October 27, 2020 : NASA Researchers Identify Cyclopropenylidene In The Atmosphere Of Titan Thanks To The ALMA Observatory

A new research work entitled « Detection of Cyclopropenylidene on Titan with ALMA », involving Conor A. Nixon and published in The Astronomical Journal on October 15, 2020 reveals the identification of cyclopropenylidene in the opaque atmosphere of Saturn's largest moon Titan thanks to data acquired with the ALMA observatory. Cyclopropenylidene whose chemical formula is C3H2 represents a relatively simple hydrocarbon molecule composed of carbon and hydrogen. Cyclopropenylidene appears to be the simplest cyclic hydrocarbon identified in Titan's atmosphere. Benzene, another cyclic hydrocarbon, had already been identified on Titan in 2003. Some researchers advance that cyclopropenylidene may represent a key molecule of organic chemistry, a prebiotic molecule or a molecule involved in the development of a potential ecosystem. The finding was made possible thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), a radio telescope observatory located in northern Chile. The planetologists analyzed the spectrum of unique light signatures obtained by the ALMA observatory in order to determine the chemical composition of the hazy atmosphere of the enigmatic moon. The way the molecules emit or absorb solar energy allows astronomers to infer the chemical composition of the atmosphere studied.

Conor Nixon who is a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland and who led the research work pointed out : « When I realized I was looking at cyclopropenylidene, my first thought was, 'Well, this is really unexpected' ». C3H2 appears to be a relatively simple molecule which has already been found in the clouds of gas and dust located in interstellar space. C3H2 can in fact form in particularly cold environments where chemical reactions are limited and where the relatively low density makes chemical reactions less frequent. Planetologists had never found that molecule in any atmosphere prior to the new finding. That's why researchers were really surprised to detect or identify that molecule in the atmosphere of that giant moon of Saturn. Titan's atmosphere is particularly dense and complex due to its haze of organics or hydrocarbons engendering numerous chemical reactions. Complex chemical reactions take shape or develop in the upper atmosphere of the Opaque Moon under the influence of ultraviolet light from the Sun in particular. Planetologists or exobiologists are particularly interested in the study of the soup of chemicals encountered in that region of Titan's atmosphere.

The team of Conor A. Nixon analyzed the spectrum of Titan's upper atmosphere and was in a position to identify key molecules and in particular C3H2 in a relatively low concentration. That was the first time that we found that molecule in the atmosphere of a moon or planetary body. How can we explain that paradox ? Researchers imagine that C3H2 can be found in environments where the mean density is relatively low or where chemical reactions are more limited. Conor A. Nixon pointed out : « Titan is unique in our solar system ». He added : « It has proved to be a treasure trove of new molecules. » A parallel can be drawn between the largest moon of the Ringed Planet and the Earth because both worlds contain a relatively significant atmosphere dominated by nitrogen. Titan's atmosphere is in fact four times denser than our own atmosphere. Titan and the Earth unveil a meteorological cycle implying evaporation processes, cloud formation or rainfall events. Lakes, seas and rivers have been found in the high latitudes of Saturn's largest moon in particular. A subsurface ocean dominated by salty water may exist beneath the external crust.

The meteorological cycle of Titan involves methane instead of water. Water can be found in the form of ice on the surface of the Orange Moon. Methane and nitrogen can interact in the hazy atmosphere to engender relatively complex molecules. That's why exobiologists focus their attention on the chemistry of Titan, a world which may tell us a lot regarding prebiotic chemistry. Rosaly Lopes who is a senior research scientist and a specialist of the Opaque Moon at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California pointed out : « We're trying to figure out if Titan is habitable. » She added : « So we want to know what compounds from the atmosphere get to the surface, and then, whether that material can get through the ice crust to the ocean below, because we think the ocean is where the habitable conditions are. » The composition of the soil and of the crust is a major field of research. We all have in mind that view of pebbles or eroded stones obtained from the Huygens module on Titan's surface on January 14, 2005. We know that the soil is relatively rich in methane and water ice but its exact composition is unknown. Are there more complex molecules for instance ?

