Comets Probably Didn't Seed Life on Earth, Cambridge Simulations Show
Comets arrive too fast and at too sharp an angle for fragile organic molecules to survive impact, making delivery at best a 'one-off fluke,' researchers report.
The idea that comets delivered the molecular building blocks of life to Earth is a long-standing one. A new study from the University of Cambridge says it probably didn't happen that way.
The researchers, led by Dr. Richard Anslow of Cambridge's Institute of Astronomy, found that cometary delivery of these molecules would have been, at best, a "one-off fluke" and not the steady rain of prebiotic material that had been theorized. The work was published in the journal Scientific Reports.
Comets are known to carry many of the carbon- and nitrogen-bearing molecules a planet would need to start life, which is why the theory has endured. "Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules, so these giant space snowballs were almost like a 'get out of jail free card' that brought all the molecules a planet would need to seed life," Anslow said. "It's kind of a romantic idea, but our results suggest it's highly unlikely."
Using computer modeling, the team found that comets travel too fast, and strike Earth's atmosphere at too sharp an angle, for fragile organic molecules to survive. They would almost certainly vaporize on impact with the surface. The researchers also found that Mars could be a slightly more promising candidate for cometary delivery of prebiotic molecules, though that is still highly unlikely.
The study builds on a 2023 paper by Anslow and several co-authors. That paper proposed that in systems where planets orbit close together, comets might slow down by bouncing from one planet to the next, delivering organic molecules along the way. The new work tested whether such a scenario could work in our own solar system, drawing on recent advances in comet impact simulations, comet-atmosphere interactions and models of how comets originate and evolve.
The team focused on hydrogen cyanide, or HCN, an important prebiotic molecule that is regularly found on solar system comets. Their simulations showed that even in the rare event that HCN survived impact with Earth, impacts delivering a useful concentration were extremely uncommon. The number of comets hitting Earth fell rapidly during the planet's first 500 million years, leaving at most a handful of chances for HCN delivery in early Earth history. Even if the molecule made it to the surface, it would not have stayed concentrated for long, so any prebiotic chemistry would have had to get going very quickly after the impact.
"It would really have to be a one-in-a-million occurrence, because not only would you need an impact at just the right speed and just the right angle, you would also need everything to happen after that point, in terms of the concentration and the speed at which the chemical origins of life got going," Anslow 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."
If comets were not the main source, the molecular seeds of life probably formed on Earth itself. The researchers say there are several possibilities for how that happened, and the result shifts attention toward chemistry that began on our own planet.
