Dark chemistry in a lab as cold as space

An international team of astrochemists has produced glycine – the simplest amino acid – in a lab that simulates the dark, cold conditions of interstellar space.

Intriguingly, they showed that the molecular structure could form without UV light. This opens up the possibility that amino acids, which are the building blocks of proteins and therefore life, could predate stars and planets.

Scientists have been ruminating on this tantalising idea for over a decade; glycine was discovered in the comet Wild 2 by NASA’s Stardust spacecraft in 2009, and in the coma of 67P/Churyumov-Gerasimenko by ESA’s Rosetta mission in 2014.

Discovering glycine in an interstellar gas cloud itself is a kind of holy grail for astronomers, but so far the molecule has been elusive.

Researchers have also produced glycine in the lab before, but the process always required UV radiation or energetic particles to induce reactions and allow radicals and ions to form more complex molecules. This suggests that radiation, like that from starlight, was necessary to form amino acids.

But this new study indicates otherwise.

The researchers, mostly based at Leiden Observatory in the Netherlands, produced glycine on the surface of icy dust grains through “dark chemistry”.

“Dark chemistry refers to chemistry without the need of energetic radiation,” explains lead author Sergio Ioppolo from Queen Mary University of London in the UK.

“In the laboratory we were able to simulate the conditions in dark interstellar clouds where cold dust particles are covered by thin layers of ice and subsequently processed by impacting atoms causing precursor species to fragment and reactive intermediates to recombine.”

The team then used astrochemical models to confirm the results and to extrapolate further, finding that glycine could be formed slowly but steadily over millions of years in the interstellar medium.

The results are published in the journal Nature Astronomy.

Astronomer Chenoa Tremblay from Australia’s CSIRO, who was not involved in the study, explains: “Laboratory experiments and molecular modelling are necessary for us to understand our astronomical observations and provide input as to how to best utilise the time we observe the sky.”

Using both approaches, according to Tremblay, is a smart move by the researchers to balance the weaknesses of each.

Experiments, she says, “are getting better at mimicking the temperatures but still have a hard time matching the gas densities of space. The models mentioned in this paper suggest that the reaction to form glycine would happen in the dense gas.”

Another independent researcher, astrochemist Courtney Ennis from the University of Otago in New Zealand, notes that the experiments discussed may be a bit removed from the models.

“For example, methylamine and molecular oxygen O₂ are key precursors in their laboratory recreations of this chemistry. However, both these starting molecules are understood to be formed through energetic processing on ice grains, even if ultimately the glycine doesn’t require energetic processing to be formed in this experiment.”

Ennis says that despite this the outcomes are “interesting in that OH radicals can initiate interesting chemical pathways at very low temperature – evidently pathways that can lead to important organics molecules for life.”

It is particularly exciting that these molecules can form in the dense interstellar medium well before the start of star and planet formation.

Harold Linnartz, co-author from Leiden Observatory, comments: “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 preserved in the bulk of ice before inclusion in comets and planetesimals that make up the material from which ultimately planets are made.”

The Royal Institution of Australia has an Education resource based on this article. You can access it here.