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Astronomers have made a startling discovery while looking for complex molecules in deep space. They have detected two of the most complex molecules yet discovered in interstellar space: n-propyl cyanide and ethyl formate. The latter gives raspberries their distinct flavour but also smells like rum. The cyanide compound (also known as butyronitrile) would be deadly poisonous to taste, so best avoided. More importantly, the research has implications for understanding the origins of life. The scientists at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the University of Cologne, Germany, and Cornell University, USA, explain that computational models of interstellar chemistry suggest that even bigger organic molecules may be present, perhaps even amino acids, such as those used to make the artificial sweetener aspartame! The team presented details of their findings during the European Week of Astronomy and Space Science at the University of Hertfordshire, UK, on Tuesday 21st April; no mention of galactic flavours or artificial sweeteners was made, although the scientists are well aware of the earthly applications of the compounds they have observed in space. MPIfR's Arnaud Belloche and colleagues used the IRAM 30-metre telescope located at Pico Veleta in the Spanish Sierra Nevada, at an altitude of 2850 metres. They investigated the emission spectra of molecules in the star-forming region Sagittarius B2, close to the centre of our Milky Way galaxy and focused specifically on a hot, dense cloud of gas known as the "Large Molecule Heimat", which contains a luminous newly formed star. Previous studies have revealed the presence of other, relatively large organic molecules in this cloud, among them including alcohols, aldehydes, and acids. Specifically, ethyl alcohol, formaldehyde, formic acid, acetic acid, glycol aldehyde, and ethylene glycol, have been detected in the Large Molecule Heimat. The discovery of ethyl formate and n-propyl cyanide represent the two most complex molecules of these classes to have been found in the cloud, esters and alkyl cyanides. "The difficulty in searching for complex molecules is that the best astronomical sources contain so many different molecules that their 'fingerprints' overlap, and are difficult to disentangle" explains Belloche. Colleague Holger Müller of the University of Cologne adds that , "Larger molecules are even more difficult to identify because their 'fingerprints' are barely visible: their radiation is distributed over many more lines that are much weaker." The survey of observations carried out by the IRAM telescope has revealed some 3700 spectral lines and the team zoomed in on just 36 lines belonging to the two new molecules. "The assignment of the 36 lines to the two molecules was based on a spectroscopic catalogue of frequencies and intensities of all spectral lines emitted by the molecules as measured in a lab (see, for instance, the Cologne Database for Molecular Spectroscopy)," Belloche told us, "We then used a chemical model to understand the processes that can lead to the formation of these molecules in the interstellar medium. Even in the rarefied world of interstellar space, chemical reactions can take place when gas molecules collide, given enough time, or when cosmic dust particles, small grains of dust suspended in the interstellar gas, act as landing sites for atoms to meet and react, producing molecules. Belloche points out that while one cannot say in general that the grains catalyse the formation of molecules they may speed up the reactions in some instances. It depends on the physical conditions, temperature and density, he adds, although temperature and density are not the only parameters that play a role. The result is that these grains gradually accumulate thick layers of ice, composed mainly of water, but also containing a number of basic organic molecules such as methanol. Sometimes these reactions go further and produce more complex molecules. Cornell's Robin Garrod explains that the chances of anything more complex forming spontaneously through such a simple aggregation process might be unlikely. "The really large molecules don't seem to build up this way, atom by atom," he says. The team's computational models suggest that instead, more complex molecules form section by section, from simple, pre-formed building blocks such as ammonia, water, methanol and other very small organic molecules. "We know that the chemistry on the surface of the grains is able to produce molecules more complex than water or methanol," Belloche adds, "What is not clear is by which processes such molecules do form. What our study shows is that building molecules atom by atom does not seem to be the favoured way on the grain surface (although it would be in principle possible), because it would produce abundance ratios (C3H7CN/C2H5CN for instance) not consistent with the ratios we measured in Sagittarius B2." Team member Robin Garrod of Cornell explains further: "During the star-formation process, the thick layers of molecular ices that have built up on the dust grains are subjected to weak ultraviolet radiation fields. This splits the ice molecules, producing highly reactive molecular radicals, such as CH2, CH3, and CN, that can diffuse around the surface of the grains to meet and react. The models show that these molecular 'functional-group' radicals can concatenate, to form chains, in a series of short steps." In this way, the large quantities of simple organic molecules present in the ices provide the building blocks for much more complex molecules. The team believes that the two newly discovered molecules were produced in this way. As the star-formation process advances, the dust temperatures increase, and all of the molecular material on the grains evaporates into the surrounding gas, allowing it to be detected using radio telescopes, which is what the team has done. These small molecules are always present on the dust grains and the models show how these precursors to molecular functional groups can concatenate, to form chains, in a series of short steps. The team believes that the two newly discovered molecules were produced in this way. "There is no apparent limit to the size of molecules that can be formed by this process," adds Garrod, "so there's good reason to expect even more complex organic molecules to be there, if we can detect them." Astronomers hope to eventually find such molecules, including the amino acid, glycine, which has a similar degree of complexity to ethyl formate and n-propyl cyanide. Indeed, last year the team used the same set of observations to identify aminoacetonitrile which is a likely chemical precursor for glycine. Belloche is uninterested in the earthly flavours of the molecules discovered. "For us, astronomers, it is unimportant. What is important is that these two molecules are quite complex compared to the other molecules discovered in space, and that their discovery suggests that even more complex molecules are likely present in the interstellar medium," he told SpectroscopyNOW. "Our discovery of these two highly complex molecules gives us great hopes for the detection of amino acids or other biologically important molecules in star-formation regions," adds Garrod, "If these molecules are detected, it implies that the molecules necessary for life on Earth (or planets in other stellar systems) are formed at a very early stage, before those planets have even formed. This means that newly formed planets may be "seeded" for life, with these prebiotic molecules already present. It would then be a very much shorter step from the formation of a planet to the inception of life on its surface."
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