Literally. The Kleinmann-Low massive star-forming region is part of Orion’s molecular cloud, and is therefore known as Orion KL. In its surroundings, the possible presence of ethyl mercaptan, a gas characterized by its smell of rotten eggs, has been detected. Hence the phrase, uttered by Marcellus’ character in the first act of Shakespeare’s Hamlet, fits to perfection in this story.
Mercaptan (better known as thiol) is a compound formed by a sulfur atom and a hydrogen atom (-SH). In the case of methyl mercaptan (CH3SH), we are talking about a colorless gas that gives off a strong odor.
But for those who investigate its presence in space, smell is not a problem. Any possible presence of molecular gas is capable of study, whatever its characteristics are (smell of flowers or sewer). In fact, methanol (CH3OH) and ethanol (CH3CH2OH), two molecules chemically similar to those already mentioned in which only one atom (OàS) changes -and which we are also going to talk about- are well known in many astrophysical environments.
The methyl mercaptan itself was detected in 1979 by Linke and collaborators at SgrB2, a molecular cloud of the galactic center known to be one of the most productive massive star formation areas in the galaxy. Subsequent studies also detected the presence of methyl mercaptan in two very different locations: in the cold molecular cloud B1 (this cloud is located in the so-called First Hydrostatic Core stadium, which is formed when the collapse phase at the star’s birth stage stops); and towards the hot core of G327.3-0.6, a massive star-forming region.
For the research team I’m going to talk about, the detection of methyl mercaptan is actually a key to try to determine whether another compound, ethyl mercaptan (CH3CH2SH), the thiol equivalent for ethanol, is present in those same areas.
Ethyl mercaptan does not leave its methyl relative behind in terms of danger: extremely irritating, corrosive and sensitizing in case of skin contact and very dangerous in case of eye contact, inhalation or ingestion. Nor is its smell very pleasant. In fact, it is one of the components added to butane and propane gas: these gases have no odor, so, to detect leaks, this odorant compound is added to them. Hence, when there’s a gas leak, it smells like rotten eggs.
In a work led by Lucie Kolesniková, involving other members of Consolider ASTROMOL (Belén Tercero and José Cernicharo, among others) has detected the possible presence of ethyl mercaptan towards Orión KL.
A survey of this area, the most active part of the Orion Nebula, carried out in the range of millimeter waves with the IRAM 30m radio telescope , resulted in the detection of more than 8,000 spectral lines unknown so far. Each spectral line is identified by a particular transition of a molecular species (the footprint left by the molecule when it passes from one energy state to another). Many of them (nearly 4,000) have already been identified as isotopologue-derived lines  and excited vibrational states  of abundant and well-known species in the interstellar environment, significantly reducing the number of unknown lines and mitigating the confusion of lines in the spectra.
Several of these lines could correspond to ethyl mercaptan transitions, but to confirm this, experiments had to be conducted in molecular spectroscopy laboratories where the frequencies of the ethyl mercaptan rotation spectrum could be measured and predicted with sufficient spectroscopic accuracy. These experiments resulted in the possible presence in Orion of the gauche and trans conformers of ethyl mercaptan.
What is a conformer? Atoms, when joined, can do so in different ways. Their simple links allow them to rotate the groups, as they have cylindrical symmetry. Because they can rotate around their link, they adopt different spatial arrangements: this is called conformation, and a specific conformation is a conformer. The gauche conformer (left, in French) has the hydrogen of the SH group at 120o and 240o with respect to the link –SH, and the trans at 0o.
Para esta especie, el gauche es el confórmero más estable de los dos y, aunque no se puede asegurar al cien por cien la presencia del confórmero trans (aún debe confirmarse con posteriores comprobaciones observacionales), es la primera vez que se detecta mercaptano de etilo en el medio interestelar. Aún así, la detección está en el límite de nuestras posibilidades de detección y el equipo recomienda ser cautos hasta que lleguen nuevas observaciones que permitan corroborarla.
For this species, the gauche is the most stable conformer of the two and, although the presence of the trans conformer cannot be guaranteed 100 percent (it has yet to be confirmed with subsequent observational checks), it is the first time that ethyl mercaptan is detected in the interstellar medium. Even so, detection is at the limit of our detection possibilities and the team recommends being cautious until new observations arrive to corroborate it.
The dust grains
Comparing the proportion of ethyl mercaptan to methyl mercaptan, the results confirm that the second would be ≃5 times more abundant than the first in the hot core of Orion KL. That difference was actually expected to be much greater, but why might it not be?
