August 27, 2021: A Team Of Researchers Simulates The Potential Mineralogy Of Titan By Mobilizing Molecules Like Acetonitrile And Propionitrile In Laboratory Experiments

A new research work entitled "Titan in a Test Tube: Organic Co-crystals and Implications for Titan Mineralogy", published by American Chemical Society and proposed by a team involving Morgan L. Cable and Tomce Runcevski reveals the potential mineralogy of Saturn's largest moon Titan. The outcome of the analytical work is presented at the fall meeting of the American Chemical Society (ACS). That meeting represents ACS Fall 2021. The researchers performed laboratory experiments involving some molecules or elements potentially present in the atmosphere and on the surface of the Orange Moon. They studied the properties of several molecules that may be present in the haze and on the surface of the giant moon. They resorted to small glass cylinders in order to recreate the potential environment of Titan. They mobilized particular molecules that are believed to be part of the mineralogy of that opaque world. They focused their attention on acetonitrile (ACN) and propionitrile (PCN). The chemical formula of acetonitrile is CH3CN whereas the chemical formula of propionitrile is CH3CH2CN. Both molecules contain the carbon element, the hydrogen element and the nitrogen element.

As Tomce Runcevski who is the project's principal investigator pointed out, the extremely harsh environment of Titan where the ambient temperature at the level of the surface is around -179 degrees Celsius, - 290 degrees Fahrenheit or 94 Kelvin allows simple organic molecules that appear in their liquid form on Earth to appear in the typical form of solid icy mineral crystals on Saturn's largest moon. The researcher who is Ph.D. also said that his team found two of the molecules that are probably widespread on the giant moon. Those particular molecules which are acetonitrile (ACN) and propionitrile (PCN) are predominantly found in one crystalline form that generates highly polar nano surfaces. Those structures could be mobilized for the self-assembly of other compounds of prebiotic interest. Titan appears to be the perfect place or world to study the chemistry of organics and hydrocarbons in a harsh environment where chemical reactions are particularly slow. The atmosphere of that moon is relatively rich in methane and methane can form clouds, fall as rain and generate rivers, lakes or seas. A parallel can be drawn between the water cycle on Earth and the methane cycle on Titan.

The Cassini-Huygens mission in the Saturn System from 2004 to 2017 has allowed researchers to realize, to a certain extent, the similarities between the meteorological cycle of the Earth and the meteorological cycle of Titan. The infrared or near-infrared views as well as the radar views captured from the Cassini orbiter have clearly revealed lakes, seas and rivers in the high latitudes or in the polar regions of the giant moon. Dynamic cloud systems can develop in the humid areas. Curiously, the low or mid-latitudes of Titan appear relatively dry. However, from time to time during the long Titan year, elongated cloud systems or storms can develop in those areas. The Huygens probe which performed an atmospheric descent on January 14, 2005 had revealed, thanks to the aerial views, bright hills composed of a network of dark channels as well as a dark plain marking a sharp contrast with the bright hills. The dark channels were probably drainage channels related to meteorological phenomena or to heavy rainfall events which must take shape from time to time. The infrared or near-infrared images obtained from the Cassini spacecraft in 2010 have clearly shown that large cloud systems engendering rainfall events can develop in the low or mid-latitudes from time to time due to seasonal factors.

The infrared or near-infrared views taken from the Cassini orbiter during its long mission in the Saturn System have demonstrated that the relatively dark areas which mark a sharp contrast with relatively bright areas in the low or mid-latitudes are not dominated by liquid methane or liquid ethane. Prior to the Cassini-Huygens mission in the Saturn System, some researchers had imagined that the relatively dark areas of the low or mid-latitudes could represent oceans or seas of methane or ethane. That's not the case apparently. The radar views obtained from the Cassini orbiter have revealed that the relatively dark areas of the low or mid-latitudes are dominated by linear and parallel dunes extending over long distances. The shape of those dunes which look like the Seif Dunes found in the Namib Desert on Earth appears to be closely related to prevailing winds. Planetologists or chemists hope to determine the exact composition of those exotic dunes whose composition must be completely different from the composition of the various types of dunes one can encounter on Earth. The dunes of the Opaque Moon must be relatively rich in hydrocarbons and organics. The global haze probably fuels those dunes.

