A long-standing theory that comets helped trigger life on Earth billions of years ago may not hold up, according to new computer simulations that contradict decades of scientific thinking. The research suggests that the fragile organic molecules supposedly carried by these icy bodies would have been destroyed on impact, casting doubt on an idea that dates back at least to 1961.
The broader notion that the ingredients for life arrived from space, known as panspermia, is even older and was proposed scientifically in 1908. Life on Earth requires complex organic molecules, but it remains unclear whether the early planet had enough raw materials for prebiotic chemistry, the reactions that produce the basic components of life. Comets are made of dust and ice but are also known to contain organic material, a composition that has earned them the nickname “dirty snowballs” and led scientists to suggest they could have deposited water and organic molecules on the young Earth.
According to the new study, published Wednesday in the journal Scientific Reports, Earth may have endured intense bombardment early in its history, more than 4 billion years ago, from more than 100,000 comets larger than 100 meters across. Most of those impacts, however, would have been too violent for the molecules needed for life to survive. The research team used the latest models of comet formation and evolution to simulate this primitive period 100,000 times.“Given what we know about the surface environments of the early Earth, our simulations revealed that, on average, there were no successful impacts,” said Richard Anslow, the study’s lead author, who carried out the work while completing his doctorate at the Institute of Astronomy at the University of Cambridge in the United Kingdom.
The simulations found that 54 percent produced no impact at all, while 46 percent recorded at least one, and a “tiny minority” produced up to three collisions, Anslow said. The comets contained large concentrations of key molecules needed for life, but the simulations showed they arrived too fast to deposit them effectively. “The typical speeds of these comets when they hit Earth meant that very few, if any, would have been able to deliver intact organic molecules, so comets do not appear to be as useful for prebiotic chemistry as previously thought,” Anslow said.
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 well-known Halley and Hale-Bopp. Made of hydrogen, carbon and nitrogen, HCN can lead to the formation of amino acids, the molecules that make up proteins, when combined with water. Other studies had already identified HCN as a possible key ingredient for the origin of life on Earth.
“When comets strike Earth, they are subjected to extreme temperatures and pressures, and recent studies have found that many of these precursor molecules can be destroyed by this high pressure and heat as well as by subsequent impacts,” Anslow said. The authors argue that even a successful impact would be unlikely to deliver the chemical in a useful concentration, and that an ocean impact would have dissolved it quickly. Many other precursor molecules, such as compounds containing phosphorus and sulfur, are also needed to set the stage for prebiotic chemistry, but these are not commonly found in comets, Anslow added.
“Comets are very good at carrying hydrogen cyanide, but since where a comet hits Earth is random, there is no guarantee that we would have everything else we need for this chemistry in that same place.” If comets did not bring life’s building blocks to Earth, Anslow said other promising avenues remain to be explored. A long-held rival theory holds that Earth’s primordial soup could have synthesized the molecules needed for life on its own. Asteroids offer another possible extraterrestrial source of organic material, since they typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings in samples from the asteroids Bennu and Ryugu, which contain most of the amino acids that form proteins.