January 19, 2021 : The Icy Particles Of The Haze Of Pluto May Be Different From The Particles Of The Haze Of Titan According To A New Study Involving Panayotis Lavvas

A new research work entitled « A major ice component in Pluto's haze », published in the journal Nature Astronomy on December 21, 2020 and involving the planetary scientist Panayotis Lavvas from the University of Reims Champagne-Ardenne in France reveals that the icy particles encountered in the haze of the Dwarf Planet Pluto may be different from the particles encountered in the haze of Saturn's largest moon Titan. Titan, Triton and Pluto which are worlds located in the Outer Solar System unveil an atmosphere composed of molecular nitrogen, methane and carbon monoxide. The presence of any atmosphere appears to be closely related to the level of gravity and to the level of energy received from the Sun or to the environmental temperature. A world on which the environmental temperature is relatively low will have a higher probability to contain an atmosphere than a world on which the environmental temperature is relatively high. A world that is more massive than another world will also have a higher probability to contain an atmosphere than the lighter world or the less massive world. Titan reveals a completely opaque atmosphere due to the presence of a haze rich in organics or hydrocarbons.

Researchers are particularly interested in the dynamics, the nature and the chemical activity of Titan's haze because that haze can produce complex molecules via the interactions between the radiations generated by the Sun and the various particles, compounds, ions or molecules found in the haze. Planetologists have noticed that there is a complex photochemical activity in the upper atmosphere of the Orange Moon under the influence of ultraviolet light from the Sun. We can clearly notice the sharp contrast between the orange ball of Titan and the blue or purple upper layers of the deep and thick atmosphere. Relatively complex hydrocarbons or organics can take shape in the upper atmosphere of Titan thanks to the photochemical activity. The atmosphere of the largest moon of Neptune Triton and of the Dwarf Planet Pluto also contains a haze or hazes engendered by photochemical processes. N2, CH4 and CO are regularly identified on the surface or in the atmosphere of the worlds found in the Outer Solar System. The images captured by the New Horizons probe had clearly revealed a field of ice rich in nitrogen ice and carbon monoxide ice for instance. Methane ice had also been identified on Pluto's surface.

The photochemical haze can regularly engender relatively heavy molecules rich in organics or hydrocarbons that can become heavy enough to fall toward the surface to form a type of snow or to form a dark or red sludge or mud known as tholin. The dark or red soil of Pluto or Titan may contain large amounts of tholin, hydrocarbons or organics. Thanks to the Cassini-Huygens mission, our knowledge upon the chemistry of Titan's atmosphere and Titan's surface has been significantly enriched. We have clearly seen cloud systems involving methane or ethane. We have clearly seen a vortex taking shape above the south polar region as well as a very large cloud system evolving above the north polar region. The meteorology of Titan is clearly complex. We imagine rainfall events on Titan like on Earth. However, the concentration of clouds on Titan remains relatively limited compared to the concentration of clouds on our planet. The meteorology of Titan involves methane and ethane whereas the meteorology of the Earth involves water. Water appears in its solid form in the harsh environment of Titan. There can't be lakes or seas dominated by liquid water on Titan's surface. However, there can be lakes, seas or rivers of methane, ethane or even propane.

The observations from the Cassini orbiter have clearly revealed that there are pools of liquids in the high latitudes of each hemisphere on Saturn's largest moon. Curiously, the low or mid-latitudes appear relatively dry. That's what we have clearly noticed during the long journey of the Cassini spacecraft in the Saturn System from 2004 to 2017. Will that configuration change over time ? In fact, the campaign of observation of Titan has only represented about half a Saturn year or Titan year. Let's recall that a Saturn year or Titan year represents almost 30 Earth years and that each season on Titan represents approximately 7 Earth years. The dichotomy in the distribution of lakes, seas and rivers on Titan can be explained by various factors such as the orbital parameters or the dynamics of methane molecules. The orbit of Saturn and Titan is elliptical so that the level of energy received from the Sun will be higher when Saturn and Titan are closer to the Sun in their orbit. The difference between the maximum distance to the Sun (Aphelion) and the minimum distance to the Sun (Perihelion) is in fact relatively significant so that the level of energy received from the Sun can significantly vary during the orbital dance.

