Life's molecules in outer space

An astronomical infrared study reveals one of the most complex organic molecules yet found in the interstellar medium - anthracene - offering possible new clues to the way the building blocks of life might have emerged.

Anthracene is a solid polycyclic aromatic hydrocarbon comprising a structure of three fused benzene rings in a row. Industrially, it can be produced from coal-tar or other residues of thermal pyrolysis and is commonly used as a starting material for dyestuffs, including the red dye alizarin. It is also used as an additive in wood preservatives, insecticides, and coating materials. Anthracene has no colour but does exhibit blue fluorescence with a peak at between 400 and 500 nanometres under ultraviolet light irradiation.

Now, in work that reveals this relatively complex small organic molecule to exist way beyond the realm of coal-tar and wood preservatives, a team of scientists from the Instituto Astrofísica de Canarias (IAC), Spain, and the University of Texas, USA, has identified its presence in the interstellar medium. This latest molecular discovery adds to the short list of organic molecules known to exist in space, which includes the much simpler methane, formaldehyde, and ketene. The present study used infra-red observations from the William Herschel Telescope at Roque de los Muchachos Observatory on La Palma in the Canary Islands and from the Hobby-Eberly Telescope in Texas.

The fact that small organic molecules containing, carbon, hydrogen, oxygen, and nitrogen, can exist in the interstellar medium and have been detected using radio, microwave, or infrared portions of the spectrum, suggests to some that the very seeds of life, precursors to the amino acid building blocks and other molecules, might be ubiquitous throughout these regions of space and may have relevance in the emergence of life on earth through deposition via a meteorite impact, for instance, or they may simply have been presence in the cloud of dust and gases from which the solar system formed.

The discovery of anthracene in the interstellar medium, however, reveals an important aspect to this field of research and could help resolve a decades-old astrophysical mystery concerning the production of organic molecules in space.

"We have detected the presence of anthracene molecules in a dense cloud in the direction of the star Cernis 52 in Perseus, about 700 light years from the Sun," explains Susana Iglesias Groth, the IAC researcher heading the study. In her opinion, the next step is to investigate the presence of amino acids. Hydrocarbon molecules like anthracene are essentially prebiotic, but when they are subjected to ultraviolet radiation in the presence of small inorganic molecules, such as water and ammonia, which can act as a source of oxygen and nitrogen, it is possible to yield amino acids and other compounds essential for the development of life. Groth worked with Rafael Rebolo of the Instituto Astrofísica de Canarias, David Lambert of the University of Texas, and others.

The presence of diffuse spectroscopic bands in the spectra of the interstellar medium has puzzled researchers since at least as early as the 1980s; there are hundreds of such bands. Their existence has hinted at the presence of small molecules in the interstellar medium, but many of those bands have remained unassigned.

"Two years ago," adds Iglesias, "we found proof of the existence of another organic molecule, naphthalene, in the same place, so everything indicates that we have discovered a star formation region rich in prebiotic chemistry." Until now, anthracene had been detected only in meteorites and never in the interstellar medium. Oxidized forms of this molecule are common in living systems and are biochemically active. The new finding suggests that a good part of the key components in terrestrial prebiotic chemistry could be present in interstellar matter.