A new Cambridge-led study challenges a century-old idea that comets seeded Earth with the molecular building blocks of life. Computer modeling suggests comets hit too fast and at too sharp an angle for fragile organic molecules to survive. Cometary delivery would be, at best, a “one-off fluke” rather than the steady rain of prebiotic molecules once theorized.Comets hit too hardComets have long been seen as a delivery route for life’s ingredients because they carry carbon- and nitrogen-bearing molecules. Lead author Dr. Richard Anslow of Cambridge’s Institute of Astronomy called it “kind of a romantic idea,” but said the results suggest it is “highly unlikely.”The team used recent advances in simulations of comet impacts, comet-atmosphere interactions and the origin and evolution of comets. Their modeling shows the molecules would almost certainly vaporize on impact with the surface.The work builds on a 2023 study in which Anslow and several co-authors proposed that comets might slow by “bouncing” between closely orbiting planets. Such bouncing comets could deliver organic molecules as they went. The new paper tested whether that could happen in the Solar System and found the scenario is extremely rare.Why hydrogen cyanide mattersThe researchers focused on hydrogen cyanide (HCN), an important prebiotic molecule regularly found on Solar System comets. Even in the rare case that HCN survived impact with Earth, impacts delivering a useful concentration were extremely uncommon.The number of comets hitting Earth declined rapidly in the planet’s first 500 million years, leaving at most a handful of chances for HCN delivery. Even if HCN reached the surface, it would not have stayed concentrated for long, so prebiotic chemistry would have needed to progress very quickly after impact.It would take “a one-in-a-million occurrence,” Anslow said, needing just the right impact speed and angle plus rapid, favorable chemistry afterward. He said it cannot be ruled out entirely but looks like, “at best, a one-off fluke.”If comets did not seed life, other sources remain. The researchers point to molecules synthesized after asteroid impacts, since asteroids hit Earth far more often than comets. Another possibility is that the chemical precursors formed through processes on Earth itself, without outside help.Mars and future searchesMars could be a slightly better candidate for cometary delivery, though still highly unlikely. It is smaller than Earth and now has a much thinner atmosphere, so comets would more likely have reached its surface when they visited more frequently.Anslow said that does not mean it happened on Mars. It would be unlikely if it happened on Earth but not Mars, he said, adding that the more likely alternative is that it happened on neither planet.The findings could help the search for life elsewhere, especially once successors to the James Webb Space Telescope, such as the proposed Habitable Worlds Observatory, come online. Identifying systems where bouncing comets could exist might be one way astronomers narrow their search.The results appear in the journal Scientific Reports.