Shock synthesis of amino acids from impacting

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Sep 15, 2013 - Impacts of icy bodies (such as comets) onto rocky surfaces, and, equally ... observed in comets; for example, Halley, Hyakutake, Tempel-1,.
ARTICLES PUBLISHED ONLINE: 15 SEPTEMBER 2013 | DOI: 10.1038/NGEO1930

Shock synthesis of amino acids from impacting cometary and icy planet surface analogues Zita Martins1† , Mark C. Price2 *† , Nir Goldman3 , Mark A. Sephton1 and Mark J. Burchell2 Comets are known to harbour simple ices and the organic precursors of the building blocks of proteins—amino acids—that are essential to life. Indeed, glycine, the simplest amino acid, was recently confirmed to be present on comet 81P/Wild-2 from samples returned by NASA’s Stardust spacecraft. Impacts of icy bodies (such as comets) onto rocky surfaces, and, equally, impacts of rocky bodies onto icy surfaces (such as the jovian and saturnian satellites), could have been responsible for the manufacture of these complex organic molecules through a process of shock synthesis. Here we present laboratory experiments in which we shocked ice mixtures analogous to those found in a comet with a steel projectile fired at high velocities in a light gas gun to test whether amino acids could be produced. We found that the hypervelocity impact shock of a typical comet ice mixture produced several amino acids after hydrolysis. These include equal amounts of D- and L-alanine, and the non-protein amino acids α-aminoisobutyric acid and isovaline as well as their precursors. Our findings suggest a pathway for the synthetic production of the components of proteins within our Solar System, and thus a potential pathway towards life through icy impacts.

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mpact-shock synthesis of organic molecules may have been one of the sources of organic carbon in our Solar System 4.5–3.8 billion years ago1–4 , just before life emerged (for example, refs 5–7). In refs 8,9, it was demonstrated that shock heating of reducing gas mixtures in the laboratory produced amino acids in high yields. Recently, ab initio molecular dynamics simulations were used to show that shock waves passed into ice mixtures representative of comets could theoretically yield amino acids by a synthetic route, independent of the pre-existing atmospheric conditions and materials on a planet10 . Amino acid precursors (such as ammonia, methanol and carbonyl compounds) have been observed in comets; for example, Halley, Hyakutake, Tempel-1, Giacobini–Zinner, Hartley 2 and Hale–Bopp11–17 , and glycine was recently detected on comet 81P/Wild 2 from samples returned by NASA’s (National Aeronautics and Space Administration’s) Stardust mission18 . Further organic materials could be synthesized within the interior of comets during shock compression and survive the high pressures and temperatures created by shock waves19–24 . In addition, the impact of a comet, or asteroid, onto an icy surface, such as the moons of Jupiter and Saturn, may result in the synthesis of complex organic compounds within the target surface, including amino acids. According to observations made by Cassini’s Visual and Infrared Mapping Spectrometer instrument, the surface of Saturn’s moons show infrared absorption features characteristic of ammonia hydrate and, within the plumes of Enceladus, water, CO2 , ammonia and methanol have all been detected25 . The surface of Enceladus is also known to be composed of water ice (pure, amorphous and crystalline), short-chain organics and CO2 (ref. 26), although the presence of methanol on its surface is yet to be confirmed. There are also detections of ammonia ice on Titan27 and methanol ice on trans-neptunian objects and Centaurs in the outer Solar System28 . Moreover, it has recently been determined that methanol can be trapped within a clathrate hydrate, providing a potential, and so far undetected, reservoir of

methanol (and possibly other volatiles) on the surface of icy bodies throughout the Solar System29 . Therefore, given the plethora of Solar System bodies that seem to have the necessary starting composition for shock synthesis of complex organics, we set out to experimentally verify whether the impact shock applied to a typical cometary ice mixture would synthesize amino acids.

Hypervelocity impact shock of a typical comet ice mixture Several ice mixture batches with differing compositions mimicking the ice present in comets11–17 were prepared (Supplementary Table S1), shocked in the University of Kent’s light gas gun24 (Methods) and were subsequently analysed to test for the presence of amino acids. The conditions to identify and quantify amino acids are described in the Methods, and in Supplementary Methods, Text, Fig. S1 and Table S2. The ice mixture that gave rise to detectable levels of amino acids was a mix of NH4 OH (a 28% by mass solution of ammonia gas dissolved in H2 O), CO2 and CH3 OH ices in the molar ratio of 9.1:8:1 impacted at 7.15 km s−1 and 7.00 km s−1 . The total mass of the shocked ice mixture was 500 g, but hydrocode modelling of the impacts implies that