Comets have long been considered potential carriers of the molecular building blocks of life to early Earth, bringing carbon- and nitrogen-bearing compounds that could have helped life emerge. However, a new study led by the University of Cambridge suggests that such delivery was extremely rare, as fragile organic molecules would mostly vaporise during impact. The findings were published in Scientific Reports.
The researchers focused on hydrogen cyanide (HCN), a prebiotic molecule found in Solar System comets. Computer simulations showed that comets approach Earth at speeds and angles that make molecular survival difficult. Even when molecules survive, impacts delivering useful concentrations are exceptionally uncommon.
Comet impacts also declined rapidly during Earth’s first 500 million years, leaving few opportunities for such delivery. Any HCN reaching the surface would have needed to remain concentrated long enough for prebiotic chemistry to progress, adding another obstacle to the theory.
Mars may have offered better conditions for cometary delivery because its smaller size and thinner atmosphere would have allowed more comets to reach the surface during the early Solar System. However, the researchers cautioned that this does not establish that the process occurred there.
Alternative explanations include organic molecules forming through asteroid impacts or chemical processes on Earth itself. The findings could also help astronomers narrow their search for life beyond the Solar System by identifying planetary systems where cometary interactions might favour the delivery of prebiotic material.