October 17, 2019 : A New Study Based On Laboratory Experiments Suggests That Cosmic Radiations May Engender Aromatics Feeding The Dunes Of Titan

A new study entitled  Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies , published in Science Advances on October 16, 2019 and proposed by a team composed of Matthew J. Abplanalp, Robert Frigge and Ralf I. Kaiser reveals that the interaction between galactic cosmic rays and acetylene ices in the harsh environment of Titan may engender aromatics fuelling the well-known dunes of Saturn's largest moon. The scientists performed laboratory simulation experiments involving radiations mimicking galactic cosmic rays and acetylene ices that can be found near the dunes of the Opaque Moon. They came to the conclusion that the interactions between galactic cosmic radiations and acetylene ices in a relatively low-temperature environment can lead to the development of Polycyclic Aromatic Hydrocarbons such as benzene, naphthalene and phenanthrene. The team of researchers believes that those hydrocarbons may be encountered in the dunes found at low or mid-latitudes on Titan or that they may represent building blocks of the exotic dunes of the giant moon.

Thanks to the Radar Mapper of the Cassini spacecraft, we know today that the relatively dark areas found in the low or mid-latitudes of Titan are dominated by Seif dunes or linear and parallel dunes extending over long distances. There are many hypotheses regarding the nature and the origin of those dunes. The prevailing theory advances that the dunes have developed on the basis of organics or hydrocarbons found in the global haze. If the molecules, compounds or particles of the haze become heavy enough, they can fall to the surface and they are likely to produce dunes over time. The hydrocarbons or organics found in the upper part of the opaque atmosphere can be engendered via complex interactions between UV light from the Sun and the elements, ions or molecules of the environment. The new research work has demonstrated another potential mechanism to generate the dunes that we can observe on Titan today. There can be a type of snowfall involving relatively complex organics or hydrocarbons engendered via the interaction between radiations from the Sun and particles or molecules from Titan's atmosphere but there can also be chemical reactions on the surface under the influence of galactic cosmic rays.

Some airless worlds of the Solar System reveal areas composed of Polycyclic Aromatic Hydrocarbons. That's the case for Ceres or Makemake for instance. The Dwarf Planet Pluto that is covered in a thin atmosphere dominated by nitrogen reveals dunes rich in organics as well. Those observations lead us to advance new hypotheses regarding the mechanisms of formation or development of that type of soil rich in Polycyclic Aromatic Hydrocarbons or organics. Cosmic rays may play a key role in the chemistry taking shape on the surface of the icy or solid worlds in the Outer Solar System or in the Kuiper Belt. One can identify carbon-rich molecules with ring-like structures in the area of Titan's dunes. Planetologists try to gather new clues regarding the chemistry of organics and hydrocarbons on Titan because the atmosphere, the clouds, the haze or the pools of hydrocarbons are likely to engender complex chemical reactions under the influence of cosmic radiations or UV light from the Sun. The chemistry of carbon or organics on Titan is likely to bring us major clues regarding the origin of life.

The dunes of Saturn's largest moon are reminiscent of the Seif dunes found in the Namib Desert in particular. But in terms of composition, they may be particularly exotic since the environmental conditions are very different from those of the Earth. Ralf Kaiser who is a chemist at the University of Hawaii at Manoa pointed out that the dunes of the Orange Moon are quite large. In fact, they can be as high as the Great Pyramid of Giza. Ralf Kaiser insists on the fact that the chemistry of carbon, the cycle of methane or hydrocarbons or the various processes involving hydrocarbons or organics can be correctly understood if we know the main origin of carbon or methane. According to specialists of Titan, the most obvious scenario for the development of Polycyclic Aromatic Hydrocarbons on Titan is the scenario of various chemical reactions in the deep and thick atmosphere under the influence of UV light from the Sun and a progressive migration of heavier molecules toward the surface where dunes can take shape or develop. However, how can we explain that a small world covered in an extremely thin atmosphere like Pluto can unveil Polycyclic Aromatic Hydrocarbons on its surface ?

