July 7, 2018 : What Do We Know About The Lakes, Seas And Rivers Of Titan Today ?

The discovery of lakes, seas and rivers on Saturn's largest moon Titan is undoubtedly among the most remarkable discoveries ever made in the exploration of worlds beyond the Earth in the Solar System. Thanks to the Voyager 2 spacecraft, we have been in a position to determine that there are exotic geysers on Neptune's moon Triton. Thanks to the Voyager 1 spacecraft, we have identified outstanding volcanoes on Io, one of the Galilean Moons of Jupiter. The Galileo spacecraft captured remarkable images of the icy world Europa, another moon of Jupiter, dominated by a fractured landscape implying the potential presence of a subsurface ocean of liquid water. More recently, the Cassini spacecraft also made major discoveries in the System of Saturn. Thanks to that orbiter, we know, now, that Enceladus, a tiny moon of Saturn which appears particularly bright and icy, unveils active geysers in the fractures of its south polar region. There may be pockets, seas or a layer of liquid water beneath the presumed icy crust of Enceladus. During the Cassini-Huygens mission, Titan has drawn our whole attention because it is really a fascinating world unveiling familiar landscapes such as canyons, hills, mountains, tectonic phenomena, dune fields and obviously rivers, lakes or seas.

Prior to the plunge of the Huygens probe into Titan's atmosphere on January 14, 2005, some planetologists had imagined that the giant moon of the Gas Giant Saturn could host lakes, seas, rivers or even oceans on its surface. The idea that there may be seas or oceans of liquid water on Titan's surface was completely ruled out simply because the environmental temperature at the level of the soil of that intriguing world is around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin. Water can only appear in its solid form on Titan's surface. The level of solar energy reaching the System of Saturn is particularly low since Saturn and Titan evolve very far away from our star at a distance of about 1.4 billion kilometers from the Sun which is almost 10 times higher than the mean distance between the Earth and the Sun. The deep, thick and opaque atmosphere of the Orange Moon allows the potential presence of liquids on Titan's surface. But what kind of liquid can be present on the surface of that exotic moon ? Pure liquid nitrogen on Titan's surface can be ruled out because the environmental temperature is too high for liquid nitrogen to appear in its liquid form even if the atmospheric pressure on the surface is remarkably high.

In fact, Titan is known to have the right combination of atmospheric pressure and environmental temperature at the level of the surface to allow the presence of liquid methane (CH4), liquide ethane (C2H6) or liquid propane (C3H8) on its surface. We know that Titan's hazy atmosphere is mainly composed of molecular nitrogen and that the second most abundant gas in that gas layer is methane which represents a relatively high fraction of the overall composition. Therefore, prior to the Cassini-Huygens mission, some researchers had advanced the possibility that the presumed lakes, seas, oceans or rivers of the Opaque Moon may be dominated by liquid methane. Prior to the historic landing of the Huygens probe onto Titan's surface, the infrared or near-infrared views obtained from the Cassini orbiter had revealed a major contrast between bright areas and dark areas on the surface of that enigmatic world. Did the dark areas found at low or mid-latitudes correspond to seas or oceans of liquid methane ? We would have to wait until the touchdown of the Huygens probe to get a clearer idea upon the nature of those dark areas. Prior to the atmospheric descent, some planetologists had advanced that the dark areas located at a relatively low latitude were not dark or uniform enough to represent seas or oceans of hydrocarbons like methane or ethane.

During its atmospheric descent toward the Shangri-La/Adiri region on January 14, 2005, the eye of the Huygens probe obtained a multitude of panoramic views of the area. And the images were remarkable since we saw bright hills composed of dark fractures or drainage channels as well as a dark or red plain contrasting with the bright hills. The aerial views of the dark or red plain were far from being obvious since they appeared in black and white and since the plain appeared quite uniform. One could interpret the dark or red plain as a sea of liquid methane even if most commentators regarded it as a dry land or a land devoid of any liquids. The views acquired from Titan's surface clearly showed that the Huygens probe had not landed onto a sea or lake. However, they unveiled eroded stones or pebbles close to the probe implying that the probe may have landed onto an ancient stream or brook. On the basis of those observations, one can advance that there may be rainfall from time to time in that region located at a particularly low latitude close to the equator. Some researchers believe that there may be monsoon events related to seasonal factors in the area. At the time of the touchdown, the southern hemisphere of the Orange Moon was experiencing the Summer season whereas the northern hemisphere was experiencing the Winter season.

