January 14, 2022: A New Study Reveals That The Haze Particles Of Pluto Are Bimodally Distributed And Confirms The Key Role Of Photochemistry In The Formation Of The Upper Haze Of Titan

A new study entitled "A bimodal distribution of haze in Pluto's atmosphere" and proposed by a team of researchers involving Siteng Fan and Peter Gao reveals that the haze particles of the Dwarf Planet Pluto are bimodally distributed and that the formation of the upper haze of the giant moon Titan is apparently related to photochemical phenomena. Several worlds of the Outer Solar System contain atmospheres that look alike to a certain extent. Those worlds are Titan, Triton and Pluto. Their atmosphere unveils a haze that is closely related to the action of the light emitted by our star the Sun. The atmosphere of Titan is completely opaque and is particularly deep and thick. On the other hand, the atmosphere of the largest moon of Neptune Triton and of the biggest Dwarf Planet Pluto is very thin with a very limited atmospheric pressure on the surface. The atmosphere of the three worlds is dominated by molecular nitrogen like the atmosphere of our own planet. One can notice that the atmosphere of Mars or Venus is part of another category of atmospheres since the atmosphere of both worlds is dominated by carbon dioxide rather than molecular nitrogen. However, molecular nitrogen appears to be the second most abundant compound of the atmosphere of those worlds.

The atmosphere of Pluto or Triton can be compared to the upper atmosphere of Titan where photochemistry tends to play a key role. In the upper atmosphere of the largest moon of the Ringed Planet, several atmospheric layers can be discerned. That's also the case in the atmosphere of Pluto which had been clearly observed from the New Horizons spacecraft during its historic flyby of Pluto and Charon on July 14, 2015. The icy or rocky worlds of the Outer Solar System tend to be rich in molecular nitrogen, carbon monoxide, methane or water ice. A molecule like methane can appear in its solid form on the surface of Triton or Pluto whereas that molecule appears as a gas in the typical environment of the Earth. Methane can also form clouds in the atmosphere of Titan. Rainfall events can be encountered on Saturn's largest moon. Therefore, rivers, lakes or seas can be encountered on that enigmatic world. On Pluto or Triton, the atmosphere is too thin for the presence of liquid methane or liquid nitrogen on the surface. A heavier atmosphere is needed on Pluto for the formation of lakes or seas of nitrogen. Strong cryovolcanic events on Pluto may allow the presence of transient lakes or rivers of nitrogen from time to time.

Planetologists are particularly interested in the chemistry of organics and in the chemistry of hydrocarbons on icy or rocky worlds found in the Outer Solar System. The atmosphere of Titan, Triton and Pluto can engender relatively complex molecules involving carbon elements, oxygen elements and hydrogen elements. A red sludge or mud known as tholin can be identified on the surface of Pluto or Titan. That substance may be fed by the atmospheric haze that can produce relatively heavy organics or hydrocarbons that will tend to fall to the surface to form that sludge or mud. Can the haze of Titan, Triton or Pluto engender prebiotic molecules like amino acids or sugars ? Can proteins appear on the surface ? That's a major topic of planetary science today ! The atmosphere of Titan, Triton and Pluto appears to be a chemically reducing atmosphere relatively rich in organics thanks to the action of ultraviolet light from the Sun in particular. The haze of those worlds is clearly linked to photochemical processes. The physical and chemical processes of the haze are closely linked to the particle sizes. The researchers admit that the haze size distribution in reducing atmospheres is a complex subject that must be deepened for a clearer view of the phenomena or the structure.

The observations of Pluto provide the evidence that the haze particles of the Dwarf Planet Pluto are bimodally distributed. The images obtained from the New Horizons spacecraft had revealed the remarkable structure of the thin atmosphere with several layers which appeared well separated. At the end of the flyby, we had captured a view of a dark disk surrounded by a bright atmosphere unveiling its complex structure. Researchers have mobilized previous simulations of the haze of Titan as well as this outcome to advance or conclude that haze particles in reducing atmospheres experience rapid shape change close to pressure levels of around 0.5 Pa and that photochemistry is a more powerful factor than dynamics to account for the formation of the detached haze layer of the Opaque Moon. The outcome of the team of planetologists also shows that both oxidizing and reducing atmospheres can generate multi-modal hazes. The result encourages researchers to reanalyze the observations of hazes on Saturn's largest moon and on Neptune's largest moon as well. The blue or purple upper atmosphere of Titan must have a relatively similar dynamics to the atmosphere of Triton and Pluto.

