A new computer study challenges a long-standing idea that comets helped kick-start life on Earth billions of years ago. Simulations published in the journal Scientific Reports suggest that the fragile organic molecules carried by icy comets would most likely have been destroyed when the objects struck the planet, undercutting an origin-of-life theory that dates back to at least 1961.
The broader notion that the ingredients for life arrived from space, known as panspermia, is older still and was first proposed scientifically in 1908. Life on Earth depends on complex organic molecules, but scientists have long debated whether the early planet had enough raw materials for prebiotic chemistry, the chemical reactions that produce the building blocks of life. Comets, often described as "dirty snowballs," are made of dust and ice and are known to contain organic material, which led researchers to propose that they could have delivered water and organic molecules to Earth's surface.
According to the study, Earth may have endured a period of heavy bombardment more than 4 billion years ago from more than 100,000 comets larger than 100 meters, or 328 feet, across. The impacts, however, were mostly too violent for life-forming molecules to survive. The research team used the latest models of comet formation and evolution to simulate this early 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 author Richard Anslow, who conducted the work 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, but they were arriving too fast to deposit them intact. "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.
In the simulations, 54% showed no impact at all, while the remaining 46% included at least one impact, and a "tiny minority" recorded up to three collisions. Anslow cautioned that a single successful delivery cannot be ruled out entirely. "We can't rule out confidently that there was never a successful impact — there could have been one of these fluke impacts." Even so, such a fluke would have had to land in a "perfect location" that already offered liquid water and the other chemicals needed to support life, a combination Anslow said would be unlikely.
The study centers on hydrogen cyanide, or HCN, a naturally occurring compound that is an important precursor in prebiotic chemistry. It has been detected in dozens of comets, including Halley's and Hale-Bopp. Composed of hydrogen, carbon and nitrogen, HCN can form amino acids, the molecules that make up proteins, when combined with water, and other studies have previously identified it as a potential key ingredient for the origin of life.
The impacts themselves are the problem, Anslow said. "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." The authors also argue that even a successful delivery would probably not have concentrated the chemical enough to be useful, and an impact into an ocean would have dissolved it quickly. Other precursors, such as compounds containing phosphorus and sulfur, are also needed but are not routinely found in comets. "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 did not supply life's building blocks, Anslow said several other avenues remain promising. One long-held competing theory holds that Earth's primordial soup could have synthesized the necessary molecules on its own. Asteroids are another possible source, as they typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings on samples from the asteroids Bennu and Ryugu, which contain most of the protein-building amino acids.
The study also raises the possibility that conditions on Mars were different. Because Mars is smaller than Earth and farther from the sun, comets would have hit it at lower speeds. "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. He added that this offers another perspective as scientists try to understand the early evolution of Mars and whether the red planet once supported life.
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