Planetary scientist Richard Anslow of the University of Cambridge led research that challenges a long-held theory about life's origins on Earth. The study, published October 28 in Scientific Reports, uses computer modeling to show that comets almost certainly did not deliver the complex organic molecules needed for life to our planet.
Comets travel too fast and enter Earth's atmosphere at angles too steep for organic molecules to survive impact, Anslow's team found. The survival threshold is below 15 kilometers per second. Complex organic molecules are mostly vaporized on impact. On the rare occasions they do survive, their abundance is too low to support prebiotic chemistry.
"It would really have to be a one-in-a-million occurrence, because not only would you need an impact at just the right speed and just the right angle, you would also need everything to happen after that point," Anslow said.
The research used hydrogen cyanide as a proxy for how complex organic molecules behave during comet impacts. Hydrogen cyanide is frequently found in comets, including the interstellar comet 3I/ATLAS.
Anslow's team modeled comet origins, orbits, atmospheric interactions, and hypervelocity impacts. The results suggest asteroids may be more likely candidates for delivering life's building blocks. The OSIRIS-REx mission recovered material from the near-Earth asteroid Bennu that contained all the building blocks of RNA.
Mars presents a different scenario. The Red Planet's thinner atmosphere and lower gravity would give complex organic molecules a slightly better chance of surviving comet impacts, though Anslow cautioned this does not mean it definitely happened there.
Anslow's previous research, published in 2023, described how comets in multi-planet systems around red dwarf stars like TRAPPIST-1 could gravitationally bounce between planets, losing velocity and potentially allowing organic molecules to survive impacts.