September 14, 2018: Planetologists Have Been Surprised By The Remarkable Asymmetry In The Distribution Of Surface Liquids Between The North Polar Region And The South Polar Region On Titan During The Cassini Mission

During its journey inside the Saturn System between 2004 and 2017, the Cassini spacecraft has obtained a huge amount of data regarding Saturn and its numerous moons, from Mimas to Enceladus, Titan or Iapetus. The Cassini orbiter was equipped with multiple instruments to observe or study the Gas Giant Saturn and its natural satellites. For instance, the Visual and Infrared Mapping Spectrometer of the probe was able to acquire infrared or near-infrared data regarding Saturn or Titan and it appeared clearly adapted for the observation of Saturn's largest moon in particular. Titan appears completely opaque from outer space in the visible spectrum. The surface of Titan as well as cloud formations become visible with a camera using spectral filters sensitive to infrared or near-infrared light. Thanks to the Visual and Infrared Mapping Spectrometer, we were in a position to observe and study the surface of the Opaque Moon as well as meteorological phenomena. The Cassini spacecraft was also equipped with a radar instrument which allowed us to observe the surface as well and which turned out to be particularly fruitful.

One of the main discoveries during the Cassini-Huygens mission is clearly the identification of lakes and seas of hydrocarbons in the high latitudes of Titan, undoubtedly one of the most fascinating worlds in the Solar System. Researchers had anticipated that we would find lakes, seas or even oceans of methane or ethane on Titan but the panoramic images acquired during the atmospheric descent of the Huygens probe on January 14, 2005 clearly showed that the brown or dark plain where the Huygens probe was going to land was devoid of any liquid during that Summer period in the southern hemisphere of Titan. Later, we found, on the basis of infrared data from the Cassini orbiter, Ontario Lacus, the first body of surface liquids identified on the giant moon. Ontario Lacus, likely composed of methane and ethane, appeared at a high latitude in the southern hemisphere. The radar data obtained from the Cassini spacecraft revealed that the dark or low-albedo areas found at low or mid-latitudes and marking a sharp contrast with relatively bright areas were dominated by Seif dunes or linear and parallel dunes extending over long distances.

Thanks to radar data acquired from the Cassini orbiter, planetologists also found a land of lakes, seas and rivers in the high latitudes of the northern hemisphere. At the time of the first observations of the north polar pools of hydrocarbons, the northern hemisphere was experiencing the Winter season. The Radar Mapper of the Cassini spacecraft appeared perfect to discern surface features or lakes, seas and rivers in the high latitudes of the northern hemisphere which was encountering the relative darkness of Winter. The lakes, seas and rivers of that humid region appeared to be mostly composed of methane and ethane. Researchers realized that the bodies of surface liquids on Titan are mostly concentrated in the high latitudes or in the polar areas. And curiously, the high latitudes of the northern hemisphere appear by far the most humid area on the giant moon. The dichotomy in the distribution of lakes, seas and rivers between the southern hemisphere and the northern hemisphere is a major subject today. Some researchers have advanced that there may be seasonal factors regarding the level or the size of the pools and that the variations in the distance between Titan and the Sun over a full Titanian year may play a key role in the movement or migration of liquid hydrocarbons.

Elizabeth ( Zibi ) Turtle of the Johns Hopkins Applied Physics Lab and the Cassini Imaging Science Subsystem (ISS) group that acquired the images of the lakes, seas and rivers pointed out :  We expected more symmetry between the southern and northern summer.  The last image of Titan's north polar pools of hydrocarbons, obtained from the Cassini spacecraft during the final distant flyby of the Orange Moon on September 11, 2017, four days before the planned crash between the Cassini probe and the Gas Giant Saturn, clearly revealed the major pools of liquids of Titan's northern hemisphere as well as a few small clouds. Planetologists had speculated that we would find more clouds than that in that humid area, at the beginning of the Summer season in the northern hemisphere. Elizabeth ( Zibi ) Turtle advanced :  In fact, atmospheric models predicted summer clouds over the northern latitudes several years ago. So, the fact that they still hadn't appeared before the end of the mission is telling us something interesting about Titan's methane cycle and weather. 

