The idea that primitive life on Earth may have been seeded by a comet or asteroid impact is controversial. Since it was suggested more than 40 years ago, however, increasingly complex organic molecules have been discovered in space. Now astronomers have detected an amino acid - one of the building blocks of proteins - in interstellar dust clouds in our galaxy. The discovery of glycine in space suggests that interstellar molecules may have played a pivotal role in the prebiotic chemistry of the Earth (Y-J Kuan et al. 2003 Astrophys. J. 593 848).
Glycine – CH2NH2COOH – is the simplest of all the 20 amino acids. Yi-Jehng Kuan of the National Taiwan Normal University and co-workers from the NASA Ames Research Center and the Polish Academy of Sciences searched for the molecule in the hot cores of three giant molecular clouds, which are regions of active star formation. They measured the spectral lines of the clouds – Sagittarius-B2, Orion-KL and W51 – over a four-year period using the 12-metre telescope at the National Radio Astronomy Observatory (NRAO) in Arizona.
The frequencies of certain transitions in glycine, which are known from experiments in the lab, provide a characteristic signature for the molecule. Knowing this spectral “fingerprint”, the researchers were able to identify 27 glycine lines at frequencies between 90 and 265 GHz in the clouds. This confirms the results of earlier searches for interstellar glycine in which tantalizing evidence was provided by a handful of spectral lines.
Observing the spectral lines in these clouds – which are tens of thousand of light-years away – is not easy because the lines are very weak. Moreover, the molecular transitions that cause them can be contaminated by emissions from “interloper” molecules in the surrounding medium. To overcome these difficulties, Kuan and co-workers concentrated on high-frequency molecular transitions that occur in warm, dense regions of the clouds. The chemical process that actually produces glycine in the interstellar medium is not understood, although lab-based experiments suggest that amino acids can be created by exposing organic molecules in interstellar ice to ultraviolet radiation.
The researchers claim that the discovery of glycine is the first step in establishing the crucial link between amino acids in space and the emergence of life in the solar system or, indeed, elsewhere in the galaxy. The molecular spectra seen in interstellar gas clouds closely matches those found in comets and meteorites, and comparing them could in principle allow astronomers – or exobiologists – to trace the origin of the Earth’s early chemistry to its parent gas cloud.