April 14, 2020 : A New Laboratory Experiment Reveals How The Methane Found On Neptune Or Titan Potentially Formed In Interstellar Space

A new research work entitled « An experimental study of the surface formation of methane in interstellar molecular clouds », published on April 13, 2020 in the journal Nature Astronomy and proposed by a team of scientists involving Danna Qasim unveils laboratory experiments upon the potential formation of methane in the harsh environment of space far from any star. The formation of methane in interstellar molecular clouds may be closely related to the presence of water ice or icy dust grains. The simulations performed in a laboratory at Leiden University in the Netherlands confirm the prevailing assumptions on the mechanisms that lead to the formation of methane in the Outer Solar System or beyond. Reproducing the harsh environment of interstellar space is clearly a challenge but the team of researchers was in a position to obtain a significant outcome regarding the potential interactions in the soup of molecules, particles or radiations encountered between stars. Methane is widespread in the Outer Solar System, from the giant moon Titan to Gas Giants or Ice Giants like Uranus or Neptune. Many worlds found beyond the Inner Solar System like Pluto or Charon may be relatively rich in organics, hydrocarbons or methane.

Methane whose chemical formula is CH4 represents the simplest stable hydrocarbon molecule. It contains one carbon atom and 4 hydrogen atoms. Methane can be encountered on Earth in the form of natural gas. In the environment of the Earth at sea level, methane can only be found as a gas because the ambient temperature is much too high for the molecule to appear in its liquid form or in its solid form for instance. Everyone knows that, in a typical environment, methane represents a flammable gas that takes shape from decaying organic molecules. That's also the case for other hydrocarbons like ethane (C2H6), propane (C3H8), butane (C4H10) or for the typical fuel of our cars. Thanks to the Cassini-Huygens mission, we know, now, that there are lakes, seas or rivers dominated by methane on Saturn's largest moon Titan. The opaque atmosphere of that moon contains a relatively significant concentration of methane. The Gas Giants are dominated by hydrogen and helium but they also contain a relatively significant fraction of methane. Astronomers have been in a position to determine that methane ice represents one of the ten most abundant ices identified in interstellar space.

The leading theory upon the way methane forms in space suggests that carbon interacts with hydrogen to form CH and, progressively, the initial molecule captures new hydrogen atoms. From CH, it becomes CH2, then CH3 and finally CH4. That process of interactions is slow in the gas phase. However, methane can be produced much faster if the chemical reactions occur on an icy dust grain because the grain tends to boost or speed up the formation process of methane. The dust grain represents, to a certain extent, a crossroad where particles, atoms or molecules can more easily interact in the vastness of interstellar space. Moreover, the dust grains can swallow or absorb the energy engendered from chemical reactions so that the new molecules like methane remain stable or don't break apart. The team of scientists from the Laboratory for Astrophysics at Leiden Observatory located at Leiden University in the Netherlands has been in a position to simulate the extreme environment of interstellar space where ambient temperatures are particularly low implying particularly slow chemical reactions and the specialists managed to generate methane molecules in that type of environment.

The interactions studied or observed by the scientists in their simulations took shape in an ultrahigh vacuum environment where the temperature was at the level of minus 263 degrees Celsius, minus 442 degrees Fahrenheit or 10 Kelvin. The group let hydrogen atoms interact with carbon atoms on an ice-cold ground. The outcome clearly revealed the formation of methane molecules. The scientists had previously managed to generate water (H2O) as well as ammonia (NH3) in a comparable way. They had mobilized oxygen atoms, nitrogen atoms as well as hydrogen atoms to study the interactions which had led to the formation of water or ammonia. The simulations involving carbon atoms turned out to be particularly challenging due to the fact that carbon is very sticky making chemical reactions in which carbon atoms are mobilized harder. Danna Qasim who is a PhD student at Leiden Observatory and who is the lead author of the research work released in Nature Astronomy argued : « It is difficult to conduct an experiment with carbon atoms. Carbon likes to stick, so it is challenging to produce a controlled beam of pure carbon atoms. At the same time, you have to make sure that after an experiment, your entire setup is not completely covered with carbon. »

