Meteorites may have kick-started life on Earth by creating organic molecules as they slammed into the ocean, say Japanese scientists who have done laboratory experiments to test the idea. 

Takeshi Kakegawa and colleagues at Tohoku University produced organic molecules under conditions replicating a meteorite hitting the sea. Their experiments validate a theory first proposed by the American astronomer Carl Sagan in the seventies, says Kakegawa. 'He just mentioned this hypothesis in one sentence in a paper, but he didn't do any experiments. Since then, many people have thought meteorites could produce organic molecules, but we are the first to succeed in showing this.'

The team simulated impacts of meteorites travelling at approximately two kilometres per second. They fired mini missiles at mixtures of graphite, iron, water, nitrogen and ammonia to generate carboxylic acids and amines, as well as small amounts of the amino acid glycine. Graphite and iron are components of chrondrites - the rocks most commonly recovered from meteorite hits. According to Kakegawa, the missiles, like their larger counterparts, create high pressure, high temperature (above 2600 degrees Celsius) conditions that promote synthesis of organic molecules.

Although most materials would be vapourised at such high temperatures, the researchers think that on early Earth large amounts of biomolecules would have been produced as water cooled following an impact. Theoretically, a succession of meteorite hits could have led to the synthesis of increasingly complex molecules.

But Jeffrey Bada, an expert in prebiotic synthesis at the University of California, San Diego, doesn't think the results are particularly impressive. 'With the exception of methyl amine and acetic and propanic acid, the yields are very, very small,' he says. 'If glycine is indeed made, the amount that would be present in the ocean from one such impact would be about 10^-30  grams per litre - hardly a meaningful concentration.'

Kakegawa admits glycine concentrations obtained in the impacts are low. However, he thinks that the sheer force of a meteorite impact is enough to suggest such events had an important role in prebiotic synthesis. 'Some people say submarine hot springs or submarine volcanoes could have produced prebiotic organic molecules but if you consider the total energy that submarine volcanoes can produce, it's very small. In comparison, meteorite energy is huge.'

Hayley Birch

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