A study published in Scientific Reports on October 28 questions whether comets supplied early Earth with the concentrated chemicals needed for life to emerge. The researchers found that atmospheric entry, impact damage and dilution made successful delivery unlikely.
The work, led by Richard Anslow at the University of Cambridge, examines the transport of prebiotic molecules: chemicals involved in reactions that can produce the building blocks of living systems. It does not test the delivery of living organisms.
Modeling the delivery of chemicals by comets
The team combined simulations of comet populations and orbits with models of atmospheric entry and chemical survival during impacts. It focused on hydrogen cyanide, or HCN, a molecule found in comets that can participate in prebiotic chemistry.
The distinction between carrying a chemical and delivering it in usable form is central to the study. The simulations assessed whether HCN could survive the journey and remain concentrated enough at the surface to support subsequent reactions.
Several barriers to delivery
The models indicate that Earth’s atmosphere destroys many incoming comets, while the heat generated by high-speed impacts damages fragile molecules. Even material that survives can become diluted in water.
The researchers also considered the limited amount of exposed land on early Earth. Impacts into oceans would disperse surviving chemicals, reducing their concentration. Under the modeled conditions, most simulation runs produced no successful delivery of HCN at the concentration used as the study’s threshold.
What the findings leave unresolved
The team does not rule out an unusually favorable comet impact. However, the Cambridge account says such an event would also need to be followed quickly by suitable chemistry before the delivered material became too diluted.
Other possible sources of the relevant chemicals include reactions occurring on Earth and processes associated with asteroid impacts. The study does not establish which pathway led to life.
The researchers also found more favorable delivery conditions for Mars in their comparison of planets. That result is not evidence that life existed there. It identifies differences in the modeled delivery process, while the conditions needed for later chemistry remain a separate question.
Sources: Scientific Reports — original research, University of Cambridge — research summary.
