Did comets really deliver the building blocks of life on Earth? A new study challenges assumptions - CNN

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.

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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.

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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.