success there would still be uncertain. Other explanations remain open: asteroid impacts may have produced or delivered important compounds, and chemical reactions on early Earth could have created some ingredients locally.
The work relies on simulations rather than direct observations of ancient impacts and does not explain how life began. Its contribution is to test a familiar idea against the physical difficulty of bringing fragile molecules intact to the ground.
