Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites—over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body. As the most ancient and pristine bulk material studied in the laboratory, primitive meteorites are the clearest windows to the birth of the Solar System. Planetary systems such as our own are believed to form from the collapse of an interstellar dense molecular cloud composed of gas and sub-micrometre sized grains. In such ‘dark’ clouds the temperatures are low (T , 50 K), and all but the most volatile species (that is, H2, He, Ne) condense onto grains, coating them with a thin layer of ice. This ice is composed primarily of amorphous H2O, but usually also contains a variety of other simple molecules, such as CO2, CO, CH3OH, and NH3. Laboratory studies and astronomical observations indicate that radiation processing of such ices can create complex organic compounds. Many of the organic molecules that are present in carbonaceous chondrites (primitive carbon-rich meteorites) and comet and asteroid dust are thought to come, at least in part, from the ice and complex compounds constructed in the interstellar medium (ISM). Perhaps the most convincing molecular evidence for the interstellar heritage of meteoritic molecules is their high deuterium (D) enrichment. At the low temperatures in dense molecular clouds deuterium fractionation is efficient and elevated D/H ratios are seen in grain mantles; such increased ratios are also found in several gas-phase interstellar molecules, including amino acid precursors, such as formaldehyde and ammonia. Although it had been accepted that the deuterium in meteoritic organics indicated that their precursors formed in the ISM, the actual hydroxy and amino acids are still commonly believed to have formed on the asteroid or comet parent body from reactions in liquid water that, at least in Murchison, was apparently deuterium poor. It is difficult to explain how these compounds retain relatively high amounts of deuterium, let alone how it is distributed. For example, it seems contradictory that the hydroxy acids in the Murchison meteorite have one-third as much deuterium as the amino acids, and yet have a lower rate of deuterium exchange in water than amino acids. If, however, the hydroxy and amino acids had formed in the ISM their deuterium enrichment would be a logical consequence of the photochemistry of already deuterium-enriched pre-solar ices. The laboratory experiments described here were designed to elucidate potential pathways from interstellar chemistry to the organic molecules extracted from meteorites. We have conducted laboratory experiments at temperatures, pressures and radiation conditions that are representative of the interstellar clouds from which planetary systems form. In a series of experiments, gases were vapour deposited onto a substrate at 15 K forming an ice film consisting primarily of amorphous H2O with other compounds over a range of concentrations (0.5–5% NH3, 5–10% CH3OH and 0.5–5% HCN, relative to H2O). These solid mixtures are