June 16, 2020 : A New Study Reveals Clues Upon The Potential Presence Of Volcanic Craters In The Wetland Of The Northern Hemisphere Of Titan

A new study entitled « Morphologic Evidence for Volcanic Craters near Titan's North Polar Region » and published in the Journal of Geophysical Research : Planets on June 10, 2020 reveals clues upon the potential presence of volcanic craters in the high latitudes of the northern hemisphere where many lakes, seas or rivers have been identified from the Cassini spacecraft during its long mission in the Saturn System. The research work, financed by a grant to the Planetary Science Institute from the Cassini mission propelled by NASA and proposed by the Senior Scientist Charles A. Wood from the Planetary Science Institute and by his collaborator Jani Radebaugh from Brigham Young University, unveils morphological clues that suggest the potential presence of lakes inside depressions related to cryovolcanism. The group of planetologists suggests that some lakes or depressions found in the high latitudes of Titan's northern hemisphere may be the outcome of explosive eruptions. The lakes which are relatively circular or round with wide edges or steep slopes at the level of the shoreline are reminiscent of calderas, volcanic craters or collapsed floors found on Earth or Mars in particular. Therefore, some lakes of Saturn's largest moon may represent cryovolcanoes.

The study of Titan's topography via the radar images obtained from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017 has allowed researchers to identify various features such as sand dunes, mountains, hills, fractures, lakes, seas, rivers or potential cryovolcanoes. The landscape of Titan unveils a remarkable diversity unlike most moons of Saturn which tend to be heavily cratered. Planetologists have been in a position to identify nested collapses, elevated ramparts, halos as well as islands. Those observations imply that the numerous small depressions located in the high latitudes of Titan's northern hemisphere may correspond to volcanic collapse craters. Charles A. Wood pointed out : « The close association of the proposed volcanic craters with polar lakes is consistent with a volcanic origin through explosive eruptions followed by collapse, as either maars or calderas. » Is the Opaque Moon still volcanically active today ? The researcher added : « The apparent freshness of some craters may mean that volcanism has been relatively recently active on Titan or even continues today. » Cryovolcanoes or geysers may spew methane, ethane, ammonia or water ice from time to time.

One of the most surprising features on Titan is the presence of a thick and deep atmosphere dominated by molecular nitrogen and containing a relatively significant concentration of methane. Titan may be massive enough to keep a relatively dense layer of gas around it in an environment where the level of energy received from the Sun is particularly low. In fact, the presence of any atmosphere implies the right combination of gravity and environmental temperature. If the environmental temperature is too high, the gases will tend to escape into outer space. If the gravity is too low, the gases will also tend to go away. The atmosphere of Titan is really impressive because it looks like our atmosphere to a certain extent and the atmosphere of Venus to a certain extent as well. Titan's atmosphere is dominated by molecular nitrogen and experiences a meteorological cycle like the Blue Planet. The atmosphere of the Orange Moon is opaque like the atmosphere of Venus and experiences super-rotation processes in its upper part like in the atmosphere of Venus. The atmosphere of Venus may have been strongly fed by volcanism. Therefore, can we advance the hypothesis according to which Titan's atmosphere may be regularly fuelled by cryovolcanism ?

The interactions between Titan's atmosphere, the surface of the moon and its interior are not well understood and fuel an endless debate. There may be a subsurface ocean beneath the icy crust. Is that ocean rich in ammonia, liquid water, liquid methane or liquid ethane ? Are there fractures within the crust where subsurface gases can escape in the form of geysers or cryovolcanoes like in the south polar region of the tiny moon Enceladus ? The presence of a thick atmosphere clearly implies the potential presence of a subsurface ocean or pockets of liquids beneath the icy crust. Ammonia whose chemical formula is NH3 contains one nitrogen atom and 3 hydrogen atoms. Therefore, NH3 could be a good candidate for the potential liquid found beneath the external crust of the giant moon. It's hard to imagine a subsurface ocean dominated by molecular nitrogen since the ambient temperature seems too high for the element to appear in its liquid form. The nitrogen present in Titan's atmosphere can be fed by a subsurface ocean rich in ammonia. The relatively significant concentration of methane present in Titan's atmosphere can be fed by a subsurface ocean rich in methane as well.

