Comets probably weren’t reliable couriers for the ingredients that helped life begin on Earth. A new Cambridge-led study found that fragile organic molecules would usually have been destroyed when the comets carrying them struck the surface.
Published in Scientific Reports, the research tracked hydrogen cyanide, a molecule involved in chemistry that can precede life. Rare successful deliveries remain possible, but the simulations offered little support for a steady supply from comets.
Why is comet delivery so difficult
In 2023, lead researcher Richard Anslow and colleagues proposed that gravitational encounters with closely spaced planets could slow comets down. A slower collision would give fragile molecules a better chance of reaching a planet’s surface intact.
The new study tested that idea in our solar system, combining models of comet evolution with simulations of atmospheric entry and impacts. Earth-bound comets generally arrived too fast and at angles too steep for those molecules to survive. Hydrogen cyanide, regularly detected on comets, provided a way to test whether their chemical cargo could withstand the collision.
Why wasn’t surviving an impact enough
Surviving hydrogen cyanide also had to accumulate in useful concentrations. There were, at most, a handful of opportunities for that delivery in Earth’s early history. Comet impacts declined rapidly during the planet’s first 500 million years, further limiting the chances.
Any hydrogen cyanide that reached the surface wouldn’t have stayed concentrated for long. Reactions leading toward life would have needed to start quickly after impact, before that temporary supply dispersed.
Where does this leave life’s origins
Asteroid impacts, which happened far more frequently than comet strikes, could have helped create the molecules needed for life. Chemical processes on Earth itself offer another possible route, without requiring those ingredients to arrive intact from space.
Mars offered somewhat better odds for comet delivery, although success there was still unlikely. Successful delivery to either planet remains unconfirmed, leaving a substantial gap between finding life’s ingredients on a comet and showing that they helped life begin.
