September 10, 2019 : Some Methane Lakes Of Titan Appearing In Craters Generated By Explosions Of Nitrogen ?

A new study entitled  Possible explosion crater origin of small lake basins with raised rims on Titan , published in Nature Geoscience on September 9, 2019 and proposed by a team involving Giuseppe Mitri, suggests that some methane lakes of Saturn's largest moon Titan may appear in craters generated by explosions of nitrogen. Thanks to radar data acquired from the Cassini orbiter, we know, now, that most lakes, seas and rivers on Titan are found in its high latitudes or in its polar regions. The north polar region appears to be the most humid area with large pools of liquid and with a multitude of small lakes. The pools of liquid tend to be dominated by methane in the north polar region. Researchers try to determine how those lakes or seas formed. Some planetologists have advanced that dissolution processes and erosional processes must play a key role in the development and in the dynamics of the lakes or seas. However, a new scenario involving explosions of nitrogen may explain the surprising morphology of some small methane lakes of the Opaque Moon.

The radar data obtained with the Radar Mapper of the Cassini spacecraft during its mission in the Saturn System have clearly shown that some small lakes found in the high latitudes of the northern hemisphere unveil steep rims that can be hundreds of feet high. That strange topography around the lakes may be closely related to explosions of warming nitrogen which would have engendered the basins in which the methane lakes have taken shape according to the new research work performed by Giuseppe Mitri and his collaborators. A methane cycle comparable to the water cycle of our planet can be observed on Titan. One can encounter evaporation processes, condensation processes, clouds and rainfall events on that enigmatic moon of the Ringed Planet. The size or the level of the lakes or seas can change over time due to seasonal factors. However, there may be internal sources of methane or ethane which could fuel the lakes or seas. Planetologists try to gather new clues regarding the nature and the origin of the pools of methane or ethane on Titan because the environment seems quite complex.

There may be several types of lakes on Titan. Planetologists have envisaged the typical lake or basin unveiling sharp boundaries which has been formed by the action of liquid mehane or liquid ethane via dissolution processes. That type of lake or basin can be compared to the karstic lake on Earth. On Titan, the environment is composed of exotic substances. The Titanian bedrock is dominated by water ice and solid organic molecules. On Earth, dissolution processes involve liquid water and can take shape in limestone. Methane will appear as a gas on our planet due to a much warmer environment than on Titan. The new approach proposed by the team of Giuseppe Mitri suggests that some small lakes of the giant moon (tens of miles across) may have taken shape in craters produced by explosions of pockets of liquid nitrogen due to a warming process likely to turn liquid nitrogen into nitrogen in its gaseous form. Liquid methane and liquid ethane would have progressively accumulated inside those craters. The radar images captured from the Cassini orbiter reveal small lakes with steep boundaries and high rims similar to ramparts like Winnipeg Lacus in the north polar region of the Opaque Moon. That type of topography or morphology can be well explained by the new theory.

The analyses of Giuseppe Mitri and his team were based, in particular, on radar data obtained from the Cassini spacecraft, proposed by NASA's Jet Propulsion Laboratory in Pasadena, California, during the last close flyby of Saturn's largest moon as the probe prepared to carry out its final plunge into the atmosphere of the Gas Giant Saturn in 2017. The radar views allowed the group of researchers, led by Giuseppe Mitri of Italy's G. d'Annunzio University, to advance that the karstic model was insufficient to account for what they saw in the images. Giuseppe Mitri pointed out :  The rim goes up, and the karst process works in the opposite way.  He added :  We were not finding any explanation that fit with a karstic lake basin. In reality, the morphology was more consistent with an explosion crater, where the rim is formed by the ejected material from the crater interior. It's totally a different process.  The theory of craters or basins related to explosions involving nitrogen is in line with some climate models of the Opaque Moon which reveal that the environment of Titan has been colder or that the moon has experienced its  ice ages .

