Comets unlikely to have delivered life’s building blocks to Earth, study suggests

Comets are unlikely to have supplied a sustained flow of the molecules needed for life to begin on Earth, according to modelling that finds successful delivery would have required an exceptional chain of events.

The Cambridge University-led research challenges a theory discussed for almost a century: that comets brought the molecular building blocks of life to the young planet. It does not rule out such a delivery, but suggests that useful quantities would rarely have survived arrival and remained concentrated long enough for the necessary chemistry.

The study, published in Scientific Reports, combined simulations of comet impacts, interactions with atmospheres and the origin and evolution of comet orbits.

Researchers found that comets generally approach Earth too fast and at angles too steep for fragile organic molecules to survive. Those reaching the surface would almost certainly be vaporised on impact.

The team focused on hydrogen cyanide, or HCN, an important prebiotic molecule regularly detected in Solar System comets. Even where simulations allowed it to survive, impacts delivering a useful concentration were extremely uncommon.

The rate of comet impacts declined rapidly during Earth’s first 500 million years, leaving, at most, a handful of opportunities for HCN delivery. Any surviving material would also have dispersed, requiring prebiotic chemistry to proceed very quickly.

Lead author Dr Richard Anslow, of Cambridge’s Institute of Astronomy, said: “We can’t rule out for certain that it happened that way. But it seems to be a really rare occurrence – at best, a one-off fluke.”

He described the combination of the necessary speed, impact angle, concentration and subsequent chemistry as a “one-in-a-million occurrence”, rather than presenting a steady supply of molecules.

In 2023, Anslow and several colleagues proposed that comets in systems with closely spaced planets might slow by “bouncing” between them, potentially delivering organic material along the way. The new study examined whether that mechanism could have operated in our Solar System and found the required circumstances to be rare.

Other possible sources of life’s precursors remain. The researchers suggest that molecules could have been synthesised following asteroid impacts, which were much more frequent than comet impacts, or through processes on Earth without an external delivery.

Mars emerged as a somewhat more promising destination for comet-borne molecules, although successful delivery there was still considered highly unlikely. Anslow said its smaller size and much thinner present-day atmosphere made it a better candidate for material reaching the surface during the Solar System’s more comet-rich early development.

“This is not to say it definitely happened on Mars, of course, but suggests it would be unlikely if it happened on Earth and didn’t happen on Mars,” he said.

“Or the other, perhaps more likely, alternative, is that it didn’t happen on either planet.”

The findings may help guide future searches for life beyond the Solar System. Anslow said identifying planetary systems where comets could slow through repeated encounters might narrow the search when successors to the James Webb Space Telescope become available.