What if comets had played a much more limited role than previously thought in the emergence of life on Earth?
For a long time, scientists have thought that these blocks of ice and dust might have brought molecules necessary for the emergence of life to our planet. We do know that comets contain various carbon-rich substances. But these molecules must first survive their arrival on Earth.
Comet C2022 E3 (ZTF) - Alessandro Bianconi - CC BY-SA 2.0
This is what a team from the University of Cambridge investigated. The researchers focused in particular on hydrogen cyanide. Despite its alarming name, this molecule can participate in chemical reactions capable of producing certain elements that are part of living organisms. It is not life, but a possible raw material for its formation.
The problem lies in the violence of the impacts. When a comet passes through the atmosphere and strikes the ground at very high speed, it is subjected to enormous heat and pressure. A large proportion of the molecules it carries can be destroyed. And even those that survive may then be too diluted in water to participate effectively in chemical reactions.
To assess the significance of these obstacles, the team ran 100,000 computer simulations of early Earth's history. They varied the conditions of the collisions: the size and speed of the comets, the angle of arrival, and the presence of water or exposed land.
The result is surprising. In 54% of these simulations, no comet manages to deliver hydrogen cyanide to the ground at a concentration considered sufficient for the reactions studied. Even when an impact succeeds, the liquid that subsequently reaches the crater can quickly dilute the molecules. There would then be only a short time for the reactions to take place.
Although the results concern mainly one particular molecule and are based on models, they make it less likely that comets alone supplied a large proportion of the ingredients necessary for the emergence of life.
Other possibilities remain. These molecules may have formed directly on Earth through chemical reactions in the atmosphere or at its surface. Asteroids, which also struck our planet, may have contributed to these supplies as well. The researchers also point out that Mars may have offered more favorable conditions for certain deliveries by comets.
The question of the origin of life is therefore far from settled. Above all, this study shows that bringing molecules from space is not enough. They must also arrive intact, in the right place, and in sufficient quantities.