October 9, 2018 : A New Study Reveals A Simulation Showing That Polycyclic Aromatic Hydrocarbons Can Form In The Low-Temperature Environment Of Titan And Can Engender The Brown-Orange Haze We Observe Today
A new study entitled « Low-temperature formation of polycyclic aromatic hydrocarbons in Titan's atmosphere », published on October 8, 2018 in the journal Nature Astronomy and proposed by a team of researchers involving Ralf Kaiser and Musahid Ahmed, reveals that polycyclic aromatic hydrocarbons can form from benzene in a low-temperature environment similar to the environment of Titan's atmosphere where a brown-orange haze is observed today. The surprising outcome based on laboratory experiments and computer simulations demonstrates that the complex haze of Saturn's largest moon Titan can result from various chemical reactions involving benzene (C6H6) and polycyclic aromatic hydrocarbons in a particularly harsh environment. The atmosphere of the Opaque Moon has always fascinated researchers because that gas blanket which is deep and thick is mostly composed of nitrogen like our atmosphere and because Titan's atmosphere is rich in methane. The surface of the giant moon can't be seen from outer space in the visible spectrum due to the presence of a haze or smog which is rich in hydrocarbons or organics.
Titan's atmosphere may represent a prebiotic natural laboratory since it is rich in organics and hydrocarbons that can engender particularly complex molecules. The new research work representing a collaboration of scientists in the Chemical Sciences Division at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), of the University of Hawaii at Manoa, of the Samara University in Russia and of the Florida International University, clearly demonstrates that a complex chemistry can take shape from benzene in a particularly harsh environment. Now, planetologists realize that chemical reactions involving benzene, at a low temperature, can produce multiple-ringed molecules which can lead to an even more complex chemistry. The complex chemistry of Titan's haze may be the outcome of that kind of chemical reactions involving benzene or other hydrocarbons. The analysis leads to the conclusion that theories which advance that high-temperature reactions are needed to produce the type of haze we observe on Saturn's largest moon today may not be accurate or may be wrong.
The team of Ralf Kaiser of the University of Hawaii at Manoa resorted to vacuum ultraviolet radiation experiments at Berkeley Lab's Advanced Light Source (ALS) and to computer simulations as well as a modeling work to unveil the potential chemical reactions which have led to the atmospheric soup or to the haze we observe today on the giant moon of the Gas Giant Saturn. The Cassini spacecraft as well as the Huygens probe have obtained a huge amount of data and key data regarding Titan's atmosphere from the start of the journey inside the Saturn System in 2004 to the crash of the Cassini spacecraft in the second half of 2017. Musahid Ahmed who is a researcher in Berkeley Lab's Chemical Sciences Division and who is a co-leader of the research work at the ALS pointed out : « We provide evidence here for a low-temperature reaction pathway that people have not thought about. » He added : « This gives rise to a missing link in Titan's chemistry. » Planetologists often say that the atmosphere of Titan at the present time looks like the atmosphere of the Early Earth. The Titanian atmosphere that is clearly dominated by nitrogen contains a relatively significant fraction of methane.
Complex hydrocarbons and organics can take shape in the exotic environment of Titan which may bring clues regarding the development of a prebiotic chemistry on any world, from the Earth to Triton or Pluto. Musahid Ahmed argued : « People use Titan to think about a 'pre-biotic' Earth – when nitrogen was more prevalent in the early Earth's atmosphere. » Multiple hydrocarbons or organics such as ethane, acetylene or benzene have been detected or identified in Titan's atmosphere thanks to the Cassini spacecraft. The researchers focused their attention on benzene, a simple hydrocarbon composed of a ring of six carbon atoms connected to hydrogen atoms. They think that benzene may represent a building block of more complex hydrocarbons containing several rings of carbon. Benzene may engender hydrocarbon molecules containing two or three rings of carbon atoms. Those molecules containing several rings of carbon may have formed other hydrocarbons and aerosol particles that appear in the hazy atmosphere of Titan today. Molecules which are composed of carbon atoms and hydrogen atoms and consisting of several rings of carbon atoms represent polycyclic aromatic hydrocarbons or PAHs.
