Did life on Earth hitch a ride on a comet? Study may have answer

By Stephen Beech

Comets probably didn’t seed life on Earth after all, suggests new research.

The study potentially overturns a century-old theory that the celestial bodies delivered the molecular “building blocks” for life on the planet.

The new research, led by Cambridge University scientists, found cometary delivery of building block molecules would be — at best — a “one-in-a-million one-off fluke” and not the steady rain of prebiotic molecules that had been theorized.

They say their findings could be useful when searching for alien life.

Using computer modeling, the research team determined that comets travel too fast and at too sharp an angle to Earth’s atmosphere for the fragile organic molecules to survive.

And they would almost certainly vaporize on impact with the surface, according to the findings published in the journal Scientific Reports.

The researchers also found that Mars could be a slightly more promising candidate for cometary delivery of prebiotic molecules — although that is still “highly unlikely.”

Study lead author Richard Anslow said: “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.

“It’s kind of a romantic idea, but our results suggest it’s highly unlikely.”

In 2023, Anslow and several co-authors of the new paper theorized that in systems where planets orbit closely together, comets might slow down by “bouncing” from one planet to the next, delivering organic molecules as they went.

Building on the earlier study, Anslow and his colleagues investigated whether such a scenario might be possible in our own solar system.

He said: “We’re learning more and more all the time about how our solar system and the Earth formed, and pulling this together with lots of different advances in modeling, which allowed us to investigate this idea that has been around for close to a century.”

But the new study shows just how rare that scenario is.

The research team used recent advances in comet impact simulations, comet-atmosphere interactions and dynamic modeling of the origin and evolution of comets to conduct the study.

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Taken together, the researchers found that the while the successful delivery of prebiotic molecules cannot be ruled out completely, it would be extremely unlikely.

The research team focused on hydrogen cyanide (HCN), an important prebiotic molecule that is regularly found on solar system comets.

Their simulations showed that even in the rare event that HCN molecules 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, 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.

Anslow, from Cambridge’s Institute of Astronomy, said: “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.

“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.”

The researchers say there are several possibilities for where the seeds of life on Earth came from, including the synthesis of the molecules following the impact of asteroids, which hit the Earth far more frequently than comets.

The chemical precursors may also have formed from processes that occurred on Earth itself — without any outside interference.

Although cometary delivery of HCN and other prebiotic molecules to Earth seems highly unlikely, the researchers found that one of our closest neighbors could be a better candidate.

Since Mars is smaller than Earth, and currently has a much thinner atmosphere, Anslow says comets would be much more likely to make it to the red planet’s surface during the period of the solar system’s development when comets were far more frequent visitors.

He added: “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.

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

The researchers say their results could be useful when searching for life in other systems, especially when the successors to the James Webb Space Telescope (JWST) come online.

Anslow said identifying systems where “bouncing” comets could exist could be one way that future astronomers could narrow their search.