Simulation Finds Comet Impacts Unlikely to Have Delivered Life's Building Blocks
A new simulation study suggests most comet impacts were too fast and steep for hydrogen cyanide to survive, challenging the idea that comets seeded life on Earth.
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A simulation study published October 28 in Scientific Reports finds that comets were unlikely to have delivered life's chemical building blocks to Earth, because most impacts would have destroyed hydrogen cyanide. Lead author Richard Anslow of the UK Met Office says only about one impact over more than a billion years could have delivered a meaningful amount.
- Simulations suggest most comet impacts were too fast and steep for hydrogen cyanide to survive.
- Only one impact over more than a billion years could have delivered a meaningful amount, requiring a comet over 100 meters wide, slow-moving, and hitting at an oblique angle.
- A surviving comet would also have needed to land on land, not ocean, to enable useful chemistry.
- Comets may have been more likely to deliver carbon-based molecules to Mars than to Earth, the simulations showed.
Simulation tests survival of a key molecule
A new simulation study published October 28 in Scientific Reports finds that comets were unlikely to have delivered life's chemical building blocks to Earth. The researchers modeled whether hydrogen cyanide, a carbon- and nitrogen-containing molecule, could survive a comet's passage through Earth's present atmosphere and its impact with the surface. The early Earth's atmosphere was probably denser, which would have been even more destructive, the study notes.
Based on the simulations, the team found that an impact preserving hydrogen cyanide would be extremely unlikely. On average, only one impact over more than a billion years could have delivered a meaningful amount. To survive, a comet would have to be larger than 100 meters across, slow moving, and traveling at an oblique angle. It would also have had to strike land rather than ocean; otherwise any surviving hydrogen cyanide would be diluted too quickly.
Narrow window and alternative sources
Lead author Richard Anslow, a planetary scientist at the UK Met Office in Exeter, said even though comets contain these molecules in high concentrations, it is not straightforward to conclude they were available for prebiotic chemistry on Earth's surface. The study does not completely rule out comets as a source, but it contradicts the long-standing idea that they seeded Earth with the chemicals necessary for life.
The simulations showed comets may have been more likely to deliver carbon-based molecules to Mars than to Earth, because Mars' thinner atmosphere and smaller size meant lower impact speeds. Gordon Osinski, a planetary geologist at Western University who was not involved in the study, said carbon-rich asteroids called carbonaceous chondrites, volcanoes, and hydrothermal vents are other possible sources of Earth's carbon-based chemicals. He added that the study will make people think twice about comets as the originator of organics.
Why might comets have been more likely to deliver organic molecules to Mars than to Earth?
Mars' thinner atmosphere and smaller size meant comets would impact at lower speeds, allowing more compounds to survive the collision, according to the simulations reported by Science News.
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