Titan's dense and opaque atmosphere is really captivating because it looks like the atmosphere of the Earth to a certain extent. The atmosphere of the Orange Moon is dominated by molecular nitrogen like the atmosphere of the Blue Planet. However, Titan's atmosphere is devoid or almost devoid of any oxygen. Water can only appear in its solid form on the surface due to the extremely harsh environment. A major particularity of Titan's atmosphere is the presence of a relatively significant concentration of methane. The meteorological cycle of Saturn's largest moon must involve methane and ethane whereas the meteorological cycle of the Earth mobilizes water. Water has engendered life on Earth. Can methane engender life or a prebiotic chemistry on Titan ? That's a good question because liquid methane could potentially act as a solvent for a biosphere based on liquid methane for instance. Liquid methane can engender relatively complex chemical reactions. In the haze or in the upper atmosphere of Titan, complex chemical reactions can also take shape thanks to the action of ultraviolet light from the Sun in particular. Methane, ethane, hydrogen cyanide or molecular nitrogen can interact to form new molecules like organics or hydrocarbons.

In the photochemical soup taking shape in the upper atmosphere, new molecules which can appear relatively complex can be heavy enough to fall toward the surface due to their relatively strong weight. Therefore, the mineralogy of Titan can be intimately linked to the haze or to the photochemical soup of the atmosphere. The energy from the Sun, the energy from the cosmos and the magnetic field of the Ringed Planet Saturn can strongly influence the chemistry of the atmosphere and the chemistry of the soil. The researchers imagine that acetonitrile (ACN) and propionitrile (PCN) can be found in the yellow haze of Titan in the form of aerosols and that they can fall as rain onto the surface to engender solid chunks of minerals. They know the behavior of those molecules in the typical environment of the Earth but the chemical properties of those molecules in the harsh environment of Titan must be studied. Thus, the team of researchers undertook laboratory experiments to simulate the environment of the Opaque Moon. The simulated environment appears in a tiny glass cylinder where various compounds or elements are introduced. Any experiment simulating Titan's environment can be dangerous on Earth !

The planetologists or chemists introduced water into the cylinder and lowered the environmental temperature in order to turn water into ice. Then, in order to simulate the seas or lakes of Titan, they introduced ethane which can be found as a liquid like methane on the surface in the harsh environment of the giant moon. Nitrogen is also added to simulate the atmosphere of Titan. The researchers also incorporated acetonitrile (ACN) and propionitrile (PCN) in order to simulate the atmospheric rain on that exotic world. During the experiment, they slightly changed the environmental temperature by lowering or increasing the value in order to be as close as possible to the real conditions on that enigmatic moon. They observed the formation of crystals that were analyzed on the basis of the synchrotron and neutron diffraction instrumentation, on the basis of spectroscopic experiments and on the basis of calorimetric measurements. The study involved calculations and simulations and mobilized the team of Tomce Runcevski from SMU (Southern Methodist University) as well as researchers from Argonne National Laboratory, the National Institute of Standards and Technology and New York University.

Tomce Runcevski advanced that the study has unveiled a lot about the structures of ices on worlds like Titan. We have discovered new things thanks to laboratory experiments. For instance, the team of researchers discovered that one crystalline form of propionitrile (PCN) does not expand in a uniform way along its three dimensions. The thermal expansion of the crystals related to variations in the environmental temperature may engender cracks in the surface of the Hazy Moon. The identification of those properties can allow us to better imagine or anticipate the potential mineralogy of the giant moon. Tomce Runcevski is now mobilizing crystals of acetonitrile (ACN), propionitrile (PCN) as well as mixtures of acetonitrile (ACN) and propionitrile (PCN) in order to acquire their detailed spectra. Thus, the team of scientists will be in a position to compare those specific spectra to the various spectra captured during the Cassini-Huygens mission in the Saturn System and which have not been identified yet. Thanks to the new spectra obtained in the laboratory, the researchers or chemists may be in a position to identify new minerals on Titan in the prospect of the Dragonfly mission. The scientists benefited from the support and the funding from the Welch Foundation and NASA.

The image above represents a portion of a radar swath of Titan obtained from the Cassini orbiter during the T-113 Flyby of September 28, 2015. Each side of the view represents about 100 km (around 62 miles). One can notice, in particular, relatively dark and parallel lines that may be the sign of dunes which appear widespread in the low or mid-latitudes. Those dunes may contain organics and hydrocarbons related to the haze of the Opaque Moon. Credit for the original view: NASA/JPL/Cassini Radar Team/PDS Image Atlas. Montage credit: Marc Lafferre, 2021.

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