Planetologists advance that over the last half-billion or billion years, methane (CH4), which represents a relatively significant fraction of the composition of Titan's atmosphere, has generated greenhouse effects in the environment of the giant moon like carbon dioxide (CO2) or water vapor (H2O) in our own environment. Therefore, the Titanian environment appears warmer than it would be without any methane in the air. Researchers have long believed that the Orange Moon has experienced epochs of cooling and warming due to the variations in the concentration of methane related to the action of solar radiations such as UV light which tends to destroy methane molecules. Over geologic time scales, the concentration of methane of Titan's atmosphere may periodically diminish and may periodically increase. Jonathan Lunine who works at Cornell University in Ithaca, New York and who is a Cassini scientist and a co-author of the research work argued that in the colder periods, nitrogen dominated more the atmosphere than today and could fall as rain onto the ice crust of the Opaque Moon. There was a cycle of nitrogen comparable to the cycle of methane today. Pockets or pools of nitrogen took shape beneath the icy surface.

Jonathan Lunine also pointed out :  These lakes with steep edges, ramparts and raised rims would be a signpost of periods in Titan's history when there was liquid nitrogen on the surface and in the crust.  Any warming process of the pockets of liquid nitrogen is likely to engender an explosion and to produce a crater because nitrogen can rapidly expand from its liquid form to its gaseous form. As a result, the upward pressure generated by nitrogen against the external crust or icy crust can engender a volcano or a dome and if the pressure is too high, a caldera can take shape. That's a physical process encountered on Earth with other molecules or compounds. Linda Spilker of JPL, who is Cassini Project Scientist pointed out :  This is a completely different explanation for the steep rims around those small lakes, which has been a tremendous puzzle.  She concluded :  As scientists continue to mine the treasure trove of Cassini data, we'll keep putting more and more pieces of the puzzle together. Over the next decades, we will come to understand the Saturn system better and better. 

Titan and the Earth are the only known worlds in the Solar System where there are stable pools of liquid on the surface. The lakes, seas and rivers of Titan fascinate us because they are closely related to a meteorological cycle comparable to our own meteorological cycle. We are in a position to better understand our own atmosphere or our own meteorology thanks to the study of Titan's atmosphere and environment. At first sight, the meteorology and the hydrology of Saturn's largest moon appear strange because clouds are rare at low or mid-latitudes and can be regularly encountered in the polar regions. Planetologists try to understand or to explain the dichotomy in the distribution of pools of liquid between the northern hemisphere and the southern hemisphere. The radar data acquired from the Cassini orbiter have clearly revealed that most lakes and seas on Titan are found in the high latitudes of the northern hemisphere. Seasonal factors must play a key role but we must keep in mind that there are probably internal sources to the methane present in Titan's atmosphere.

Is the north polar region composed of cryovolcanoes or calderas where lakes of methane and ethane have taken shape ? If there is a layer of liquid methane or liquid ethane beneath the presumed icy crust of the northern hemisphere of the Opaque Moon, one can easily imagine geysers or fractures within the crust. Like the ice pack in the Arctic on Earth, the presumed icy plate of the north polar region of Titan can be thinner or more fragile in some places so that lakes of methane or ethane can take shape if the portion of fractured plate sinks for instance. The composition of lakes and seas on Titan can vary over time. Planetologists try to determine the exact composition of the pools of liquid. There may be several layers of hydrocarbons within some lakes or seas. Nitrogen may also be present in the liquid dominated by hydrocarbons. An increase in the concentration of nitrogen in the presumed subsurface layer of hydrocarbons is likely to engender fractures or an explosion within the crust. Therefore, some pools of liquid found in the polar areas may be the outcome of an explosion related to a hot spring involving nitrogen.

The image above represents an artistic view of one of the small lakes of Titan where steep and elevated rims have been observed in radar data obtained from the Cassini orbiter. That's the case for Winnipeg Lacus in the high latitudes of the northern hemisphere for instance. Has Winnipeg Lacus taken shape in a crater produced by an explosion involving warming nitrogen ? Image credit: NASA/JPL-Caltech.

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