April 16, 2019 : Some New Analyses Reveal Small And Remarkably Deep Lakes Rich In Methane On Top Of Hills As Well As Lakes Which May Have Evaporated In The North Polar Region Of Titan During The Cassini Mission

Two new studies published in Nature Astronomy on April 15, 2019 unveil new clues or conclusions regarding the structure, the nature or the dynamics of the lakes of Saturn's largest moon Titan. The first study entitled  Deep and methane-rich lakes on Titan  and led by Marco Mastrogiuseppe, who is a Cassini radar scientist at Caltech in Pasadena, California, reveals the identification of deep lakes rich in methane on top of hills in the high latitudes of the northern hemisphere of the Opaque Moon. The second study entitled  The case for seasonal surface changes at Titan's lake district  and led by Shannon MacKenzie, who is a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, reveals the identification of lakes which may have evaporated during the Cassini mission in the north polar region of that enigmatic moon. The research work was partly based on data obtained during the final close flyby of Saturn's largest moon performed by the Cassini orbiter on April 22, 2017. During that flyby, we obtained radar data unveiling the small lakes of hydrocarbons found in the northern hemisphere.

The new analyses reveal or confirm that the small lakes identified on top of hills in the high latitudes of the northern hemisphere are remarkably deep and are composed of methane. The small pools of hydrocarbons studied by the planetologists can be more than 300 feet or 100 meters deep. Thanks to these analyses, researchers are in a position to have a better understanding of the dynamics of Titan's lakes and of the hydrological cycle of the Hazy Moon. The new clues allow scientists to better analyze the processes of evaporation, condensation and precipitation as well as the way the exotic liquid seeps into the soil. Researchers can draw a parallel between the hydrology of Titan and the hydrology of our planet even if the hydrology of Titan is mainly based on methane whereas the hydrology of the Earth is based on water. In fact, to our knowledge, the Earth and Titan are the only worlds in the Solar System containing stable pools of liquid on their surface. Cyclones, cloud systems and rainfall can take shape on Titan like on Earth. The key molecules of the hydrological cycle of Titan appear to be methane and ethane which can appear in their liquid form on the surface.

The environmental temperature of Titan is particularly low due to the relatively low level of energy received from the Sun. The environmental temperature at the level of the surface is around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin. Titan has the right combination of atmospheric pressure on the surface and environmental temperature for methane, ethane or propane to appear in their liquid form on the surface. On our planet, methane can be encountered as a gas due to the relatively high environmental temperatures. Radar data and infrared or near-infrared data obtained from the Cassini orbiter have allowed researchers to determine that the lakes, seas or rivers are mostly located in the high latitudes of each hemisphere of the giant moon. Planetologists have been in a position to determine that the northern seas or lakes are mainly composed of methane. Regarding the smaller pools of liquid located in the north polar region, they were surprised to determine that those lakes were mostly composed of methane.

Ontario Lacus, a giant lake or a sea which had been identified in the high latitudes of the southern hemisphere in the first half of the mission, turns out to have a different composition from that of the north polar lakes or seas. We had measured that Ontario Lacus which looks like a foot contained a roughly equal mix of methane and ethane at the time of the observation. Ethane whose chemical formula is C2H6 appears to be less volatile than methane whose chemical formula is CH4. Marco Mastrogiuseppe pointed out :  Every time we make discoveries on Titan, Titan becomes more and more mysterious.  He added :  But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan.  The specialists of Titan are surprised or amazed to come to the conclusion that the hydrology on one side of the northern hemisphere is completely different from that of the other side of the same hemisphere. The landscape of Titan unveils similarities to the landscape of our planet but there are also major differences which make Titan very exotic. Various liquids with different mechanical properties engendering relatively strong erosional processes can evolve on the surface of Titan.

Regarding the geographical dichotomy in the composition of lakes on Titan, Jonathan Lunine who is a Cassini scientist, who works at the Cornell University in Ithaca, New York and who took part in the research work upon the characteristics of the small lakes found on hills on Titan, pointed out :  It is as if you looked down on the Earth's North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does.  The eastern side of the Opaque Moon unveils big seas with low elevation as well as islands and canyons. By contrast, the western side of the giant moon contains small lakes. The new measurements reveal that the small pools of liquid are located on top of big hills and plateaus. The new radar measurements are in line with the previous discoveries revealing that the small pools of liquid are found far above sea level but they also bring us new information regarding the topography. One can observe for instance mesas and buttes peaking hundreds of feet above the surrounding terrain and containing deep lakes of hydrocarbons at their top.

