New study casts doubt on theory about the role comets played in kick-starting life on Earth

ATLANTA — The popular theory that comets were crucial in kick-starting life on Earth billions of years ago may not hold up, according to new computer simulations that counter decades of scientific thinking.

The finding suggests the fragile organic molecules supposedly carried by the icy objects would have been destroyed on impact, upending an established origin of life theory that dates back to at least 1961. The broader idea that the ingredients for life arrived from space, known as panspermia, is even older and was scientifically postulated in 1908.

Life on Earth requires complex organic molecules, but it's unclear whether our early planet had enough of the raw materials needed for prebiotic chemistry — the chemical reactions that produce the building blocks of life. Comets are made of dust and ice but are also known to contain organic material. This composition, which has earned the celestial bodies the nickname "dirty snowballs," prompted scientists to suggest that comets could have deposited water and organic molecules on Earth's surface.

Early in its history — over 4 billion years ago — Earth may have experienced a period of heavy bombardment from more than 100,000 comets larger than 100 meters (328 feet), according to the new study, published Wednesday in the journal Scientific Reports. Those impacts, however, were mostly too heavy for life-starting molecules to withstand.

The team used the latest models of comet formation and evolution to simulate this early planetary period 100,000 times. "Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts," said lead study author Richard Anslow, who conducted the research while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom.

The comets contained large concentrations of key molecules needed for life, but the simulation found the objects were coming in too fast to successfully deposit them. "The typical velocities of these comets when they impact the Earth meant that actually very few, if any, would have been able to deliver intact organic molecules, so comets actually don't seem to be as useful for prebiotic chemistry as was previously thought," Anslow said.

More precisely, 54% of the simulations showed no impact at all, while the remaining 46% had at least one and a "tiny minority" returned up to three collisions, Anslow said. "We can't rule out confidently that there was never a successful impact — there could have been one of these fluke impacts." However, such flukes had to occur in a "perfect location" that already met several important conditions, including the presence of liquid water and other chemicals needed to support life. That combination makes the overall process unlikely, he added.

Alternative explanations

The study focuses on a naturally occurring chemical called hydrogen cyanide or HCN, an important precursor in prebiotic chemistry that has been detected in dozens of comets, including the notable Halley's and Hale-Bopp comets.

Composed of hydrogen, carbon and nitrogen, HCN can lead to the formation of amino acids — molecules that make up proteins — when combined with water; other studies have previously identified HCN as a potential key ingredient for the origin of life on Earth.

"When comets strike Earth, they're shocked to high temperatures and pressures, and recent studies have found that a lot of these precursor molecules can be destroyed by this high pressure and temperature as well as the following impacts," Anslow said.

The study authors argue that even if a successful impact occurred, it's unlikely that it would have delivered the chemical in a useful concentration, and in the case of an ocean impact, it would have quickly dissolved.

To set the stage for prebiotic chemistry to occur, many other precursor molecules are also needed, such as compounds that contain phosphorus and sulfur, but these are not routinely found in comets, Anslow said. "Comets are really good for delivering hydrogen cyanide, but given it's random where a comet impacts onto Earth, we're not going to be guaranteed to have everything else that we need for this chemistry right in that spot."

If comets didn't deliver the building blocks of life on Earth, Anslow said there are several other promising avenues to explore. Another long-held, competing theory is the possibility that Earth's primordial soup could have synthesized the molecules needed for life on its own. An alternative cosmic source of organic material could have been asteroids, which typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings on samples from asteroids Bennu and Ryugu, which contain the majority of protein-building amino acids.

The recent study also raises another intriguing possibility: While Earth may not have had the ideal conditions to foster HCN from comet impacts, ancient Mars might have been better suited. "Because Mars is much smaller than Earth and further away from the sun, the typical impact velocity of comets is much lower, so while these impacts on Earth were destroying most of the HCN, this wasn't the case so much for Mars," Anslow said.

This idea could add another dimension to the long-standing question of whether the red planet once supported life.

"It's just another perspective to have in the back of our minds when we're trying to understand the early evolution of Mars in more detail," Anslow said.

Comet theory could still stand

While the authors closely analyzed how HCN could have survived comet impact, the chemical may have undergone reactions before the icy bodies entered Earth's atmosphere, said Queenie Chan, an associate professor of Earth sciences at Royal Holloway University of London, who was not involved with the study. This process could have created biologically important compounds, such as those protein building blocks, that are less volatile and more likely to survive.

"Therefore, the inefficiency of delivering intact HCN does not completely rule out a potential role for comets in supplying other important organic compounds that may have contributed to Earth's prebiotic organic inventory before the emergence of life," Chan wrote in an email.

Nicolle Zellner, a professor of physics at Albion College in Michigan, said in an email that she wasn't surprised by the findings. While she is also reluctant to eliminate comets as a possible contributor to the emergence of life, she acknowledged that "the authors have definitely put some good parameters around the likelihood."

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