During the long mission of the Cassini spacecraft, we have monitored and studied Titan's atmosphere in order to try to better understand its dynamics and its meteorology. We know, now, that there is a complex methane cycle on Titan. Our analysis of Titan's atmosphere required time because the year and the seasons on Titan are very long. A Titanian year represents almost 30 Terrestrial years and a Titanian season lasts about 7 Terrestrial years indeed. Thanks to the radar data taken from the Radar Mapper of the Cassini spacecraft during its journey in the Saturn System, we have been in a position to determine that the dark areas found in the equatorial or tropical region are dominated by linear and parallel dunes extending over long distances under the influence of prevailing winds. Therefore, those low-albedo areas, located at relatively low latitudes and contrasting with bright areas, don't represent the seas or oceans we had previously imagined. However, the aerial views from the Huygens probe have clearly shown that, at least, active rivers can form in the bright hills of Adiri.

Are there seasonal rainfall events at low or mid-latitudes on Titan ? Were there seas or oceans of liquid methane in the dark areas of the low or mid-latitudes in the past ? The presence of Seif dunes or linear and parallel dunes extending over long distances in the dark areas of Titan's low or mid-latitudes suggests the potential presence of ancient oceans or seas of methane. Over time, processes of erosion and dissolution related to liquid methane or liquid ethane may have engendered the fields of dunes clearly observed in radar images. If there are seasonal rainfall events in the area, one can imagine that evaporation processes, condensation processes and winds can play a key role in the formation and in the development of those dunes. The dunes found at low latitudes are likely rich in organics or hydrocarbons. Those molecules may also result from the complex chemistry taking shape in the haze of Titan's atmosphere which can engender tholins. Tholin represents a dark or red sludge which is closely related to the photochemistry involving UV light from our star. There are complex interactions between UV radiations from the Sun and molecules, particles or ions present in Titan's upper atmosphere. In the chemical soup of Titan's haze, complex hydrocarbons or organics can take shape and fall toward the soil of the Orange Moon.

To a certain extent, we were quite disappointed to notice that the dark areas of the low or mid-latitudes of Titan are not dominated by seas or oceans of methane or ethane. However, thanks to infrared or near-infrared data acquired from the Cassini spacecraft, we rapidly identified a pool of liquids on Titan's surface and that pool of liquids appeared in a surprising location that is to say at a particularly high latitude in the southern hemisphere of the Orange Moon. That body of surface liquids known as Ontario Lacus turned out to be the first stable extraterrestrial lake or sea identified in the Outer Solar System. Later, thanks to radar data obtained from the Radar Mapper of the Cassini orbiter, we discovered a multitude of lakes, seas and rivers in the high latitudes of the northern hemisphere. At the time of the first observations, the northern hemisphere was experiencing the Winter season. In fact, we have determined that the north polar region of the giant moon represents the most humid area of Titan. How can we explain that surprising distribution of bodies of surface liquids on Titan ?

Planetologists believe that the evolution in the distribution and in the size of the lakes, seas and rivers on Titan may be closely related to seasonal factors. The lakes, seas and rivers present in the north polar region may be richer in methane than the south polar lake or sea Ontario Lacus which may have a higher concentration of ethane. Due to the high concentration of nitrogen in Titan's atmosphere, researchers assume that the surface liquids on Titan may contain dissolved nitrogen mixed with methane and ethane. The composition of the lakes, seas or rivers may vary via photolysis. Thus, liquid methane can engender more complex hydrocarbons like ethane via the action of UV light from the Sun. A study entitled  Laboratory measurements of nitrogen dissolution in Titan lake fluids , published in Icarus in 2017 and proposed by a team involving Michael J. Malaska, had revealed the complex interactions between methane, ethane and nitrogen in a simulated lake or seas of Titan. The specialists had shown that on heating, high volumes of nitrogen gas was released with lots of bubbles and that on cooling, high volumes of nitrogen gas was absorbed. When methane and ethane were mixed, large amounts of nitrogen were released as well. One can imagine the behavior of a potential pool whose temperature is around 91 Kelvin in an environment whose temperature is around 89 Kelvin.

The image in the upper part of the table unveils a portion of Kraken Mare, the largest pool of surface liquids on Titan. The image in the lower part of the table represents a Google Earth view of a portion of the coast of Greece. Both views are at the same scale and are 100 km wide and 100 km high. The view of a region of Kraken Mare corresponds to a portion of a radar swath acquired from the Radar Mapper of the Cassini probe during the T30 Flyby performed on May 12, 2007. The original color of the image showing a portion of Greece has been removed in order to make the comparison easier. One can notice a lot of islands or peninsulas in the image of Kraken Mare as well as in the image of Greece. Water can't appear in its liquid form on the surface of Titan but a layer of liquid water where extremophiles can take shape may be hidden beneath the icy crust. Kraken Mare may be dominated by liquid methane.

Credit for the original image of Kraken Mare: NASA/JPL/Cassini RADAR Team/Jason Perry.
Credit for the original image of the portion of Greece: Google Earth.
Credit for the image adjustments: Marc Lafferre, 2018.

- To get further information on that news, go to: https://www.sciencedirect.com/science/article/pii/S001910351630375X.



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