A University of Cambridge-led study suggests comets were unlikely to have delivered the molecular building blocks for life on Earth. Using computer modelling, the researchers found that comet impacts would usually destroy fragile organic molecules rather than preserve them, making the once-popular idea of a steady cometary “rain” of prebiotic material highly unlikely.
Key facts
- The study was led by researchers at the University of Cambridge and published in Scientific Reports.
- The team modelled comet impacts, comet-atmosphere interactions and the origin and evolution of comets.
- They focused on hydrogen cyanide, or HCN, a prebiotic molecule found on Solar System comets.
- The researchers say successful delivery to Earth cannot be ruled out completely, but would be extremely rare.
- Mars was identified as a slightly more promising target than Earth, though the researchers still say delivery there is highly unlikely.
What does this mean for origin-of-life research?
For researchers studying how life began, the practical takeaway is that comets now look like a poor explanation for Earth’s first supply of prebiotic molecules. The study points instead to other possibilities, including chemistry driven by asteroid impacts or processes that happened on Earth itself.
The findings also affect how scientists think about life in other planetary systems. The authors say future observatories could use the possibility of “bouncing” comets as one way to identify systems worth closer study, but only where the orbital setup makes such delivery plausible.
- Best-fit scenario in this study: a one-off fluke, not a steady source of prebiotic material.
- Most relevant molecule examined: hydrogen cyanide.
- Comparative implication: cometary delivery may have been more plausible for Mars than for Earth, but still unlikely.
Why do the simulations rule out comet delivery so strongly?
The researchers say comets generally hit Earth too fast and at too steep an angle for fragile organic molecules to survive. Even in the rare case that some hydrogen cyanide made it through impact, the number of suitable impacts in Earth’s early history appears to have been very small.
They found that comet impacts declined rapidly during Earth’s first 500 million years, leaving at most a handful of chances for useful delivery. Any molecules that did survive would also have needed to stay concentrated long enough for prebiotic chemistry to continue, which the team says would have been difficult.
Where could those molecules have come from instead?
The study does not identify a single replacement explanation, but it does outline two main alternatives. One is that asteroids, which hit Earth far more often than comets, may have helped synthesize the needed molecules. Another is that the chemical precursors formed on Earth without any outside delivery at all.
The researchers say the comet hypothesis is not impossible in a strict sense. But their overall conclusion is that it would have required an exceptionally rare set of conditions: the right impact speed, the right angle, the right chemistry, and the right timing.
Could Mars have been a better target?
Yes, at least in principle. The team found that Mars, because it is smaller than Earth and has a thinner atmosphere, would have given comets a better chance of reaching the surface during the era when comet impacts were more common.
That does not mean the process happened there. The researchers say it is possible that comet delivery was unlikely on Earth and also did not happen on Mars. But if a rocky planet were to receive prebiotic material from comets, Mars appears to be the slightly more promising candidate.
What does this matter for future searches for life?
The authors say the results could help future astronomers narrow their search for life in other systems. As successors to the James Webb Space Telescope and proposed missions such as the Habitable Worlds Observatory come online, scientists may look for planetary arrangements where comets could “bounce” from one world to another.
That kind of system would still be unusual, based on the modelling in this paper, but the study suggests it could be one clue in deciding where to look next.
