November 4, 2020 : A New Study Involving Manuel Scherf Unveils The Potential Origin Of The Nitrogen Atmosphere Of Titan, Triton And Pluto

A new study entitled « Nitrogen Atmospheres of the Icy Bodies in the Solar System », involving Manuel Scherf and recently published in Space Science Reviews reveals the potential origin of the nitrogen atmosphere of Titan, Triton and Pluto, three major worlds located in the Outer Solar System. The research work brings key information upon the potential origin and upon the potential evolution of the atmosphere of Titan, Triton and Pluto. The atmosphere of those worlds is known to be dominated by nitrogen. The observations reveal that most moons located in the Outer Solar System are devoid of any significant atmosphere. Titan is really an exception among the numerous moons found beyond Mars due to its remarkable atmosphere. How did the atmosphere of Saturn's largest moon form ? Is the atmosphere of the Orange Moon stable over time ? Is the high atmospheric pressure on Titan's surface related to cryovolcanic events for instance ? How did the atmosphere of Triton and Pluto form ? Is the atmosphere of Triton and Pluto stable over time ? Will it completely disappear in the near future ? Are there cycles in the evolution or in the development of the atmosphere of Titan, Triton or Pluto ?

The atmosphere of Titan looks like the atmosphere of the Earth to a certain extent because the atmosphere of both worlds is dominated by molecular nitrogen. Where do the nitrogen molecules or elements in Earth's atmosphere and in Titan's atmosphere come from ? Researchers can bring major clues regarding the origin or the development of the atmospheres mainly composed of molecular nitrogen by studying, in particular, the 14N/15N ratio in the gas blanket. That ratio can tell us a lot regarding the building blocks of the atmosphere or the building blocks of the surface. The 14N/15N ratio can also tell us a lot regarding the various fractionation processes that engendered or fuelled the atmosphere. In fact, researchers study several isotopic or elemental ratios to better understand the dynamics or the origin of the atmosphere. The astronomers can also analyze the 12C/13C ratio or the Ar/N ratio in the atmosphere in order to deduce the potential history of the atmosphere in particular. Planetologists can mobilize the key data of Titan's atmosphere, Triton's atmosphere or Pluto's atmosphere in order to better understand the potential history of those icy or rocky worlds, to determine the potential mechanisms taking shape in the gas blanket or to determine the potential influence of solar activity upon the chemistry of the atmosphere.

The deep, dense and opaque atmosphere of Titan is really surprising because the atmospheric pressure on the surface is higher than that of Mars or the Earth. The contrast between Titan, the largest moon of the Ringed Planet Saturn, and Ganymede, the largest moon of Jupiter, is remarkable because Ganymede is devoid of any significant atmosphere whereas Titan contains a relatively significant atmosphere. Yet, Ganymede is more massive than Titan. How can we explain that contrast ? Is that contrast related to a different history between the development of the System of Jupiter and the development of the System of Saturn ? Is that contrast related to the level of energy received from the Sun ? A lower level of energy received from the Sun or a lower environmental temperature favor the development of any atmosphere. Titan evolves much farther from the Sun than Ganymede so that the environmental temperature on Titan is lower than the environmental temperature on Ganymede. The lower level of energy received from the Sun at the level of the Saturn System may have favored the development of the significant atmosphere of the Opaque Moon even if Titan is less massive than Ganymede.

The atmosphere of Saturn's largest moon most likely took shape on the basis of ammonia ices in particular. The molecule of ammonia whose chemical formula is NH3 contains one nitrogen atom and 3 hydrogen atoms. Photochemical activity involving the radiations from the Sun and the various molecules of the surface or the various molecules of the atmosphere can engender the formation of various molecules, atoms, ions or particles. In that configuration, ammonia may have engendered nitrogen atoms or molecules in the relatively long history of Titan. Refractory organics or hydrocarbons may also have played a key role in the formation or in the development of the dense and hazy atmosphere we observe today on that giant moon of the second largest Gas Giant in the Solar System. Researchers can clearly notice the relatively high 14N/15N ratio of Titan's atmosphere compared to that of our own atmosphere since it represents 167.7. Let's note however that that level slightly evolved over the geological time scale due to the phenomena of atmospheric escape and photodissociation of molecular nitrogen or N2. Concretely, lighter elements tend to escape into space over time.

