Methyl isocyanate in space and in comets
One of the translation errors (from English to Spanish) that has caught my attention the most has been that of the expression “on the rocks”. I’ll never forget that Hollywood classic where someone asks another character for a whiskey and says, “Yes, on the rock” (literally, put the glass on the stone rock!) … In Spanish the translation should have been, “Yes, only with ice.” Equally curious is the translation of “Shaken, not stirred” for the Martini, which brings translators crazy, although it seems this expression has been the most used by James Bond. The erroneous expression in Spanish “Sí, sobre la roca” is very useful to talk about not whisky, but gas: the gas that is deposited on the “rock” (whether a grain of dust or a comet) and which, later, may end up being transformed into ice. All very “on the rocks” and very mixed.
Today, detecting new molecular species in space is something relatively normal. It is very complex, because molecules emit in the range of the least energetic electromagnetic spectrum, and that is why very sensitive instruments are necessary. To date, a large number of molecular species have been detected, and about 30% of these detections have been carried out by Spanish research teams.
One of the goals in all astrochemical research is to understand how chemical phenomena take place in space: is it important that a molecule found in a comet is also present in the interstellar environment? How can the mixture of molecular species condition the emergence of life or the future characteristics of a planet? What amounts are needed? Is there any relationship between the chemistry of the primitive Solar System and the current one?
Dense interstellar clouds are the places where stars and planets form. Most of its mass is essentially molecular gas with a small fraction of tiny grains of dust .
On the other hand, dust grains usually have a nucleus of silicates on which the molecules of the gas phase are adhering and accumulating, forming ice sheets on the grain. This occurs during the gravitational collapse of the clouds of gas and dust, clouds from which new stars and planetary systems like ours will form, resulting in giant gaseous planets and rocky bodies such as Earth, asteroids and comets.
Our Solar System was formed 4.5 billion years ago from an interstellar cloud of gas and dust and, therefore, the composition of the bodies that emerged from it is closely linked to the composition of the interstellar cloud from which they were born. Thus, it is considered that, for example, the icy surface of comets is a repository of information that tells us about the composition of the gas and dust that was in the primitive solar nebula.
Comet 67P/Churyumov-Gerasimenko and the Orion Cloud
The recent analysis of the composition of the icy surface of comet 67P/Churyumov-Gerasimenko  by its lander, Philae, revealed the existence of a significant number of complex organic molecules, most of them already detected in gas phase in interstellar clouds.
But among the species detected on the surface of the comet there was one that had not previously been observed in interstellar clouds: methyl isocyanate (CH3NCO).
Philae detected the molecule with a mass spectrograph, but to detect a molecular species on a comet, techniques other than those used in the interstellar medium are used. In fact, to confirm its presence in the interstellar medium, a thorough analysis had to be performed consisting of obtaining the rotational spectrum of molecules in a molecular spectroscopy laboratory, so that frequencies and lines corresponding to that molecule could be obtained.
After hard laboratory work that began in 2010, an international research team, led by José Cernicharo (from the Molecular Astrophysics Group of the Madrid Institute of Materials Science (ICMM) of the Higher Council for Scientific Research (CSIC)), discovered, in the clouds of Orion, methyl isocyanate. In fact, from this observation work, carried out with the data obtained with the IRAM 30-meter radio telescope and the ALMA interferometer, 400 lines of this molecule have been characterized and detected.
This result, together with previous analyses of other comets studied from the ground, has led to the development of important work in the search for a possible connection between interstellar and cometarian molecular abundances.
Methyl isocyanate (CH3NCO) could play an important prebiotic role in the formation of peptides that could be important in the chemical evolution of primitive Earth. It is known that, at room temperature, methyl isocyanate reacts with water and with many substances containing N-H or O-H groups , common in the gas phase in Orion.
Although it is a potentially relevant molecule in the chemistry of the interstellar medium, it had so far not been included in any chemical model and has not been released until now in astrophysical journals. However, as Cernicharo states, “We intuit its presence by similarity to other previously detected species and finally confirm it. To our surprise, it is one of the most abundant molecules with a methyl group and an isocyanate group.”
