Life, Unbounded
Interstellar Environments May Breed Complex Organic Molecules
If biologically important organic molecules like amino acids could form in interstellar space, the implications would be enormous. On the Earth we find plenty of amino acid species inside certain types of meteorites, so at a minimum these compounds can form during the assembly of a proto-stellar, proto-planetary system (at least this one) and end [...]
By Caleb A. Scharf on September 30, 2014
Hub of complex chemistry? Our galactic center (MSX/IPAC/NASA)
If biologically important organic molecules like amino acids could form in interstellar space, the implications would be enormous.
On the Earth we find plenty of amino acid species inside certain types of meteorites, so at a minimum these compounds can form during the assembly of a proto-stellar, proto-planetary system (at least this one) and end up intact on the surface of rocky planets. The implication is that there may be a 'starter-mix' of pre-biotic chemistry for young planets that comes from off-world, and which could play a role in the origins of life.
However, it's not been clear whether this chemical heritage might extend further back in time and space, to the interstellar environment. Detecting complex molecules through astronomical spectroscopy is a tricky business, dealing in radio to infrared wavelengths, and with poorly understood spectral features that can all run together into a great big mess when peered at from afar. Although the majority of identified interstellar molecular species are organic, confirming the presence of chains and structural forms with more than a handful of carbon atoms is tough.
So a paper published in Science last week by Belloche and colleagues is particularly intriguing, because it claims the detection of what's called a 'branched alkyl' - the molecule iso-propyl cyanide (i-C3H7CN) - deep inside the most massive nebula region of star formation in our galaxy, Sagittarius B2, some 27,000 light years away in the galactic core. Using the Atacama Large Millimeter/Submillimeter Array (ALMA), together with laboratory work on the spectral signature of this kind of molecule, the researchers identify both the branched and 'straight' version of propyl cyanide (shown left to right in the figure below).
Left: 'branched' iso-propyl cyanide, Right: 'straight-chain' normal-propyl cyanide.
What's very exciting is that this kind of branch structure is a key feature of side-chain molecules like amino acids, but until now only the straight-chain structure has been detected in similar interstellar molecules.
The likely reaction pathways to either molecular structure involve the environments present on the surface of ice-covered dust grains, but it seems that it may only be in the denser and more energy-rich environment of a place like Sagittarius B2 that conditions allow for branched isomers to form, and possibly dominate.
In other words, rich, star-forming regions may indeed have the right conditions to generate increasingly complex organic molecules (including amino acids) long before they end up mixed into the intense environment of a youthful planetary system. Unless all of that deep-space chemistry is undone - which seems unlikely - some of these molecules will be the very ones we find in meteorites and which must have painted the surface of a young Earth.
A next step for the astronomers is to start looking for an increased level of complexity; the butyl cyanides (n-C4H9CN), which boasts 3 branched isomers. The hunt is well and truly on for what could be our ancestral molecules.
Caleb A. Scharf
Caleb A. Scharf is director of astrobiology at Columbia University. He is author and co-author of more than 100 scientific research articles in astronomy and astrophysics. His work has been featured in publications such as New Scientist, Scientific American, Science News, Cosmos Magazine, Physics Today and National Geographic. For many years he wrote the Life, Unbounded blog for Scientific American.
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