A comet's tale of life on earth

Ab initio molecular dynamics simulations and detailed analysis hint at how conditions on the early earth might have been ripe for a cometary impact to generate small organic molecules. The study suggests that the formation of molecules akin to the simple amino acid glycine may have been viable. Follow-up spectroscopic studies may demonstrate the validity of the hypothesis.

NASA scientists discovered glycine, the simplest amino acid, in samples of comet Wild 2 returned by NASA's Stardust spacecraft in 2009. More recently scientists have added yet more organic molecules to the list of those present in cosmic dust, including the buckminsterfullerene molecule the silver anniversary of whose discovery by Sir Harry Kroto et al was celebrated in 2010.

However, the delivery of prebiotic compounds to the early Earth by a cometary impact - or even panspermia - has been seen by many scientists as an unlikely mechanism for the origins of life. They point out that the unfavourable chemical conditions on the planet coupled with the intense heat (thousands of degrees) from such an impact would not result in a pre-biotic chemical legacy as any "seed" compounds would be instantly pyrolysed. Ice from comets are mainly composed of water but also contain several small molecules that one might consider important to prebiotic aqueous chemistry, including carbon dioxide and methanol as carbon sources, and ammonia as a nitrogen source for the construction of primordial amino acids.

Nir Goldman, Evan Reed, Laurence Fried, William Kuo and Amitesh Maiti of the Physical and Life Sciences Directorate, at Lawrence Livermore National Laboratory, in California, beg to differ. They have carried out atomistic simulations of a shocked compressed cometary ice mixture impacting a planet at an oblique angle. The sidelong impact can generate lower pressures and temperatures that could allow for survival of organic species. Upon expansion to ambient conditions, Goldman et al. observed chemical complexes closely resembling the amino acid glycine.

The researchers point out that other work on shock-compression experiments on cometary mixtures has shown that a high percentage of pre-existing amino acids can survive relatively low pressure conditions and that mixtures resembling carbonaceous chondrites can produce a variety of organic compounds at pressures of about 6 gigapascals, which is significantly lower pressure than the current study.

These ab initio molecular dynamics simulations demonstrate how shock waves can drive forward the synthesis of transient carbon-nitrogen bonded oligomers at relatively high temperatures and pressures (thousands of degrees Kelvin up to about 60 GPa in pressure). Following the hypothetical impact, quenching to lower pressures leads to the breaking apart of these oligomers, which could then form amino acids such as glycine.

"We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet," the team explains. The team concludes that their simulations provide "a possible mechanism for the 'shock synthesis' of prebiotic molecules on the early Earth." The advent of experimental methods capable of monitoring complex time-dependent chemical reactivity in shock-compressed systems hold much promise of verifying this synthetic route, they say.

Such experiments are unlikely to settle the arguments between those who suspect life arose spontaneously from the Earth's primordial soup and those who suspect the seeds were transported to our planet from elsewhere in the solar system.

"Our work is not intended to show that the origin of prebiotic compounds is exclusively from comets," Goldman told SpectroscopyNOW. "Rather, our work discusses the plausibility of one specific hypothesis, namely that an impacting comet could have helped create prebiotic materials on early Earth. We show that the actual impact event could have created a unique synthetic route for the creation of protein building amino acids."