Orion’s massive star formation region is the prototype of “hotcore”, the most promising areas to search for CH3NCO. Its most active part is the Kleinmann-Low nebula (Orion-KL) where a group of newborn stars, deeply embedded in the region, interacts with their surrounding material: the fact that it has been detected in hot nuclei and not in dark and cold clouds suggests a chemistry dominated mainly by the activity in the mantle of dust grains. That is, the evaporation of the ice sheets of the dust grains produces a very rich chemistry (when the original gas molecules mix with the ones that arise from that evaporation).
On the other hand, we assume that the frozen surface of comets maintains memory of the composition of the dust grains of the primitive solar nebula. These dust grains, if similar to Orion’s, will expel molecules as soon as they are heated by radiation or impacts with cosmic rays.
It will be of great interest to observe the comet’s coma to learn about the abundances of gas phase species and to obtain information on how molecules that survived the ejection of the comet’s surface have been identified. In addition, laboratory experiments on ice are essential to learn about CH3NCO formation processes on these surfaces. Knowing its original composition will help us to know more about what are the conditions necessary for systems similar to ours to emerge, systems that start being simply “gas on the rocks”.
In 2006, this research team initiated an in-depth survey of lines in Orion KL’s millimeter domain (80-280 GHz) with the IRAM 30m radio telescope with the aim of fully characterizing its chemical composition. However, due to the high kinetic temperature ofthe gas , there were many rotational and vibrational levels of abundant species that produced a forest of spectral lines (i.e., there was a huge amount of information “overlapping”, difficult to decipher).
The number of unidentified lines was too large to perform a realistic search for new molecular species. Initially, about 15,000 spectral lines were detected, of which 8,000 were unknown. It was necessary to initiate systematic work in spectroscopic laboratories to characterize all the isotopologues and vibrationally excited states of the most abundant species in Orion-KL in order to identify unknown lines.
Numerous isotopologues and vibrational states were characterized in the laboratory, later identifying them in the data and reducing the number of unidentified lines to 4,000, some of them particularly strong .
Of the expected 523 lines of CH3NCO in the data obtained by the team, 282 are not mixed with others and 119 are partially mixed with other species (without this preventing them from being identified on the line profile). The other 122 lines are completely mixed with lines of other more abundant species, most of them in the 1.3 mm (197-280 GHz) wavelength domain, where the density of lines in Orion grows enormously.
 The fraction of dust grains is ~1/200. The most abundant molecular species is molecular hydrogen (H2), followed by CO. More than 180 complex molecules are added to this list in different proportions.
 The COSAC (Cometary Sampling and Composition) experiment, aboard the Rosetta mission’s Philae lander, has measured in situ the abundances of the main surface components of comet 67P/Churyumov-Gerasimenko.
 CH3NCO was responsible for the deaths in the Bhopal industrial disaster.
 TK ~ 100-300 K
 A large number of isotopologues containing 13C, 15N, 18O and vibrationally excited states of species such as CH2CHCN, CH3OCOH, CH3CH2CN, and NH2CHO among others, were fully characterized in the laboratory and identified in the data. New molecules such as ammonium, NH3D+, methyl acetate, CH3COOCH3 and CH3OCH2CH3, methyl ethyl ether, were also detected.
Paper: “A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks”, Astronomy and Astrophysics Journal.
Orion Nebula in the infrarred. This wide-field view of the Orion Nebula (Messier 42), lying about 1350 light-years from Earth, was taken with the VISTA infrared survey telescope at ESO’s Paranal Observatory in Chile. The new telescope’s huge field of view allows the whole nebula and its surroundings to be imaged in a single picture and its infrared vision also means that it can peer deep into the normally hidden dusty regions and reveal the curious antics of the very active young stars buried there. This image was created from images taken through Z, J and Ks filters in the near-infrared part of the spectrum. The exposure times were ten minutes per filter. The image covers a region of sky about one degree by 1.5 degrees. Credits: ESO/J. Emerson/VISTA. Grading: Cambridge Astronomical Survey Unit.
Artistic impression that takes us on a 3D journey through the Orion Nebula. Credit: ESO/M. Kornmesser. Original video link: https://www.eso.org/public/spain/videos/eso1006e/
Originally published in Spanish on the Naukas website: “Gas on the rocks: mezclado, no agitado”(2016/03/08).