At least one prebiotic molecule, an ingredient for building life, can form in the harsh environment of interstellar space, far from stars and planets, new research shows.
The amino acid glycine - the simplest amino acid, without which life can't exist - was thought to require irradiation from stars to form. But new laboratory experiments show that it can form via what is known as "dark chemistry", which takes place without energetic irradiation.
Of particular interest is its presence in the atmosphere of Comet 67P/Churyumov-Gerasimenko, hinting that the molecule could form independently of the Sun or planets.
But laboratory experiments and modelling had suggested that glycine forms when interstellar ice is bathed in radiation - ultraviolet, cosmic, thermal, X-ray - in the later stages of star formation.
At high enough energies, radiation can destroy amino acids, so a team of astronomers led by astrochemist Sergio Ioppolo of Queen Mary University London in the UK set out to see if there were alternative formation routes.
And they found one.
"In the laboratory," Ioppolo said, "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 research started with methylamine, an amine precursor to glycine.
Although we have no proof of the presence of glycine in the interstellar medium, astronomers have found methylamine - and methylamine was also detected on Comet 67P/C-G. In an independent set of experiments, the researchers showed that methylamine can form non-energetically in interstellar conditions.
Next, the researchers used a methylamine-enriched ice to determine whether glycine can form under similar conditions.
They deposited this in gas form into an ultra-high vacuum system called SURFRESIDE2, designed specifically for investigating surface reactions in interstellar space. The system was cooled to an interstellar temperature of 13 Kelvin (-260 degrees Celsius, or -436 degrees Fahrenheit) to allow ice to form.
Chemical reactions in the ice did indeed result in the formation of glycine, the researchers found. And that ice was essential to the process.
Next, they used astrochemical modelling to validate their findings. They extrapolated their experimental results, obtained in the timeframe of just one day, to the millions of years that cosmic processes can extend. And they found that glycine should be able to form in interstellar space, in small but significant quantities, given enough time.
It's not likely that these molecules can develop much further towards life in the freezing cold space vacuum.
What the research does mean is that glycine and methylamine can form in space before star formation (and, subsequently, planet formation) kicks off. Which, in turn, means that there's potentially a lot of prebiotic molecular material out there, trapped in ice, that then gets accreted onto meteorites, comets, planetesimals and ultimately planets.
"Once formed, glycine can also become a precursor to other complex organic molecules," Ioppolo said.
"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. In the end, this enriched organic molecular inventory is included in celestial bodies, like comets, and delivered to young planets, as happened to our Earth and many other planets."
The research has been published in Nature Astronomy.