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Comet impacts may have led to life on Earth — and perhaps elsewhere


Dr. Koichi Mimura, from Nagoya University and Dr. Haruna Sugahara, from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in Yokohama performed a sequence of experiments to mimic the conditions of comet collisions on the early Earth around 4 billion years ago, at the time when life first appeared.
They took frozen mixtures of water ice, amino acid and silicate (forsterite) under cryogenic conditions (77 K), and used a propellant gun to reproduce the shock of a comet impact. After analyzing the post-impact combination with gas chromatography, they discovered that some of the amino acids had joined into short peptides of up to 3 units long (tripeptides).

C/2006 P1 Comet McNaught, the ‘Great Comet of 2007′, as seen from Swift’s Creek, Victoria, Australia on 23 January 2007.

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Image credit: Fir0002 / Flagstaffotos / Wikimedia Commons CC BY-SA 3.0.

Based on the experimental data, the scientists were able to approximate that the total of peptides produced would be around the same as had been thought to be emitted by typical terrestrial processes.
According to Haruna Sugahara, “Our experiment showed that the cold conditions of comets at the time of the impacts were key to this synthesis, as the type of peptide formed this way are more likely to evolve to longer peptides. This finding indicates that comet impacts almost certainly played an important role in delivering the seeds of life to the early Earth. It also opens the likelihood that we will have seen similar chemical evolution in other extraterrestrial bodies, starting with cometary derived peptides. Within our own solar system the icy satellites of Jupiter and Saturn, such as Europa and Enceladus are likely to have undergone a similar comet bombardment. Indeed, the NASA Stardust mission has shown the presence of the amino acid glycine in comets. The production of short peptides is the key step in the chemical evolution of complex molecules. Once the process is kick-started, then much less energy is needed to make longer chain peptides in a terrestrial, aquatic environment. Comet impacts are normally associated with mass extinction on Earth, but this works shows that they probably helped kick-start the whole process of life in the first place.”
Professor Mark Burchell (University of Kent, UK), said: “This is a new piece of work which adds significantly to the exciting field of the origin of complex molecules on the Earth. It has long been known that ices under shock can generate and break bonds in complex organics. The detection of amino acids on comet 81P/Wild 2 by the NASA Stardust mission in the last decade, and the now regular exciting news from the Rosetta mission to comet 67P/Churyumov-Gerasimenko indicates that comets are a rich source of materials. Two key parts to this story are how complex molecules are initially generated on comets and then how they survive/evolve when the comet hits a planet like the Earth. Both of these steps can involve shocks which deliver energy to the icy body. For example, Zita Martins and colleagues recently showed how complex organic compounds can be synthesized on icy bodies via shocks. Now, building on earlier work, Dr. Sugahara and Dr. Mimura have shown how amino acids on icy bodies can be turned into short peptide sequences, another key step along the path to life.”

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