Gas: Of the organic interstellar kind

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  • Published: Oct 1, 2014
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Gas: Of the organic interstellar kind

Stellar array

Dust and molecules in the central region of our Galaxy: The background image shows the dust emission in a combination of data obtained with the APEX telescope and the Planck space observatory at a wavelength around 860 micrometers. The organic molecule iso-propyl cyanide with a branched carbon backbone (i-C3H7CN, left) as well as its straight-chain isomer normal-propyl cyanide (n-C3H7CN, right) were both detected with the Atacama Large Millimeter/submillimeter Array in the star-forming region Sgr B2, about 300 light years away from the Galactic center Sgr A*.  © MPIfR/A. Weiß (background image), University of Cologne/M. Koerber (molecular models), MPIfR/A. Belloche (montage).

A carbon-based molecule that is not the usual fodder of interstellar dust has been detected in a giant gas cloud some 27000 light years from earth. The discovery offers a new hint as to the origins of the complex organic molecules that were the building blocks of life in our planet's primordial soup.

Astronomers from Cornell University, New York, the Max Planck Institute for Radio Astronomy and the University of Cologne, Germany have used radio observations from the Atacama Large Millimeter/submillimeter Array, known as the ALMA Observatory, a group of radio telescopes funded partially through the National Science Foundation, to study the gaseous, star-forming region Sagittarius B2. The data reveal the presence of a branched organic molecule isopropyl cyanide. The complexity of this molecule in the seeming simplicity of a gas cloud lends weight to the evidence that the complex organic molecules necessary for the emergence of life on Earth may have their origins between the stars.

Clear view

The organic molecules that have been observed in these star-forming regions previously are usually straight-chain organics. However, there are two branches in the structure of isopropyl cyanide. Cornell's Rob Garrod, of the University's Center for Radiophysics and Space Research points out that this is the first branched compound to be detected in interstellar space. The finding detection gives us a new frontier in the complexity of molecules that can be formed and that might ultimately find their way to the surfaces of planets, Garrod suggests.

Branched carbon chains, are certainly a common feature among the molecules from which life is constructed, the amino acid building blocks of proteins, for instance. Amino acids have been found in meteorite material, verifying that they can form in space. Garrod, along with lead author Arnaud Belloche and Karl Menten, both of the Max Planck Institute for Radio Astronomy, and Holger Mueller, of the University of Cologne, were well aware that the Sagittarius B2 region held promise as this part of the Milky Way’s galactic centre had revealed a rich seam of other interstellar organic molecules.

However, with ALMA, the group was able to conduct a full spectral survey - looking for the characteristic fingerprints of previously unobserved interstellar molecules. ALMA offered sensitivity and resolution some ten times greater than that possible with earlier surveys. The ALMA Observatory has array of 66 sensitive radio antennas sited in the high elevation and dry air of northern Chile's Atacama Desert - near-perfect conditions for peering through the Earth's atmosphere at distant portions of space with little interference from water vapour and other problems.

Organic progression

"Understanding the production of organic material at the early stages of star formation is critical to piecing together the gradual progression from simple molecules to potentially life-bearing chemistry," explains Belloche. The team found approximately 50 individual features characteristic of isopropyl cyanide (and 120 for normal-propyl cyanide, the straight-chain sibling of this compound) in the ALMA spectrum of the Sagittarius B2 region. These two molecules are the largest non-cyclic molecules detected so far in any star-forming region.

The team explains the link between this finding and amino acids in their paper in the journal Science: Our astrochemical model indicates that both isomers are produced within or upon dust grain ice mantles through the addition of molecular radicals," they explain. However, the mechanisms of formation are different for each. "The production of iso-propyl cyanide appears to require the addition of a functional group to a non-terminal carbon in the chain," they add. Crucially, this kind of chemistry is exactly what would be required to the formation of the side-chain structure of amino acids, the researchers suggest.

"The numerical simulations we (my collaborator Robin Garrod) performed to understand the chemical processes that lead to the formation of the branched molecule iso-propyl cyanide suggest that we could expect branched molecules to be fairly abundant in regions where stars are being formed," Belloche told SpectroscopyNOW. "We would like to test this hypothesis by detecting the next member of the alkyl cyanide series, butyl cyanide (C4H9CN). This molecule exists in four distinct forms (structural isomers), three of them having a branched structure. Once the spectroscopic information from the laboratory will be available, we will start looking for them in the spectra we collected with ALMA. But we may need to perform more sensitive observations with the full ALMA in the near future to eventually detect them. Ultimately, we would be delighted to detect glycine, the simplest amino acid, but my expectation is that we will need another order of magnitude improvement in sensitivity to be able to detect this molecule in the interstellar medium."

Related Links

Science, 2014, 345, 1584-1587: "Detection of a branched alkyl molecule in the interstellar medium: iso-propyl cyanide"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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