This study found that, in Orion KL, methanol (CH3OH) is 30 times more abundant than ethanol (CH3CH2OH), from which it could be inferred that, as methyl mercaptan (CH3SH) and ethyl mercaptan (CH3CH2SH) are chemically similar to the previous ones, the difference should also be similar. However, as we have already pointed out, the relationship between CH3SH/CH3CH2SH was only 5, creating a somewhat surprising value.
Researchers believe the difference could be due to methanol and methyl mercaptan remaining for different times on the dust grains before evaporation. If methyl mercaptan sticks to dust grains longer than methanol, more chemical processes could take place, changing the abundances between methyl (CH3-) and ethyl (CH3CH2-) species.
Considering that the emission of the species −OH in Orion KL comes mainly from a compact area at about 150 K of kinetic temperature (called compact ridge) while the ethyl and methyl mercaptan emit from the hot core at a temperature of more than 200 K, the relative abundance differences between the methyl and ethyl species could also be due to chemical differentiation between these two regions within Orion KL.
More sensitive observations are needed to derive the possible amounts of the two ethyl mercaptan conformers and study their spatial distribution in Orion. This will be possible with interferometers such as ALMA, the crème de la crème of the cold universe and a tool that will undoubtedly bring great answers to the field of Astrochemistry.
The universe never ceases to amaze us by its richness. Recently, the discovery of methyl acetate and the gauche conformer of ethyl formate, among others (another result obtained by members of Consolider ASTROMOL) were released. Many of the lines observed in space remain unidentified, and the more sensitive our instruments are, the more abundant and defined those lines are. Confirmation of new molecular species, based on precise laboratory measurements, remains a field of research of great activity. And while important steps have been taken, there is still work ahead to confirm whether something actually smells rotten in Orion KL.
 The IRAM 30m telescope is a millimeter telescope equipped with a series of receivers that allow it to make high-resolution spectroscopy, thus studying the role that chemistry plays in the formation of stars, both within giant molecular clouds of the Milky Way and in nearby galaxies, or off, on farthest known galaxies of the young universe. The telescope is located on Pico Veleta, in Sierra Nevada (Granada, Spain), at an altitude of 2,850 m. More than 50 molecules have been detected with this telescope, most of them by scientists from this Spanish team (Consolider ASTROMOL).
 When we have two atoms of the same element with the same number of protons, but with a different amount of neutrons in their nucleus, we have two isotopes. When, on the other hand, we have a molecule that has at least one isotope, then we will have an isotopologue of that molecular species.
 Molecules have different energy levels: electronic, vibrational and rotational. Because the energy is quantized, we can know what kind of transition has taken place when a molecular species is excited or deexcited. Within a particular electronic state, the molecule can reach different types of vibrational states (those produced by the vibration of the atoms that make up the molecule) and, in turn, within the same vibrational state, the molecules rotate, producing a rotation spectrum that can be detected with radio telescopes in the domain of millimeter and submillimeter waves.
These results were published in the scientific paper “Spectroscopic characterization and detection of Ethyl Mercaptan in Orion“, in the Astrophysical Journal, 784, L7.
The authors are L. Kolesniková (Molecular Spectroscopy Group (GEM), Spectroscopy and Biospectroscopy Laboratories, Unidad Asociada CSIC, University of Valladolid, Spain); Belén Tercero (Astrophysics Department, Astrobiology Center CAB, CSIC-INTA, Spain); José Cernicharo (Astrophysics Department, Astrobiology Center CAB, CSIC-INTA, Spain); J. L. Alonso (Molecular Spectroscopy Group (GEM), Spectroscopy and Biospectroscopy Laboratories, Unidad Asociada CSIC, University of Valladolid, Spain); A. M. Daly (Molecular Spectroscopy Group (GEM), Spectroscopy and Biospectroscopy Laboratories, Unidad Asociada CSIC, University of Valladolid, Spain); B. P. Gordon (Division of Natural Sciences, New College of Florida, Sarasota, Florida, USA); and S. T. Shipman (Division of Natural Sciences, New College of Florida, Sarasota, Florida, USA).
In the box we can see the protagonist area of our story: Orion KL. Credits: (NASA, ESA, Robberto (STScI/ESA), Orion Treasury Project Team).
About ethyl mercaptan in the list of molecules discovered in space, maintained by the Institute of Physics of the University of Cologne.
Originally published in Spanish on the Naukas website: Algo huele a podrido… en Orión KL (2014/07/17).