Several worlds devoid of any atmosphere unveil, on their surface, molecules rich in carbon with ring-like structures. Ceres found in the Asteroid Belt or Makemake found beyond Saturn are among those worlds. The team of Ralf Kaiser tried to solve that paradox. Are Polycyclic Aromatic Hydrocarbons exclusively related to atmospheric phenomena or chemical reactions within the atmosphere ? Those observations suggest that there may also be direct interactions between the soil and the radiations from the Sun or the Cosmos. The planetologists have been in a position to gather a major clue regarding that puzzle. They noticed that the areas of the dunes on the giant moon of Saturn are devoid of any significant amount of hydrocarbon ices. Yet, hydrocarbon ices are widespread elsewhere. Therefore, the researchers formulated a new hypothesis. They imagined a new process between hydrocarbon ices like acetylene ices and galactic cosmic rays at the level of the surface. Those photochemical reactions may engender Polycyclic Aromatic Hydrocarbons thanks to the relatively high level of energy mobilized.

Thus, in order to anticipate what type of chemicals or molecules can take shape in the fields of dunes on Titan, Ralf Kaiser and his collaborators performed a laboratory simulation involving acetylene ice and radiations that imitate the cosmic radiations that go through the atmosphere or that reach the enigmatic soil. The experiment simulated the effects of cosmic radiations on Titan's surface for a hundred-year period. At the end of the experiment, the planetologists were in a position to determine the amounts of various molecules or compounds that had taken shape. They realized that several types of Polycyclic Aromatic Hydrocarbons had been generated in that simulated environment where the mean temperature is particularly low. Therefore, without any significant atmosphere, the interaction between cosmic radiations and acetylene ices can lead to the formation or the development of Polycyclic Aromatic Hydrocarbons like benzene. Ralf Kaiser advanced that the process studied by his team is a universal process that can take shape on any world, Dwarf Planet, moon, asteroid or comet in the Solar System or in another Star System.

Titan is a natural laboratory for the chemistry of organics and hydrocarbons or even for prebiotic chemistry but other worlds devoid of any significant atmosphere can apparently engender complex chemical reactions thanks to radiations from the Sun and radiations from the Cosmos or the Milky Way. Ralf Kaiser also pointed out that grains of interstellar dust can bring complex organics or hydrocarbons like Polycyclic Aromatic Hydrocarbons. A major goal of the team is to understand the detailed mechanisms that lead to the formation of the Polycyclic Aromatic Hydrocarbons. The analysis of the potential mechanisms or processes is not easy because the ionizing radiation the researchers mobilized to simulate cosmic galactic radiations contains several simultaneous processes. Michael Malaska who is a planetologist studying planetary ices at NASA's Jet Propulsion Laboratory in California explained that the phenomena are captivating aesthetically and scientifically. He advanced that the study is in line with the idea that the sand of Titan is particularly exotic and can unveil nice colors under UV radiations. One can imagine red or brown dunes rich in organics and hydrocarbons at low or mid-latitudes.

Planetologists can refine their understanding of the chemistry of multiple types of environments by studying new worlds like the Moon, Mercury, Venus or Mars. One can find dunes on Mars, on the Earth or even on Venus and similarities between the composition of those dunes can be identified. However, the composition of the dunes found on icy or Terrestrial worlds beyond Mars may be relatively different because worlds like Titan, Triton or Pluto are richer in lighter elements like hydrogen or nitrogen. For instance, the atmosphere of Titan is dominated by nitrogen like the atmosphere of the Earth. The second most abundant gas in Titan's atmosphere is methane and methane is clearly a key molecule of the complex meteorological cycle of the Opaque Moon. The haze of Titan, which interacts with UV light from the Sun and which is rich in organics or hydrocarbons, likely fuels the dunes found in the relatively dark areas at low or mid-latitudes. Tholins that must dominate some worlds in the Outer Solar System or in the Kuiper Belt can be generated without any atmosphere thanks to the chemistry engendered by UV light from the Sun or by cosmic radiations.

The image above represents a portion of a radar swath of Titan's surface obtained from the Cassini spacecraft on May 28, 2008 during the T44 Flyby. The landscape portion is 100 km wide and 100 km high. One can clearly notice the relatively sharp contrast between bright areas and dark areas. The dark areas are dominated by relatively parallel and linear dunes extending over long distances. The dunes of Saturn's largest moon may be rich in organics and hydrocarbons like benzene. Credit for the original radar view: NASA/JPL/Cassini RADAR Team/Jason Perry. Credit for the montage: Marc Lafferre, 2019.

- To get further information on that news, go to: https://advances.sciencemag.org/content/5/10/eaaw5841.full.



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