The particular topography of the pools or lakes found on the western side of the Opaque Moon brings significant clues about the local geology. Those small pools of liquid are small but remarkably deep. They are only tens of miles across. That configuration leads researchers to the conclusion that those pools of liquid probably took shape when the presumed surrounding bedrock of ice and solid organics or hydrocarbons chemically dissolved and collapsed. That type of landscape is reminiscent of karstic terrain on our planet. On Earth, one can evolve on a soil composed of limestone. Water tends to seep into the soil made up of limestone or into the limestone bedrock. Subsurface rivers or caves can take shape beneath that kind of bedrock. Those small and deep lakes found on Titan may resemble the karstic lakes one can encounter on Earth in countries like Germany, Croatia or the United States or in areas like the region of Quercy in France. Planetologists are particularly interested in the properties of liquid methane and in the potential interactions between liquid methane and the crust of Titan.

The analysis regarding the findings upon the small and deep lakes of the high latitudes of the northern hemisphere of Titan has been accompanied by another study focusing on the seasonal dynamics of lakes on Saturn's largest moon. The seasonal cycle of methane on Titan is clearly a major topic of research and planetologists have been in a position to determine, on the basis of Cassini data, that the level of several lakes had significantly changed during the Cassini mission in the Saturn System. In fact, the specialists have apparently observed, on the basis of radar and infrared data, transient lakes or lakes whose level has significantly decreased over a relatively short period of time. The level of lakes may be intimately linked to seasonal factors. Liquid methane may be particularly sensitive to variations in environmental temperatures or in the strength of solar radiations. The Summer season started in 2017 in the northern hemisphere of Titan and one can imagine that there may be relatively strong evaporation processes in the humid area of that hemisphere during that period of the long Titanian year. Shannon MacKenzie pointed out :  One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface. 

The outcome on the relationship between seasonal factors and the dynamics of the relatively small lakes as well as the finding on the particular topography of small lakes which appear surprisingly deep and which appear well above sea level tend to strengthen the hypothesis that there is rain of hydrocarbons feeding or fueling the lakes and the rivers, that there are condensation processes in the atmosphere, that there are evaporation processes and that the exotic liquid can infiltrate into the soil to engender reservoirs of liquid beneath the surface. The long mission of the Cassini spacecraft in the Saturn System which started in 2004 and which ended in 2017 with the targeted crash against the Gas Giant Saturn allowed researchers to map more than 620,000 square miles or 1.6 million square kilometers of lakes and seas on the surface of the Hazy Moon. The mapping work was performed on the basis of the radar instrument and on the basis of two imaging systems. The Radar Mapper which sent out radio waves and captured a return signal or an echo brought us key information about the topography or the nature of the surface. We were in a position to determine the size, the depth, the composition or the dynamics of the lakes, seas and rivers. In parallel, the two imaging systems which can see through the opaque atmosphere down to the surface allowed us to discern surface features such as mountains, craters or lakes and seas as well.

The close flyby of the Orange Moon performed on April 22, 2017 allowed researchers to gather the crucial data and to make their discoveries or to produce their new analyses. That flyby represented the last flyby of Titan with the Cassini orbiter and it was fruitful in terms of scientific data regarding the smaller lakes of the giant moon. The planetologists managed to gather major clues upon the dynamics of the lakes, the properties of liquid methane or liquid ethane and the nature of the soil. The idea that there may be caves with pools of methane or subsurface rivers of methane on Titan is clearly captivating. The engineers performed a remarkable work to determine the right trajectory in the orbit of the Cassini probe within the Saturn System. The last targeted flyby of Titan led to major discoveries regarding the hydrology of the enigmatic world. Janathan Lunine pointed out :  This was Cassini's last hurrah at Titan, and it really was a feat.  Will the level or the size of the lakes or seas found in the high latitudes of the northern hemisphere decrease in the coming years as the Summer season develops in the northern hemisphere ?

The image above reveals a near-infrared view of the disk of Titan unveiling, in particular, the land of lakes in the north polar region of Saturn's largest moon. The view was obtained with the Narrow-Angle Camera of the Cassini orbiter on June 9, 2017. At the time of the observation, the northern hemisphere of the Orange Moon was experiencing the start of the Summer season whereas the southern hemisphere was experiencing the start of the Winter season. The Solstice which leads to the beginning of the Winter season in the southern hemisphere and to the beginning of the Summer season in the northern hemisphere had occurred on May 24, 2017. A Titanian season is particularly long since it represents about 7 Terrestrial years and a Titanian year represents almost 30 Terrestrial years. Image credit: NASA/JPL-Caltech/Space Science Institute.

- To get further information on that news, go to: https://solarsystem.nasa.gov/news/904/cassini-reveals-surprises-with-titans-lakes, https://www.nature.com/articles/s41550-019-0714-2 and https://www.nature.com/articles/s41550-018-0687-6.



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