The haze of Titan can engender relatively heavy or complex molecules thanks to the action of ultraviolet light from the Sun in particular and the heavier molecules may fall toward lower altitudes where they can be involved in even more complex chemical processes. The dunes which are widespread in the low or mid-latitudes of Saturn's largest moon can be fed by relatively complex molecules falling from the haze or smog. In other words, one can imagine snowfall events on Titan. What is the nature of that snow ? Is it a bright snow like the typical snow one can encounter on Earth ? Are there several types of snowfall events on Titan ? The dunes of the giant moon are generally found in the dark areas of the low or mid-latitudes. Therefore, the presumed snow of Titan may represent a snow made up of hydrocarbons or organics so that it might be less reflective than the typical snow one can encounter on our planet. The sharp contrast between relatively bright areas and relatively dark areas in the low or mid-latitudes of the Opaque Moon is remarkable because the surface of the other moons of Saturn is generally relatively uniform. That sharp contrast may imply a different composition of the soil between the relatively bright areas and the relatively dark areas.

Similarities between the atmosphere of Pluto and the upper atmosphere of Titan have been clearly noticed. Pluto's atmosphere, dominated by molecular nitrogen, appears blue like the atmosphere of the Blue planet, dominated by molecular nitrogen as well, and like the upper atmosphere of Titan. The upper atmosphere of Titan contains several layers which can be clearly discerned. The atmosphere of Pluto is also composed of several layers. The atmosphere of Pluto is extremely thin compared to the atmosphere of the Earth or compared to the atmosphere of Titan. However, the atmosphere of Pluto has a remarkable impact on its landscape. The haze of Pluto can feed some areas and produce blankets of methane or nitrogen ice for instance. Some clouds on Triton had been observed from the Voyager 2 spacecraft during its historic flyby in 1989. The same types of clouds are likely to take shape in the thin atmosphere of Pluto. It is clear that photochemistry plays a key role in the atmosphere of Titan, Triton and Pluto. On Titan, the haze makes the atmosphere completely opaque which is not the case on Triton or Pluto where the atmosphere is much thinner than that of Mars for instance.

On Titan, the clouds can engender rainfall events mobilizing methane or ethane but on Triton and Pluto where environmental temperatures are much lower, rainfall events involving liquid methane or liquid ethane can't be envisaged because the environmental temperature is too low and because the atmosphere is too thin. Snowfall events involving methane ice or nitrogen ice can be envisaged, however, on Triton and Pluto. The researchers of the new study reveal that the hazes of the Dwarf Planet Pluto may contain a major organic ice component dominated by C4H2 ice and produced via the direct condensation of the primary photochemical compounds generated in the atmosphere. That particular configuration may imply that the haze on Pluto has a more limited action in the processes regarding the thermal balance of the atmosphere compared to the haze of Saturn's largest moon Titan. The atmosphere of the Orange Moon is known to generate greenhouse effects as well as anti-greenhouse effects which can be relatively complex. In the past, we could have anticipated higher environmental temperatures at the level of the Titanian surface due to the relatively high weight of the atmosphere or due to the relatively high atmospheric pressure on the surface.

The researchers of the study anticipate, on the basis of their models or calculations, that the haze composition of Triton is dominated by C2H4 ice. Triton remains a mysterious world where the environmental temperature is extremely low. However, remarkable phenomena were observed during the historic flyby of 1989. One can mention, in particular, the geysers, cryovolcanoes or Black Smokers. Can those cryovolcanic phenomena regularly fuel the atmosphere with hydrocarbons, organics or nitrogen molecules ? Some presumed cryovolcanoes have also been observed on Pluto. Therefore, the internal activity of many worlds located in the Outer Solar System may be relatively active today. Is the stability of the atmosphere closely related to the internal activity of the world ? The new work shows that lower environmental temperatures can have strong implications for the composition of the components of the haze. Molecules like hydrogen cyanide or HCN can contain layers of icy particles, hydrocarbons or organics thanks to lower environmental temperatures or lower levels of energy in the atmosphere. The chemistry found in the Outer Solar System is clearly surprising.

The image above reveals the disk of Titan and the disk of Pluto at scale. One can clearly notice the blue upper layers of the atmosphere of Titan as well as the various layers of the blue atmosphere of the Dwarf Planet Pluto. The original view of Titan which represents a raw image whose file name is W00091693.jpg was obtained from the Cassini orbiter on February 12, 2015 on the basis of the CL1 filter and of the VIO filter. The original view of Pluto was generated on the basis of data acquired from the New Horizons spacecraft during its historic flyby on July 14, 2015. The colors of the disk of Titan are artificial. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Pluto: NASA/JHUAPL/SwRI. Credit for the montage: Marc Lafferre, 2021.

- To get further information on that news, go to: https://www.nature.com/articles/s41550-020-01270-3.



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