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Origins of life

Shock synthesis

When asteroids or comets smash into Earth, they form craters such as the one at Gosses Bluff, Australia (pictured). But could such an impact have kick-started life on our planet? Nir Goldman and colleagues have modelled what happens inside cometary ice when it smashes into a planet (N. Goldman et al. Nature Chem. doi:10.1038/nchem.827; 2010). They find that, under certain circumstances, complexes form that can act as precursors to glycine, the simplest amino acid.

Credit: S. ALVAREZ/NATL GEOGR. STOCK

The idea that comets delivered organic molecules to the early Earth is disputed, because the extreme heat generated by the impact would probably have incinerated such a cargo. An alternative theory is that the heat and pressure of cometary impacts could have caused material on Earth's surface to react to form organic compounds. But the early Earth's chemical environment isn't thought to have been conducive to such reactions.

Impacts in which Earth was struck glancingly, however, would have generated much lower temperatures within cometary ice, allowing organic matter to survive. Goldman et al. therefore used quantum molecular dynamics to simulate events inside cometary ice during such collisions.

The authors modelled a mixture of water, methanol, ammonia, carbon monoxide and carbon dioxide under conditions of shock compression, and found that many products were formed — including oligomers that contained the carbon–nitrogen bonds required for amino acids. Subsequent quenching of the model system to lower pressures and temperatures broke the complexes into smaller fragments, including a glycine–CO2 complex.

Of course, we may never know how life began on Earth. But Goldman et al. calculate that the probability that any particular cometary impact will generate shock waves of the size modelled is 17% — well within the realms of possibility.

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Mitchinson, A. Shock synthesis. Nature 467, 281 (2010). https://doi.org/10.1038/467281a

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