The gas between stars has now been found to contain a 13-atom sulfur ring that adds to a long-standing gap between the simple chemistry that astronomers regularly observe in space and the more complex sulfur compounds observed in meteorites.
Sulfur is in a difficult position in astrochemistry. It is the 10 th richest element in the universe and on earth it forms the basis of amino acids, proteins and enzymes. However, thick molecular clouds, cold reservoirs of gas and dust which eventually give rise to stars and planets have recurrently appeared deficient in sulfur in discernible forms. The incompatibility of that has been referred to as the sulfur depletion problem: there should be plenty of sulfur there, but it usually seems to be bound up in carriers which telescopes can hardly detect.
The novel molecule is advantageous in that it cuts across an empirical line. Hundreds of interstellar molecules have been catalogued by astronomers, although sulfur-containing species have been small. In comparison, there is a broad variety of thick sulfur organic matters in extraterrestrial rocks. The observation of the six-membered carbon ring 2,5-cyclohexadiene-1-thione (C6H6S) containing sulfur proved that even large, cyclic sulfur chemistry can be formed in advance of the presence of planets to concentrate and preserve it.
The long, long ago, sulfur arrived on Earth due to space, claimed Mitsunori Araki of the Max Planck Institute of Extraterrestrial Physics, the leader of the research. But there is a very small quantity of sulfur bearing molecules that we have detected in space, which is unusual. It must be there in very big quantities, yet it is very hard to obtain.
The molecule was identified in G + 0.693 -0. 027, a molecular cloud located in the centre of the Milky Way approximately 27, 000-light-years away. These clouds have been referred to as stellar nurseries: dust grains and gas condensate, clump and, due to the pull of gravity, turn into raw material of new stars and ultimately planetary systems. According to Valerio Lattanzi, one of the coauthors of the work, the ingredients that will be present in the molecular cloud will be passed over to the planets. The chemistry of a cloud in that sense is not an astronomical curiosity, but rather a list of the inventory of future worlds.
The team needed to construct this specific compound on earth before getting it. The molecule was generated by applying an electric discharge to thiophenol and then a very finely resolved radio fingerprint of the rotational spectrum was measured. This laboratory reference was compared with current survey data of the IRAM 30-m and Yebes 40-m radio telescopes, which displayed the same spectral features in emission with G+0.693.
External scholars pointed to the importance of that workflow. Kate Freeman of Penn State had called the work “an exciting detective story made possible by powerful radio telescopes and a really good search strategy.” Meteorites do have huge sulfur compounds, although it has been hard to identify where their chemical antecedents were formed; an interstellar find provides a reasonable upstream origin.
Other sources of laboratory data indicate a further concealment of sulfur: ices on dust grains. Radiation processing experiments on cold clouds have also formed chains of sulfanes and even rings of elemental sulfur, indicating that some of the missing sulfur is also being transformed into forms that are not easily observable using standard infrared techniques. What this means is that individual molecules will not solve the problem of depletion, but rather interstellar sulfur chemistry will be divided between gases, ice and less-obvious products, each of which will demand a different detection instrument.
It has one practical implication that is methodological. The same study reports that, similar mass compounds tend to confound identifications by mass-spectrometry in extra-solar samples, that is, some sulfur rings may have been present in solar-system material and not identified as such. To the engineers of instruments as well as to the astrochemists, this is the message that a way to go through clouds to comets to planets can only be rebuilt when laboratory spectra, telescope surveys and sample analysis come to an accord.
