Comets may have destroyed life's ingredients instead of delivering them to Earth

For nearly a century, scientists have considered the possibility that comets helped make life on Earth possible by delivering essential organic molecules from space. Now, new research suggests this long-standing explanation is extremely unlikely.

Comets are known to contain many of the chemical compounds associated with the origins of life. These substances, often called prebiotic molecules, can participate in the chemical reactions that eventually produce the complex ingredients needed for living organisms. For decades, researchers have proposed that comets striking the early Earth supplied a steady stream of these compounds.

However, a new study led by the University of Cambridge challenges that theory. Using advanced computer simulations, researchers found that comets generally approach Earth at speeds and angles that make it extremely difficult for delicate organic molecules to survive. Instead of delivering their chemical cargo intact, most would likely destroy it during their violent arrival.

The findings, published in Scientific Reports, suggest that successful delivery of life's building blocks by comets would have been an extraordinarily rare event. Interestingly, the researchers also found that Mars may have offered somewhat better conditions for receiving these molecules, although the likelihood remains very low.

"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."

Why Comets May Have Failed to Deliver Life's Building Blocks

The idea that comets could transport the ingredients for life received renewed attention in 2023, when Anslow and several colleagues explored how these icy objects might behave in planetary systems where worlds orbit relatively close together.

Their earlier research suggested that comets could potentially lose speed as they passed between neighboring planets. Through a series of gravitational encounters, a comet might effectively bounce from one planet to another, eventually traveling slowly enough to deliver organic molecules without destroying them.

The team wanted to determine whether a similar process could have occurred in our own Solar System, potentially explaining how essential chemicals reached the young Earth.

"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," he said.

To investigate, the researchers combined several sophisticated modeling techniques. These included simulations of comet impacts, calculations of how comets interact with planetary atmospheres, and models tracing how comet populations formed and changed as the Solar System developed.

The results revealed a major obstacle. Although a comet might occasionally approach Earth under conditions favorable for preserving organic compounds, such encounters would have been exceptionally uncommon. Successful delivery could not be completely ruled out, but the simulations indicated that it was highly improbable.

A Critical Ingredient for Life Faces Nearly Impossible Odds

To better understand what happens to the molecules carried by comets, the team concentrated on hydrogen cyanide (HCN), a chemical commonly detected in comets throughout our Solar System.

Despite its reputation as a highly toxic substance, hydrogen cyanide is important in research on the origins of life. It contains carbon and nitrogen and can participate in reactions that produce more complex organic compounds, making it a potentially valuable ingredient in the chemistry that preceded living organisms.

The simulations showed that even when hydrogen cyanide managed to survive a comet's arrival, another major problem remained. The impact would need to deposit enough of the molecule in one location to support meaningful chemical reactions. Events capable of producing those useful concentrations were extraordinarily rare.

Timing created yet another difficulty. Comet impacts were much more common during Earth's earliest history, but their frequency declined sharply over the planet's first 500 million years. According to the researchers, there may have been only a handful of opportunities during that entire early period for comets to deliver hydrogen cyanide under favorable conditions.

Even a successful delivery would not necessarily have helped life develop. Hydrogen cyanide deposited on Earth's surface would likely have dispersed or become diluted relatively quickly, leaving little time for the chemical reactions associated with life's origins to get underway.

For the comet theory to work, several unlikely events would therefore have needed to occur together. A comet would have had to strike at just the right speed and angle, preserve its organic cargo, deposit enough material in a suitable location, and trigger important chemical processes before those molecules dispersed.

"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."

If Not Comets, Where Did Life's Ingredients Come From?

The findings raise a fundamental question about Earth's earliest history. If comets were unlikely to supply the chemical building blocks of life, what was responsible?

One possibility involves asteroids, which collided with the early Earth far more frequently than comets. The researchers suggest that asteroid impacts could have created conditions in which important prebiotic molecules formed through chemical reactions associated with those collisions.

Another explanation is that Earth produced these compounds on its own. Natural chemical processes operating on the young planet may have generated the ingredients necessary for increasingly complex chemistry, without requiring organic molecules to arrive from space.

These possibilities do not establish exactly how life began. Instead, they highlight alternative ways that the raw materials for life could have become available on Earth.

Why Mars May Have Had a Better Chance Than Earth

While the simulations cast serious doubt on cometary delivery to Earth, they also revealed an intriguing possibility involving Mars.

The Red Planet is smaller than Earth and currently has a much thinner atmosphere. These characteristics make it a potentially more favorable destination for cometary molecules, since incoming objects could be more likely to reach its surface without completely destroying their chemical cargo.

According to the researchers, Mars may therefore have been better positioned to receive prebiotic compounds during the Solar System's early development, when comet encounters were considerably more frequent.

However, the simulations do not demonstrate that such deliveries actually occurred. Even on Mars, the conditions required for a successful transfer of life's chemical ingredients appear difficult to achieve.

"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," said Anslow. "Or the other, perhaps more likely, alternative, is that it didn't happen on either planet."

What the Discovery Means for the Search for Alien Life

Beyond changing how scientists think about Earth's early chemistry, the findings could help guide the search for potentially habitable planets in other star systems.

Future space observatories may be able to investigate distant worlds in greater detail, searching for atmospheric clues that could indicate environments suitable for life. Among the proposed missions is the Habitable Worlds Observatory, a potential successor to the James Webb Space Telescope (JWST).

The researchers suggest that planetary systems where comets can move between closely spaced planets while gradually losing speed may deserve particular attention. In such environments, the unusual comet delivery mechanism explored in their earlier research could be more likely to operate.

Identifying these systems could help astronomers focus future observations on planets where essential organic molecules may have had a better chance of surviving their journey through space.

Although the new findings make comets an unlikely explanation for how life's building blocks reached Earth, they do not eliminate the possibility that cometary delivery played a role elsewhere. Understanding where these chemical transfers are possible could provide valuable clues as scientists continue investigating how life emerges across the universe.

The research received partial support from the Royal Society, the Leverhulme Centre for Life in the Universe, and the Science and Technology Facilities Council (STFC), which is part of UK Research and Innovation (UKRI).