The dynamics and the weather of Titan's atmosphere are major subjects of research. Liquid water can't exist on the surface of Titan due to the extremely harsh environment of the Opaque Moon where ambient temperatures evolve around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin. However, a parallel can be drawn between the meteorological cycle of the Earth and the meteorological cycle of Titan. There is a meteorology involving methane on Titan. Thus, there is a methane cycle on the Orange Moon where methane can appear in its liquid form on the surface and where methane can form clouds and fall as rain. Like on Earth, there are evaporation processes, condensation processes and precipitation processes on Titan. That's why we have seen rivers, lakes, seas or canyons on Titan. At the beginning of the Cassini-Huygens mission, we had observed dynamic clouds or storms in the high latitudes of the southern hemisphere in the area of Ontario Lacus. The southern hemisphere was experiencing the Summer season when we observed the bright patches of the clouds in the infrared or near-infrared spectrum.

The study of Titan's meteorology implies a long study of Titan's atmosphere since a Titanian year lasts almost 30 Terrestrial years and since a Titanian season lasts approximately 7 Terrestrial years. From the start of the Cassini mission inside the Saturn System in mid-2004 to its end on September 15, 2017, the Cassini spacecraft has obtained a multitude of infrared or near-infrared images and radar images of Titan during three different Titanian seasons. And researchers have been in a position to partly analyze the dynamics and the concentration of clouds in the southern hemisphere during the Summer season and in the northern hemisphere during the Summer season. They can advance now that cloud activity in the high latitudes of the southern hemisphere was apparently more intense during the Summer season than that in the high latitudes of the northern hemisphere during the Summer season. Let's point out however that the Summer season was only starting at the time of the last Cassini observations of Titan's lakes and seas located in the north polar region.

The last images of Titan captured on September 11, 2017 combined to form a mosaic were obtained with the ISS Narrow-Angle Camera of the Cassini spacecraft, on the basis of a spectral filter sensitive to wavelengths of near-infrared radiations centered at 938 nanometers. The mosaic was produced on the basis of data acquired at a distance of about 87,000 miles or 140,000 kilometers from the Opaque Moon. The view corresponds to an orthographic projection centered on 67.19 degrees north latitude and 212.67 degrees west longitude. The orthographic view roughly represents the image observed by a distant viewer looking through a typical telescope. The mosaic clearly unveils, in particular, the major pools of liquids located in the high latitudes of the northern hemisphere that is to say Kraken Mare, Ligeia Mare and Punga Mare. Kraken Mare which represents the largest body of surface liquids on Titan is about 730 miles or 1,200 kilometers wide whereas Ligeia Mare is about 300 miles or 500 kilometers wide. Punga Mare is a little bit smaller than Ligeia Mare since the pool is about 240 miles or 390 kilometers wide. A multitude of relatively small lakes found in the right part of the mosaic can also be clearly seen in the image.

Cloud features close to the dark patch of Ligeia Mare or in the dark patch of Ligeia Mare can be discerned. Is there a process of net evaporation during the Summer season in the high latitudes of the northern hemisphere ? Will the level and the size of Kraken Mare, Ligeia Mare and Punga Mare go down as the Summer season advances in the northern hemisphere ? Are those pools of liquids exclusively related to meteorological phenomena ? Are there internal sources to the pools of liquids found in the north polar region ? Is there an internal network of liquid methane beneath the crust of the north polar region ? Researchers suspect the presence of a layer of liquid water beneath the presumed icy crust of Saturn's largest moon. Researchers advance that the haze of Titan may engender a dark or red sludge on the surface of Titan. Can it engender a prebiotic chemistry ? Can it engender an exotic lifeform ? Is there a methane-based life on Titan ? Elizabeth ( Zibi ) Turtle argued :  Titan is a fascinating place that really teases us with some of its mysteries.  Titan appears to be an ideal place to study the chemistry of carbon or organics since there are surface liquids dominated by methane and ethane which can stimulate chemical reactions.

The image above corresponds to a mosaic of near-infrared images of Titan obtained from the ISS Narrow-Angle Camera of the Cassini probe on September 11, 2017. One can notice in particular Kraken Mare, Ligeia Mare and Punga Mare which may be filled with a mixture of methane and ethane incorporating dissolved nitrogen. All those pools of liquids are found in the high latitudes of the northern hemisphere of that intriguing moon. Image credit: NASA/JPL-Caltech/SSI.

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