Thanks to the Cassini-Huygens mission, we have captured a huge amount of data regarding Titan's atmosphere, Titan's surface or even the structure of that intriguing moon. The Cassini spacecraft had performed numerous flybys of Titan from 2004 to 2017. The Huygens probe had even landed on the surface of Titan at a relatively low latitude on January 14, 2005. Many panoramic views of Titan's landscape were acquired from the Huygens module during its atmospheric descent. We became aware that the relatively dark areas which mark a sharp contrast with relatively bright areas at low or mid-latitudes are not oceans or seas of methane or ethane. We had observed bright hills composed of a network of dark drainage channels as well as a brown or dark plain. In the period of the atmospheric descent and of the landing phase, that plain was devoid of any lake or sea. However, it is not unlikely that the monsoon events or that the heavy rainfall events occur in the area from time to time. Thus, the dark or brown plain may become a transient sea from time to time during the long Titanian year. A year on Titan represents almost 30 Terrestrial years and a season on Titan represents approximately 7 Terrestrial years.

The atmosphere of Saturn's largest moon is likely to tell us a lot about the stability or the history of our own atmosphere. How do atmospheres evolve over time ? That's obviously a big question and researchers can mobilize their knowledge about the various atmospheres found in the Solar System to infer the potential past or the potential future of the atmosphere of the Blue Planet. Scientists want to know the precise origin of our oceans of liquid water. How did our seas or oceans form during the long history of the Earth ? Why is Venus devoid of any ocean of liquid water despite the fact that the size and the mass of Venus are relatively similar to the size and the mass of the Earth ? Is the contrast related to the higher level of solar energy received by Venus ? That's a strong hypothesis because a higher environmental temperature at the level of Venus may have engendered a relatively strong greenhouse effect likely to become stronger and stronger over time. Some planetologists advance that Venus may have had oceans of liquid water in the past but the ancient oceans may have completely vaporized due to increasing greenhouse effects. The environmental temperature of Venus, today, prevents the formation or the development of any ocean of liquid water on the surface.

A higher atmospheric pressure may imply a higher environmental temperature at the level of the surface but reality is in fact more complex than that. The atmospheric pressure on the surface of Venus is much higher than that of the Earth at sea level and the environmental temperature of Venus at the level of the ground is also much higher than that of the Earth at sea level but on the surface of Titan, the greenhouse effect is obviously lower than on the surface of Venus for instance. In the environment of Titan, there are greenhouse effects and anti-greenhouse effects so that the mean greenhouse effect on the surface is relatively limited. On the surface of Mars, the greenhouse effect is also relatively limited because the atmospheric pressure on the surface is particularly weak compared to that of the Earth at sea level. On worlds like Triton or Pluto, the atmospheric pressure on the surface is much weaker than that on the surface of the Red Planet. Thus, any greenhouse effect may be extremely limited on those small icy or rocky worlds. An ancient lake of liquid nitrogen may have been found on Pluto implying the potential presence of a thicker atmosphere on Pluto in the past.

Similarities between the upper atmosphere of Titan and the atmosphere of Pluto have been observed by researchers. Several layers which are relatively parallel can be clearly observed in those particular environments. Planetologists try to gather new clues regarding those atmospheres because we have obtained a limited amount of data regarding those atmospheres. Thus, we must be cautious regarding the current theories regarding the origin or the evolution of Triton's atmosphere and Pluto's atmosphere. Planetologists advance that both atmospheres probably originated from protosolar nebula or from comets. Let's remember that the New Horizons spacecraft only performed a flyby of Pluto and Charon on July 14, 2015 but the images of the atmosphere and of the surface were really astonishing. Researchers want to know the potential interactions between the surface and the atmosphere in particular. Triton is also a key target for planetologists because Neptune's largest moon has a relatively young surface and geysers or cryovolcanoes. A recent project for a mission known as the Trident mission has been proposed to study the System of Neptune and its moons. That would be a great opportunity to study the typical atmosphere of a small icy world.

The image above unveils Titan, Triton and Pluto at scale. Those icy or rocky worlds contain an atmosphere dominated by molecular nitrogen like the atmosphere of our planet. The view of Titan represents a colorized version of an image obtained with the Narrow-Angle Camera of the Cassini spacecraft on April 13, 2013. The original view of Triton was acquired on August 22, 1989 from the Voyager 2 spacecraft. The original view of Pluto was taken from the New Horizons spacecraft in the period of its historic flyby of Pluto and Charon in 2015. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Triton: NASA/JPL. Credit for the original view of Pluto: NASA/Johns Hopkins APL/Southwest Research Institute. Montage credit: Marc Lafferre, 2020.

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