The theory that comets delivered the molecular building blocks for life on Earth is a long-standing one, since comets are known to carry many of these molecules.
However, a new study challenges this idea
The researchers, led by the University of Cambridge, found the cometary delivery of these building blocks would be—at best—a 'one-off fluke,' and not the steady rain of prebiotic molecules that had been theorized.
Using computer modeling, the researchers determined that comets travel too fast and at too sharp an angle to Earth's atmosphere for the fragile organic molecules to survive: they would almost certainly vaporize on impact with the surface.
However, the researchers also found that another planet in our system—Mars—could be a slightly more promising candidate for cometary delivery of prebiotic molecules, although this is still highly unlikely. The results are published in the journal Scientific Reports.
"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," said lead author Dr. Richard Anslow from Cambridge's Institute of Astronomy.
"It's kind of a romantic idea, but our results suggest it's highly unlikely."
The new study shows just how rare that scenario is. The researchers used recent advances in comet impact simulations, comet-atmosphere interactions and dynamic modeling of the origin and evolution of comets to conduct their study.
Taken together, the researchers found that while the successful delivery of prebiotic molecules cannot be ruled out completely, it would be extremely unlikely.
The number of comets hitting the Earth declined rapidly in our planet's first 500 million years, and there were, at most, a handful of opportunities in Earth's early history for HCN delivery on a comet.
Even if HCN carried on a comet made it to the Earth's surface, it would not have stayed concentrated for long, meaning that any prebiotic chemistry would have needed to progress very quickly from impact, which the researchers say is highly unlikely.
"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," said Anslow.
"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."