The long-held belief that comets played a pivotal role in sparking life on Earth billions of years ago is now in jeopardy, as new computer simulations challenge this established view, reports BritPanorama.
These findings indicate that delicate organic molecules transported by comets would have been eradicated upon impact, contradicting a theory of life’s origins that has persisted since at least 1961. The broader concept that life’s essential ingredients were delivered from space, known as panspermia, has been considered even longer and was first proposed in 1908.
Comets, composed of ice and dust, may also contain organic material essential for life. This composition, which led to their being dubbed “dirty snowballs,” spawned theories suggesting they deposited water and organic compounds on the early Earth. Some estimates indicate that during a period over 4 billion years ago, the planet could have endured bombardment by more than 100,000 comets exceeding 100 meters (328 feet) in size, according to a new study published Wednesday in the journal Scientific Reports. However, these impacts were generally too severe for any life-sustaining molecules to withstand.
The research team employed advanced models of comet formation and evolution, running simulations of the early Earth period approximately 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,” stated lead author Richard Anslow, who conducted this research while pursuing his doctorate at the Institute of Astronomy at the University of Cambridge.
The simulations demonstrated that while comets carry a significant amount of essential compounds for life, their velocities upon impact were too great to allow for effective delivery. “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 explained.
Importantly, the research found that 54% of the simulations indicated no impact at all, with the remaining 46% recording at least one instance, and a “tiny minority” experiencing up to three collisions. “We can’t rule out confidently that there was never a successful impact — there could have been one of these fluke impacts,” Anslow acknowledged. Still, he noted that such successful impacts would require “a perfect location” that included both liquid water and other essential chemicals for life, making them highly unlikely.
Alternative explanations
The study highlights hydrogen cyanide (HCN), a vital component in prebiotic chemistry detected in various comets, including Halley’s and Hale-Bopp. Composed of hydrogen, carbon, and nitrogen, HCN can result in the formation of amino acids when combined with water; other studies have recognized HCN as a crucial ingredient in life’s genesis.
Anslow remarked, “When comets strike Earth, they’re shocked to high temperatures and pressures, and recent studies have discovered that many of these precursor molecules can be obliterated by such conditions.” He added that even if an impact managed to occur, it remains improbable that HCN would have been delivered in sufficient concentrations, as it would rapidly dissolve in ocean waters.
The research points out that while comets are proficient at delivering HCN, they lack many other necessary precursor molecules, such as phosphorus and sulfur compounds. Anslow emphasized, “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 serve as the primary source of life’s building blocks on Earth, Anslow mentioned several alternative avenues for exploration. One competing theory suggests that Earth’s primordial environment could have generated the necessary molecules independently. Another potential source of organic material may be asteroids, which typically travel slower and strike with greater frequency. Recent findings from asteroids Bennu and Ryugu reveal the presence of amino acids essential for proteins.
This latest study also opens up the intriguing notion that if Earth lacked the ideal conditions for HCN from comet impacts, ancient Mars might have been better suited. Given Mars’ smaller size and its distance from the sun, the velocities of comet impacts there were comparatively lower, likely allowing HCN to survive in greater quantities.
“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 remarked.
Comet theory could still stand
While the authors investigated how HCN might endure comet impacts, Queenie Chan, an associate professor of Earth sciences at Royal Holloway University of London, suggested that reactions could have taken place before comets entered Earth’s atmosphere, potentially forming significant biologically relevant compounds that are better equipped to withstand such impacts.
“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 noted in correspondence.
Nicolle Zellner, a professor of physics at Albion College in Michigan, expressed that she was not surprised by the findings. Although she is cautious about dismissing comets as a possible contributor to life’s origins, she acknowledged that “the authors have definitely put some good parameters around the likelihood.”