A world that evolves farther from the Sun will tend to retain the gases of its atmosphere more easily if that world contains any atmosphere obviously. A world that is more massive than another world will also tend to retain its atmosphere more easily if that world contains any atmosphere. Titan is bigger than Triton and Pluto so that it is easier for Titan to retain its atmosphere even if the giant moon is much closer to the Sun than Triton and Pluto. The atmosphere of Titan looks like the atmosphere of Venus to a certain extent because both atmospheres appear completely opaque in the visible spectrum. Yet, the composition of Titan's atmosphere is radically different from that of the atmosphere of Venus. The carbon dioxide of the atmosphere of Venus will tend to appear in its solid form on the surface of Titan due to the extremely low environmental temperatures. In terms of dynamics, one can observe that the atmosphere of both worlds undergoes super-rotation phenomena. To a certain extent, the atmosphere slides above the surface whose relative movement speed is lower than that of the atmosphere. The upper atmosphere of Titan contains several layers that appear roughly parallel as if the atmospheric particles followed a curve or a straight line.

The analytical comparison of Titan's atmosphere, Triton's atmosphere and Pluto's atmosphere appears particularly relevant if we compare the atmosphere of Titan above the altitude of 400 km and the global atmosphere of the other worlds because the atmospheric pressure at about 400 km above the ground on Titan is roughly similar to that of Triton and Pluto on the surface. At first sight, the dynamics of Titan's upper atmosphere seems to be closer to the dynamics of Triton's atmosphere or Pluto's atmosphere than its lower layers. The atmosphere of the Earth contains several types of layers for instance from the Troposphere to the Stratosphere, the Mesosphere, the Thermosphere and the Exosphere. The Voyager 2 spacecraft had revealed clouds or haze features in the atmosphere of Triton during its historic flyby of Neptune and its moons in 1989. The presence of clear signs of clouds in the atmosphere of Pluto has not been clearly observed during the historic flyby of the New Horizons spacecraft in 2015 but planetologists believe that some clouds can take shape in the atmosphere of the second most massive Dwarf Planet in the Solar System. A very thin atmosphere can engender clouds !

A parallel can be drawn between the atmosphere of Titan and the atmosphere of the Earth which look alike to a certain extent. There is a dynamic meteorology on Earth and on Titan but the meteorology of Titan doesn't mobilize water, a molecule that tends to appear in its solid form on the surface of Saturn's largest moon. The meteorology of Titan is based on methane, a molecule that can be present in its liquid form on its surface. In the harsh environment of the Opaque Moon, methane can form clouds, lakes, seas or rivers. Like on Earth, there are evaporation processes, condensation processes and precipitation processes on Titan. The infrared or near-infrared views obtained from the Cassini spacecraft have clearly revealed the presence of dynamic clouds in the high latitudes of the northern hemisphere, in the high latitudes of the southern hemisphere as well as in the lower latitudes. The low-latitude clouds or the mid-latitude clouds are scarcer than the high-latitude clouds. The fraction of the globe of Titan that is covered with clouds is much smaller than the fraction of the globe of the Earth that is covered with clouds. Curiously, the low or middle latitudes of Titan appear relatively dry compared to its high latitudes.

There is a dichotomy in the distribution of lakes, seas and rivers between the southern hemisphere and the northern hemisphere on Titan. There must be seasonal factors, physical factors and orbital factors to explain the surprising dichotomy in the distribution of lakes, seas and rivers. The most humid area on Titan is currently the north polar region where three major lakes or seas can be found and where a myriad of drainage channels and small lakes can be discerned. Kraken Mare, found in the high latitudes of the northern hemisphere, is clearly the largest pool on Titan and can be regarded as a sea. Like in any sea or ocean on Earth, islands can be found in Kraken Mare. The infrared or near-infrared views obtained from the Cassini spacecraft at the end of its mission in the Saturn System, during the Summer season in the northern hemisphere, had revealed dynamic clouds above the north polar region. The observation of those clouds demonstrates the potential evaporation processes from the lakes or seas. The first extraterrestrial pool of liquids, Ontario Lacus, had been found in the high latitudes of the southern hemisphere of Titan during its Summer season and multiple cloud systems in the area of Ontario Lacus were also observed from the Cassini orbiter in the infrared or near-infrared spectrum.

The image in the upper part of this table reveals a portion of Titan's atmosphere in which the upper atmosphere can be clearly discerned. The image whose file name is N00099119.jpg represents a raw view acquired from the Cassini spacecraft on December 20, 2007 on the basis of the CL1 filter and of the UV3 filter. At the time of the observation, the view had not been validated or calibrated and a validated or calibrated version was going to be archived with the Planetary Data System proposed by NASA. The view in the lower part of the table represents a colorized version of the original image. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2022.

- To get further information on that news, go to: https://arxiv.org/abs/2201.04392 .



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