The conditions of the experiments or simulations were modified by the team of scientists in order to correctly understand the process involving the interactions between carbon and hydrogen atoms that leads to the formation of methane. Various interactions can take shape and methane is one of the molecules obtained. The researchers also wanted to know the probability of methane formation in the soup of elements or compounds. They were in a position to determine that methane ice is easier to produce in a water-rich environment. Their outcome or conclusion is in line with astronomical observations which suggest that methane ice and water ice are likely to take shape in parallel in space. The new study reveals a potential reality of the interstellar environment or the interstellar molecular clouds. The stars and planets can take shape from disks of dust, ice and gas where molecules of water, molecules of ammonia or molecules of methane can be widespread due to the typical reactions involving elements like hydrogen, oxygen, nitrogen or carbon. The Sun as well as the four Gas Giants of the Solar System tell us a lot regarding the potential chemistry of the cloud of dust, ice and gas that led to the formation of the Solar System.

The team of Danna Qasim strongly believes, on the basis of the new simulations, that methane was probably present in our environment of the Milky Way long before the formation process of the Solar System. Planetologists observe relatively significant concentrations of methane in the atmosphere of worlds like Titan, Uranus or Neptune and they also observe that the surface of worlds like Titan or Pluto can be rich in organics, methane and other hydrocarbons. The UV light from the Sun may play a key role in the soup of chemical reactions taking shape in the upper atmosphere of worlds like Titan, Triton or Pluto for instance. Over time, methane tends to be destroyed by ultraviolet light from our star. Are there internal sources to the relatively high concentration of methane observed in the atmosphere of Saturn's largest moon for instance ? Is there a cycle in the production of methane on worlds like Titan, Uranus or Neptune ? The internal structure of Gas Giants or Ice Giants like Uranus or Neptune remains a mystery today. Therefore, our level of understanding regarding the potential cycles is particularly limited.

A world like Titan represents, to a certain extent, a paradise for the study of organics and hydrocarbons. A meteorological cycle involving methane on Titan has been widely studied during the journey of the Cassini-Huygens spacecraft in the Saturn System. A parallel between the water cycle on Earth and the methane cycle on Titan can be drawn. Evaporation processes, condensation processes and precipitation processes can be encountered on both worlds. However, on Titan, clouds are relatively scarce in the low or mid-latitudes. Curiously, the most humid areas on the Opaque Moon are found in the high latitudes or in the polar regions. The first pool of liquids identified on Titan was Ontario Lacus, a lake or sea located in the high latitudes of the southern hemisphere. Radar images acquired from the Cassini orbiter have clearly shown that the most humid areas are found in the high latitudes of the northern hemisphere. Is that dichotomy in the distribution of lakes, seas and rivers related to the orbital or physical configuration of Titan or is that dichotomy related to internal phenomena ?

The haze of Titan represents a photochemical soup where complex hydrocarbons or organics regularly take shape. In the soup of molecules present in the exotic atmosphere of Saturn's largest moon, methane can be destroyed engendering new elements, ions or molecules. The new particles, elements or molecules produced in that soup can engender new molecules. Heavier molecules will tend to go down toward the surface. The dunes encountered in the dark areas of the low or mid-latitudes may be composed of grains rich in hydrocarbons. They may be fed by molecules engendered in the haze. Is there a layer of liquid methane hidden beneath the external crust of that giant moon ? Is the atmosphere of Titan stable over time ? Will the methane of Titan completely disappear in the long run due to the photochemical action of UV light from our star ? Is there a cycle between a subsurface ocean of methane and the atmosphere of the Orange Moon ? Are there interactions between various hydrocarbons or organics in the soil or in the crust that can engender methane molecules or ethane molecules for instance ?

The image above reveals Neptune, Titan and Uranus, three worlds of the Outer Solar System containing relatively significant concentrations of methane in their atmosphere. Methane represents, approximately, 1.4 percent of the composition of Titan's atmosphere for instance. Credit for the original view of Neptune: NASA/JPL. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Uranus: NASA/JPL. Montage credit: Marc Lafferre, 2020.

- To get further information on that news, go to: https://www.universiteitleiden.nl/en/news/2020/04/first-time-methane-ice-formed-in-leiden-under-space-conditions and https://www.nature.com/articles/s41550-020-1054-y.



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