New research casts doubt on the long-standing hypothesis that comets delivered the organic chemistry from which life on Earth emerged, concluding that such deliveries would have been exceedingly rare events.
Comets are known to carry organic compounds, including the amino acid glycine, phosphorus, and hydrogen cyanide (HCN), a molecule considered a key feedstock for prebiotic chemistry. The European Space Agency's Rosetta mission visited comet 67P/Churyumov-Gerasimenko in 2014 and detected glycine and phosphorus, helping to generate the idea that comets could have seeded early Earth with the chemistry necessary for life.
New research published in the journal Scientific Reports argues against that idea. Led by Dr. Richard Anslow of the University of Cambridge's Institute of Astronomy, the study combined simulations of comet impacts, knowledge of how comets interact with atmospheres, and dynamic modelling of short-period comets, focusing on HCN because of its critical role in prebiotic synthesis.
The results suggest the cometary bombardment of early Earth did not last long enough, and that too few molecules survived the impacts. Smaller comets are often torn apart as they enter the inner Solar System, and many are destroyed during atmospheric passage. "Taken together, the rapid decline in Earth's cometary impact bombardment and the efficient destruction of fragile cometary organics indicate that the successful delivery of prebiotic feedstock molecules was an exceedingly rare event," the authors write.
"Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules, so these giant space snowballs were almost like a 'get out of jail free card' that brought all the molecules a planet would need to seed life," said Anslow. "It's kind of a romantic idea, but our results suggest it's highly unlikely."
The researchers also discounted the idea that impacts could have created subterranean hydrothermal systems conducive to life, since rapid inflow of groundwater would have quickly diluted surviving HCN below prebiotically relevant concentrations. The hypothesis is not widely accepted, but has not been conclusively disproven. Previous work has suggested the survivability of HCN in impacts has been overestimated, with only the most favorable impacts, at low angle, low velocity and small radius, avoiding destructive temperatures.
Analysis
Why This Matters
- The study challenges a popular explanation for how life's raw materials reached early Earth, pointing researchers toward other possible sources of prebiotic chemistry.
- It identifies a very narrow set of conditions under which fragile molecules such as HCN could have survived an impact, sharpening the debate over abiogenesis.
- Because the same logic applies to any planet, the findings could inform how scientists think about the ingredients for life elsewhere in the Solar System and beyond.
Background
Comets are known to be rich in volatile organic molecules, many of which are thought to be limiting for prebiotic chemistry. The idea that an early period of cometary bombardment supplied the early Earth with reactive bioessential feedstock molecules has long been speculated, and it gained support when the Rosetta mission detected glycine and phosphorus at comet 67P in 2014. Hydrogen cyanide has emerged as a key feedstock molecule in this picture, because it can drive high-energy reactions central to prebiotic synthesis. The new study confronts that picture with simulations of impacts, atmospheric interactions, and the dynamics of short-period comets.
Key Perspectives
Study authors, led by Dr. Richard Anslow: argue that the successful delivery of prebiotic feedstock molecules by comets was an exceedingly rare event, because Earth's cometary bombardment declined rapidly and fragile organic molecules were efficiently destroyed.
Proponents of the cometary-delivery hypothesis: held the view, described in the paper as not widely accepted but not conclusively disproven, that comets carried carbon- and nitrogen-bearing molecules and could have acted almost like a "get out of jail free card," bringing all the molecules a planet would need to seed life.
What to Watch
- Whether future simulations identify any combination of impact angle, velocity and comet size that could deliver HCN in meaningful concentrations.
- Whether researchers extend the analysis to other sources of prebiotic feedstock molecules on early Earth.
- Whether the field's assessment of the cometary-delivery hypothesis shifts as the modelling is debated.
Checked
12 claims in this story checked against 1 source: 12 supported.
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- SupportedComets were unlikely to have delivered life's building blocks to early Earth, according to the study.Universe Today: “But new research shows that it is unlikely that comets seeded Earth with the chemistry that led to life.”
- SupportedThe summary says modelling suggests organic molecules were destroyed in impacts or diluted below useful concentrations.Universe Today: “A salient outcome of high-velocity cometary impacts, irrespective of the delivery of HCN, is thus the rapid influx of local groundwater into the newly-formed crater-lake, rapidly diluting surviving cometary HCN below prebiotically relevant concentrations”
- SupportedThe research challenges the long-standing idea that comets supplied early Earth with prebiotic chemistry and concludes successful delivery was exceedingly rare.Universe Today: “Taken together, the rapid decline in Earth’s cometary impact bombardment and the efficient destruction of fragile cometary organics indicate that the successful delivery of prebiotic feedstock molecules was an exceedingly rare event,”
- SupportedThe authors wrote that successful delivery of prebiotic feedstock molecules was “an exceedingly rare event.”Universe Today: “Taken together, the rapid decline in Earth’s cometary impact bombardment and the efficient destruction of fragile cometary organics indicate that the successful delivery of prebiotic feedstock molecules was an exceedingly rare event,”
- SupportedAnslow said scientists historically thought comets were important for life's origins because they carried carbon- and nitrogen-bearing molecules, likening them to a “get out of jail free card.”Universe Today: ““Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules, so these giant space snowballs were almost like a ‘get out of jail free card’ that brought all the molecules a planet would need to seed life,” said lead author Anslow .”
- SupportedAnslow said, “It's kind of a romantic idea, but our results suggest it's highly unlikely.”Universe Today: ““It’s kind of a romantic idea, but our results suggest it’s highly unlikely.” - Dr. Richard Anslow, Institute of Astronomy, University of Cambridge.”
- SupportedComets carry organic compounds including glycine, phosphorus and hydrogen cyanide.Universe Today: “Scientists have found that comets carry important organic compounds. That has led to the hypothesis that comets could have seeded Earth with chemistry that led to abiogenesis . These include the amino acid glycine, phosphorous, and hydrogen cyanide (HCN), a chemical compound of particular importance.”
- SupportedRosetta visited comet 67P/Churyumov–Gerasimenko in 2014 and detected glycine and phosphorus, helping generate the idea that comets could seed early Earth with life's chemistry.Universe Today: “The ESA's Rosetta mission, which included the Philae Lander, visited the comet 67P/Churyumov–Gerasimenko in 2014 and detected glycine, an amino acid, and phosphorous, an organic chemical. ... These findings helped generate the idea that comets played a role in seeding the early Earth with the chemistry necessary for life.”
- SupportedThe research was published in Scientific Reports.Universe Today: “The research is titled " Cometary impacts an unlikely pathway towards the origins of life on Earth ," and it's published in the journal Scientific Reports.”
- SupportedThe study was led by Dr Richard Anslow of the University of Cambridge's Institute of Astronomy.Universe Today: “The lead author is Dr. Richard Anslow from University of Cambridge’s Institute of Astronomy.”
- SupportedThe study combined comet-impact simulations, information about comet-atmosphere interactions and dynamic modelling of short-period comets, focusing on HCN because of its importance.Universe Today: “The researchers worked with simulations of comet impacts, discoveries about how comets interact with atmospheres, and dynamic modelling showing where short-period comets originate and how they evolve. They focused on HCN, since it plays such a critical role.”
- SupportedThe researchers discounted the idea that impacts could create life-conducive subterranean hydrothermal systems because groundwater would rapidly dilute surviving HCN below prebiotically relevant concentrations.Universe Today: “A salient outcome of high-velocity cometary impacts, irrespective of the delivery of HCN, is thus the rapid influx of local groundwater into the newly-formed crater-lake, rapidly diluting surviving cometary HCN below prebiotically relevant concentrations”