Researchers often say that Titan represents a prebiotic laboratory because relatively complex molecules involving carbon, hydrogen and nitrogen can take shape. The environment of Titan, today, may look like the environment of the Early Earth which probably contained relatively high concentrations of methane and which was devoid of any significant amounts of oxygen. That's a configuration which may have been encountered on our planet between 3.8 and 2.5 billion years ago. Can the complex chemistry of Titan engender a basic lifeform or prebiotic molecules ? Melissa Trainer who is a NASA Goddard astrobiologist and who is the Dragonfly mission's deputy principal investigator and lead of an instrument on the Dragonfly rotorcraft whose goal is to study the composition of the surface of Saturn's largest moon advanced : « We think of Titan as a real-life laboratory where we can see similar chemistry to that of ancient Earth when life was taking hold here. » More complex molecules than C3H2 or C6H6 will be probably found thanks to the Dragonfly mission in particular. Earth-based telescopes like the ALMA observatory are also likely to allow us to identify new molecules on Titan.

Melissa Trainer pointed out : « We'll be looking for bigger molecules than C3H2. » However, the first step is to understand how the various molecules take shape and develop in Titan's atmosphere. In other words, researchers need to understand the « mechanics » of the chemistry of Titan's atmosphere. What types of complex organics or hydrocarbons fall to the surface in particular ? Cyclopropenylidene appears to be the second « cyclic » or closed-loop molecule identified in the deep and thick atmosphere of the giant moon. The first cyclic hydrocarbon identified on Titan was benzene or C6H6, a molecule resembling a ring. C3H2 is not mobilized in known biological reactions but the compounds which represent closed-loop molecules can form the backbone rings for the nucleobases of DNA and RNA. DNA is clearly a building block of life since it represents the genetic code of the various species of our biosphere. RNA also plays a key role in the development of the various species of our biosphere. Alexander Thelen who is a Goddard astrobiologist who collaborated with Conor A. Nixon to identify C3H2 advanced : « The cyclic nature of them opens up this extra branch of chemistry that allows you to build these biologically important molecules. »

Large and highly sensitive telescopes located on Earth are mobilized by researchers like Alexander Thelen or Conor Nixon in order to identify the simplest potential prebiotic molecules containing carbon atoms in the atmosphere of the Orange Moon. Thus, astronomers have been in a position to identify the closed-loop molecule C3H2 that is simpler than C6H6, a molecule already detected or identified on Titan. Conor Nixon and his collaborators had mobilized the ALMA observatory in 2016 to study the chemistry of Saturn's largest moon. Among the numerous molecular light signatures acquired, Conor Nixon managed to identify a surprising or strange chemical fingerprint which appeared to be cyclopropenylidene. Conor Nixon was in a position to confirm that the mysterious molecule was really C3H2 by studying the research papers related to analyses of data obtained from the Cassini spacecraft which had performed 127 close flybys of the Orange Moon between 2004 and 2017. Conor Nixon was willing to know whether the data captured by the mass spectrometer of the Cassini orbiter could confirm his finding. The mass spectrometer of that spacecraft had obtained data related to many intriguing molecules in the environment of Titan.

Planetologists have not been in a position to identify relatively complex molecules captured by the Cassini orbiter in Titan's atmosphere. Conor Nixon could see that the Cassini spacecraft had found evidence for an electrically charged version of the same compound that is to say C3H3+. The fact that cyclopropenylidene is only found in the atmosphere of Titan among the numerous worlds in the Solar System encourages researchers to attempt to better understand that molecule and the various interactions it can have with other particles, molecules or compounds in the hazy atmosphere of Saturn's largest moon. Michael Malaska who is a JPL planetary scientist who has worked in the pharmaceutical industry prior to his new challenge in the field of planetary exploration pointed out : « It's a very weird little molecule, so it's not going be the kind you learn about in high school chemistry or even undergraduate chemistry. » He added : « Down here on Earth, it's not going be something you're going to encounter. » Michael Malaska explained that C3H2 can tell us a lot regarding the complex chemistry of Titan. He concluded : « Every little piece and part you can discover can help you put together the huge puzzle of all the things going on there. » The Dragonfly mission may allow us to perform a great leap forward in the field of organic chemistry in particular.

The image above reveals a colorized view of the various layers of Titan's upper atmosphere. That portion of Saturn's largest moon was obtained in the ultraviolet spectrum with the Narrow-Angle Camera of the Cassini spacecraft at the start of the Cassini-Huygens mission in the Saturn System. The limb unveiled here is found at approximately 10 degrees south latitude in the equatorial area. The color contrast between the upper atmosphere and the lower atmosphere is remarkable. Complex photochemical reactions take shape in the upper atmosphere under the inflence of ultraviolet light from our star in particular. Image credit: NASA/JPL/Space Science Institute.

- To get further information on that news, go to: https://www.nasa.gov/feature/goddard/2020/nasa-scientists-discover-a-weird-molecule-in-titan-s-atmosphere and https://iopscience.iop.org/article/10.3847/1538-3881/abb679 .



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