Published online 26 March 2010 | Nature | doi:10.1038/news.2010.152

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Shock waves could force amino-acid forming chemistry.

Hitting a planet at the right angle could trigger the formation of molecules necessary for life on Earth. NASA

Striking a glancing blow to a planet could create the perfect conditions in a comet's icy core to create amino acids — molecules that are vital to forming life on Earth.

This shock-compression theory for making amino acids has been developed by Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in Livermore, California. Goldman presented their results on 24 March at the American Chemical Society meeting in San Francisco, California.

The researchers wanted to find out what chemical events might occur in an ice grain trapped inside a comet glancing off a planet. They used around one million computer hours on the powerful Atlas computer cluster at Lawrence Livermore to simulate the possible chemical processes occurring in a single ice grain during such an impact. In particular, they were looking for amino acids — markers of potential life.

Previous theories for how amino acids on Earth might have come into being include lightning strikes on a primordial soup of simple molecules or the ultraviolet irradiation of interstellar dust grains, but none of the theories proposed so far is definitive.

Goldman's simulations included 210 molecules: a mixture of water, methanol, ammonia, carbon dioxide and carbon monoxide. This mix is commonly used by scientists to represent ice in comets.

First impact

When a comet strikes a planet, a shock wave travels through it as it comes to a sudden halt. This, Goldman explains, compresses the comet, and the compression wave travels through the comet faster than the speed of sound. As a result, the molecules inside deform and bonds break.

Goldman's group based its models on the impact that a comet travelling at 29 kilometres per second would be likely to experience. The impact had to be a side-on blow because a head-on impact would probably destroy everything inside.

To unpick the chemistry going on inside the ice, the researchers used density functional theory simulations, a quantum mechanical treatment of the electrons in a molecule. In the model, if the electrons around the atoms come close enough to those around other atoms a bond will form.

The first and weakest shock compression that Goldman and his colleagues modelled had a pressure of 10 gigapascals and reached a temperature of 700 kelvin. The grain was compressed by 40%. The team noticed that molecules with carbon–nitrogen bonds were forming, including an unstable molecule called carbamide. This was a hint that amino-acid-forming processes were possible. "Under these sorts of conditions everything's very reactive, so if you have one sort of morsel that has an essential component like a C–N bond you can imagine more carbons adding to it and getting a complicated amino acid," says Goldman.

In further simulations, in which the pressures and temperatures were higher, the scientists saw more chemistry. They focused on a simulation at 47 gigapascals and a temperature of 3,141 kelvin for the first 20 picoseconds of the impact. They saw many complex molecules forming, including large molecules with carbon–nitrogen bonds.

And relax

After the initial impact, the compressed comet relaxes, cools down and expands, events that Goldman and his co-workers recreated in the next stage of their simulation. After 50 picoseconds of relaxation Goldman's group saw just five types of molecule with carbon–nitrogen bonds including hydrogen cyanide and more carbamide. There were also several hydronium ions — water plus a hydrogen ion. More interestingly, the team also saw what looked like the amino acid glycine with carbon dioxide stuck to it.

Goldman is certain that glycine would ultimately be formed in such a collision, although the simulation was too complex to run for long enough to show this. The glycine/carbon dioxide complex would react spontaneously with a hydronium ion to produce glycine, water and carbon dioxide, he says. "We see things that are one step away from forming glycine. What that really means is if it were easier to run our simulations for longer, let's say up to a microsecond, the glycine would form readily."

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This is the first suggestion, says Goldman, that a shock impact could drive interesting chemistry within a comet.

The work is theoretically sound, says Murthy Gudipati, an expert in interstellar ice at NASA's Jet Propulsion Laboratory in Pasadena, California. However, he would like to see further calculations to work out the probability of these amino-acid-forming events actually happening. Goldman's simulations could also be tested experimentally, he says.

That the ingredients for making amino acids are held within comets has been known for some time, says Gudipati, and laboratory studies have demonstrated that radiation can trigger the formation of amino acids in comet-like ice. The new study is "the icing on the cake", Gudipati says, although is not the only process to consider. "A comet heading towards early Earth may already have been loaded with prebiotic molecules."