Glycine in space produced by dark chemistry

Dark chemistry

Comets are the most pristine material in our Solar System and reflect the molecular composition at a time when our Sun and planets were about to form from the chemically treated material in the interstellar medium. The detection of glycine in the comet of comet 67P / Churyumov-Gerasimenko and in the samples returned to Earth by the Stardust mission strongly suggests a prestellar origin of the amino acids. Until recently, it was thought that the formation of glycine occurred through energy radiation, placing clear constraints on the environment in which it can form. New results from scientists working at the Leiden Observatory astrophysics laboratory in the Netherlands show that glycine can form on the surface of frozen dust grains through “dark chemistry”.

Ice sheets

“Dark chemistry means chemistry without the need for energy radiation,” says Sergio Ioppolo (Queen Mary University, London), lead author of the paper that appeared this week in Nature Astronomy. “ In the laboratory, we simulated the conditions in dark interstellar clouds: 10-20 K of cold dust particles are covered with thin layers of abundant ice – CO, NH3, CH4 and H2O frozen – and subsequently processed by atoms that cause development. of precursor species. fragment and reactive intermediates to be recombined. “In this way, the formation of the first methylamine, a precursor species of glycine, was demonstrated and also detected in the coma of comet 67P. Using a unique ultra-high vacuum configuration, equipped with a series of atomic beam lines and accurate diagnostic tools, Ioppolo and colleagues were able to show that glycine can also form, and the presence of ice water is essential in this process.

Create planets

“The important conclusion of this work is that the molecules considered constitutive elements of life are already formed at a stage that is well before the start of the formation of stars and planets,” says Harold Linnartz, director of the astrophysics laboratory at the Observatory. from Leiden. “ Such an early formation of glycine in the evolution of star-forming regions implies that this amino acid can be formed more ubiquitously in space and is conserved in most ice before inclusion in the comets and planetesimals that make up the material from which eventually the planets are made. ‘

Millions of years

The experiments were performed under fully controlled laboratory conditions and show that a non-energetic surface formation pathway for glycine at low temperatures is possible, different from previous work that required UV radiation to produce this molecule. Astrochemical models support this finding and allow for extrapolation of the data obtained on a typical laboratory time scale of only one day under interstellar conditions, spanning millions of years. “From this, we find that low but substantial amounts of glycine can be formed in space over time,” says Herma Cuppen (Radboud University, Nijmegen), who was responsible for some of the modeling studies presented in the Nature Astronomy publication.

Precursor of life

“Once formed, glycine can also become a precursor of other complex organic molecules”, concludes Sergio Ioppolo. “Following the same mechanism, in principle, other functional groups can be added to the glycine backbone, resulting in the formation of other amino acids, such as alanine and serine in dark clouds in space.” Eventually, this enriched organic molecular inventory is included in celestial bodies, such as comets, and delivered to young planets, as has happened to our Earth and many other planets.

Publication

S. Ioppolo et al. A non-energetic mechanism for the formation of glycine in the interstellar medium. Nature Astronomy, November 16, 2020