Building blocks of life can form in interstellar clouds through ‘dark chemistry’
Glycine, an amino acid, can form in interstellar clouds without the heat of a star.
The molecule glycine is a favourite among astrophysicists looking for the origins of life out among the stars. And now, scientists have shown that glycine can form in cold, dark interstellar clouds before any stars or planets.
Glycine is an amino acid – that is, a type of molecule that can be strung together to form proteins. In fact, glycine is the simplest example of an amino acid, which is what makes its presence in space so exciting: by the same process that glycine is formed, extra molecular chunks can be tacked on to form more complex amino acids.
"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,” said Dr Sergio Ioppolo of Queen Mary University of London, and lead author of the paper. “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."
Glycine has been found in space before. NASA's Stardust mission, launched in 1999, was the first to return a sample of a comet to Earth. It scooped up some molecules of glycine in its samples of the comet Wild 2. ESA's Rosetta mission orbited the comet 67P/Churyumov-Gerasimenko, and found some in its coma.
Since our Solar System’s comets formed just before the Sun and the planets, these findings showed that glycine could be produced in an interstellar cloud before a star was formed. However, it was assumed that it did need some outside energy in the form of UV light.
Read more about the origins of life:
- Phosphorus, a key ingredient of life, could have been brought to Earth on a comet
- Cosmic origins: the miraculous journey from dust clouds to life
Now, an international team of scientists have shown that glycine can form through ‘dark chemistry’, i.e. without external energy. "Dark chemistry refers to chemistry without the need of energetic radiation,” Dr Ioppolo explained.
The team created the conditions of an interstellar cloud in the lab: glacial temperatures of 10-20 Kelvin (-263 to -253°C), and dust particles covered in thin layers of different types of ice. First, this formed a precursor to glycine, methylamine, which was also found in comet 67P. The team then showed that glycine also formed – but only in the presence of water ice.
"The important conclusion from this work is that molecules that are considered building blocks of life already form at a stage that is well before the start of star and planet formation," said Prof Harold Linnartz, Director of the Laboratory for Astrophysics at Leiden Observatory.
"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."
Reader Q&A: How do atoms ‘know’ what other atoms to bond to?Asked by: Paul Layfield, via email
Atoms bond with each other in order to make their arrangement of negatively-charged electrons more stable. These electrons lie in so-called ‘shells’ around the positively charged nucleus, and each shell becomes stable once it contains a certain number of electrons, as dictated by quantum theory. Bonding allows atoms to achieve this stability by swapping or sharing electrons with other atoms until each has their shells filled.
So, for example, sodium and chlorine atoms bond because the outer shell of sodium can become stable by losing an electron, while the latter can do so by gaining an electron. The two atoms are held together because losing an electron makes the sodium atom positively charged, while gaining an electron makes the chlorine atom negatively charged – and opposite charges attract.
This is known as ‘ionic bonding’. Another way of bonding is to share electrons – ‘covalent bonding’. For example, oxygen atoms need two more electrons for their outer shells to become stable, and they can do this by sharing two electrons with another oxygen atom. Sharing then makes both atoms stable, and the resulting bond creates an oxygen molecule.