One can also envisage a subsurface ocean representing a mixture of ammonia and water beneath the external crust of the Opaque Moon. The infrared or near-infrared views acquired from the Cassini orbiter have allowed planetologists to identify some features which may represent cryovolcanoes or some features which may represent cryovolcanic craters. The low or mid-latitudes seem relatively dry today. The dark areas of the low or mid-latitudes which mark a sharp contrast with bright areas tend to be dominated by Seif dunes or linear and parallel dunes extending over long distances. Those dunes are probably rich in organics or hydrocarbons and may be fed by the haze which makes the atmosphere opaque from outer space. The Huygens probe may have landed onto an ancient brook or river on January 14, 2005 since the famous color image obtained from the surface reveals eroded stones or pebbles. The aerial views acquired from that parachuted probe had clearly shown the presence of a network of dark channels on bright hills which seem to represent drainage channels related to rainfall events. But they may also represent cryolava rivers or fractures where gases can escape from the presumed subsurface ocean.

Researchers have been surprised to notice that the lakes, seas and rivers are mostly found in the high latitudes of each hemisphere. There is also a dichotomy in the distribution of lakes or seas between each hemisphere. The north polar area appeared to be the most humid area on Titan during the Cassini-Huygens mission. Is the asymmetry related to seasonal factors or is it related to orbital factors ? Is the asymmetry related to cryovolcanism ? The new study reveals strong clues for the presence of cryovolcanic lakes or cryovolcanic depressions in the high latitudes of the northern hemisphere. Seasonal variations in the level of energy received from the Sun must play a major role in the dynamics or in the level of the lakes but those lakes may be fed by internal sources or by pockets of liquids. The liquids may be rich in methane or ethane. Methane, ethane or propane can appear in their liquid form on Titan's surface. That's not the case for liquid nitrogen because the environmental temperature is too high or not low enough at the level of the surface. Water can only appear in its solid form on the surface. The bright areas of Titan may be rich in water ice like on most moons of Saturn.

Can we envisage the presence of geysers of water on Titan ? The bright moon Enceladus demonstrates that we can't rule out that possibility. Charles A. Wood argued : « We demonstrate that there is also evidence for internal heat, manifest at the surface as cryovolcanoes, made from melting the water ice crust into liquid water that erupts onto Titan's surface. » The depressions or the lakes observed by the team of researchers appear roughly round with raised edges. Several depressions, pits or roughly circular lakes can be interconnected. A parallel can be drawn between their shape and the shape of volcanic landforms generated via explosion, excavation and collapse on Earth or on the Red Planet Mars. A moon like Io also unveils volcanic depressions. Huge volcanic lakes on the incredible moon of Jupiter had been identified from the Galileo spacecraft. The presumed cryovolcanic lakes of Titan probably imply very different compounds from the compounds involved in the volcanism of Io. The internal activity of Titan may be fed by the gravitational interaction of Saturn and the other moons. That's also the case for Io or Enceladus.

The strange distribution of lakes on Titan may also be closely related to variations in the thickness of the external crust. In the north polar region, a thinner crust may allow the development of explosive cryovolcanoes. On Earth, volcanism that is closely related to the phenomenon of plate tectonics can take several forms. A volcano like Mount Saint Helens in the United States can be explosive whereas a volcano like « Le Piton de la Fournaise » in the island of La Réunion is relatively soft with a relatively fluid lava. One can imagine several types of cryovolcanism on Titan as well. Charles A. Wood pointed out : « That these features are at the polar regions, near the lakes of methane, may indicate methane, nitrogen or some other volatile may power them. The features appear relatively fresh, meaning they could still be forming today. » Many mysteries remain in the exploration of Saturn's largest moon and in particular the phenomenon of the Magic Island. The Magic Islands represent, in radar views, transient bright features appearing and rapidly evolving in the major pools of liquid of the high latitudes of the northern hemisphere. Are they related to bubbles associated with cryovolcanic explosions beneath the lake or sea ?

The image above reveals a mosaic of images containing in its upper left part a radar view of Titan's surface where topographic depressions or lakes can be identified as well as nested craters on Mars in its upper right part and volcanic craters in its lower part. The relatively circular or round shape of some lakes of Saturn's largest moon suggests that those lakes evolve on volcanic depressions or craters. Source of the image: Planetary Science Institute.

- To get further information on that news, go to: https://psi.edu/news/titancraters and https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006036.



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