In the research work, the planetologists mixed two gases at the ALS. These gases representing hydrocarbons are naphthyl radical (C10H7) which is a short-lived two-ring PAH and vinylacetylene (C4H4) which is a relatively simple hydrocarbon. The chemical reaction generated three-ring PAHs. The team of Ralf Kaiser believes that the two types of molecules mixed in the reaction are present in Titan's atmosphere on the basis of its known chemical composition. The chemical reactions took shape in a small reaction chamber during the ALS experiments. The specialists resorted to a detector known as a reflectron time-of-flight mass spectrometer to determine the mass of molecular fragments generated in the reaction of the two types of hydrocarbon molecules. The measurement of the mass of these fragments brought information regarding the chemistry of the three-ring polycyclic aromatic hydrocarbons that is to say phenanthrene and anthracene. The experiments at the Advanced Light Source (ALS) mobilized a chemical reactor to simulate the chemical reaction as well as a beam of vacuum ultraviolet radiation to identify the molecules or particles engendered in the reaction. In parallel, the planetologists performed calculations and simulations to demonstrate that the reaction producing the multi-ring PAHs does not require high temperatures.
Polycyclic aromatic hydrocarbons such as the molecules analyzed at the ALS unveil properties that make them remarkably hard to identify in outer space as Ralf Kaiser explained. He argued : « In fact, not a single, individual PAH has been detected in the gas phase of the interstellar medium, » which represents the molecules or particles that fill the space between stars like our Sun or Proxima Centauri. He advanced : « Our study demonstrates that PAHs are more widely spread than anticipated, since they do not require the high temperatures that are present around carbon stars. This mechanism we explored is predicted to be versatile and is expected to lead to the formation of even more complex PAHs. » Polycyclic aromatic hydrocarbons are believed to play a key role in the formation of molecular clouds which are regarded as « molecular factories » of more complex molecules like amino acids, proteins or more complex organics like the precursors of our biosphere. Ralf Kaiser advanced : « This could open up theories and new models of how carbon-containing material in deep space and in the rich atmospheres of planets and their moons in our solar system evolve and originate. »
Alexander M. Mebel, another scientist of the team who is a chemistry professor at Florida International University and who is a co-leader of the research work, performed calculations that revealed how the two molecules which were incorporated into the reaction chamber can naturally assemble and engender new compounds at very low temperatures. He pointed out : « Our calculations revealed the reaction mechanism. » He added : « We showed that you don't need any energy to drive the reaction of naphthyl and vinylacetylene, so the reaction should be efficient even in the low-temperature and low-pressure atmospheric conditions on Titan. » The detailed modeling of the reactor cell where naphthyl and vinylacetylene interacted appeared to be a major aspect of the research work. Alexander M. Mebel was in a position to notice that modeling of the energies and simulations of the gas-flow dynamics taking shape within the reactor allow researchers to monitor the evolution of the reaction within the reactor. In this way, the specialists were able to establish a close connection between the theoretical outcome and the experimental observations.
The group of scientists at Samara University led the modeling work which allowed the team to predict the type of molecules or compounds generated in the reactions on the basis of the initial compounds or gases and of the temperature and pressure of the heated chamber where the compounds interacted and underwent the influence of the vacuum ultraviolet beam. Alexander M. Mebel argued : « This verification of the model, by comparing it with experiments, can also be helpful in predicting how the reaction would proceed in different conditions – from Titan's atmosphere to combustion flames on Earth. » Ralf Kaiser pointed out that a goal of the continuing study is to unveil the details of the process or chemistry that can lead to the development of carbon-containing molecules with similar structures to DNA or RNA in environments which can be extreme or particularly harsh. Thus, the study of Titan's atmosphere and Titan's surface which likely contain remarkably complex organics or hydrocarbons may allow us to better understand the chemistry of life or the chemistry that can produce key molecules of life such as amino acids, lipids, proteins or sugars.
The natural color image above shows a portion of Titan's complex atmosphere. One can notice in particular the contrast in terms of colors between the lower part of the atmosphere which appears orange and the upper part of the atmosphere, composed of several layers, which appears blue. The surface of the enigmatic moon can't be seen in the visible spectrum from outer space due to the presence of a complex haze of organics and hydrocarbons. A remarkably complex chemistry is taking shape in Titan's atmosphere in a harsh environment, under the influence of ultraviolet light from the Sun. The image was generated on the basis of data acquired from the Cassini spacecraft on March 31, 2005. The images combined to produce the final view were obtained with green, red and blue spectral filters. Image credit: NASA/JPL/Space Science Institute.
- To get further information on that news, go to: https://www.astrobio.net/also-in-news/scientists-present-new-clues-to-cut-through-the-mystery-of-titans-atmospheric-haze, http://newscenter.lbl.gov/2018/10/08/new-clues-chemistry-mystery-titans-atmospheric-haze or https://www.nature.com/articles/s41550-018-0585-y.