March 4, 2021: Titan May Contain Several Types Of Lakes, Seas Or Oceans

Saturn's largest moon Titan represents the only world beyond the Earth, in the Solar System, known to harbor stable pools of liquids on its surface. That's why that giant moon of the Ringed Planet represents a key target in terms of exploration for planetologists. Some researchers and engineers are currently preparing Dragonfly, a new mission to Titan. The Dragonfly probe will be in a position to fly and to explore its environment when it lands on Titan in the 2030's. Thanks to the Cassini spacecraft and thanks to the Huygens probe, we have gathered a huge amount of data regarding the atmosphere, the surface, the chemistry or the upper layers of the Orange Moon. The long mission of the Cassini spacecraft and the intense mission of the Huygens probe have allowed us to better understand the exotic atmosphere of that icy world. We have become aware that Titan is really a complex world which can be compared to a puzzle. Researchers try to gather new clues in order to improve our level of understanding of Saturn's largest moon. Prior to the Cassini-Huygens mission, some researchers had imagined that there may be seas or oceans of methane or ethane in the harsh environment of the Opaque Moon. Was the Earth the only world in the Solar System to contain oceans ?

The Cassini-Huygens mission has brought us clear answers regarding the potential presence of stable lakes, seas, oceans or rivers on Saturn's largest moon. The radar views as well as the infrared or near-infrared views obtained from the Cassini orbiter have clearly shown that there are lakes, seas or rivers in the high latitudes of each hemisphere on the Opaque Moon. Researchers did not expect the surprising distribution of lakes, seas or rivers on that enigmatic world. The Huygens probe obtained, during its atmospheric descent on Titan on January 14, 2005, remarkable panoramic views unveiling a sharp contrast between relatively bright areas and relatively dark areas. One could clearly identify a network of dark drainage channels within the bright hills observed from the parachuted probe. One could also discern a dark or brown plain marking a sharp contrast with the bright hills. Some specialists or researchers had thought, on the basis of the black-and-white views, that the dark or brown plain was in fact a sea of liquid methane. The color view obtained from the surface clearly showed that the probe had not landed onto a sea or ocean. Was the area of the landing site an ancient sea or ocean ?

The Huygens probe had landed at a particularly low latitude in the southern hemisphere of Titan. The area of the landing site was the dark or brown region known as Shangri-La close to the bright area of Adiri. Some aerial views captured from the Huygens probe were reminiscent of a coastline on our planet like the coastline of the French Riviera. The eroded stones or pebbles observed at the level of the soil near the Huygens probe clearly suggest that the probe may have landed onto an ancient stream, brook or river. The network of dark channels observed in the bright hills as well as the view of strongly eroded stones or pebbles imply that there may be regular rainfall events in the area. Are there strong rainfall events related to seasonal factors in that region during the long Titan year ? The network of dark channels is likely related to rainfall events of methane or ethane. However, the network may also be related to cryovolcanism or tectonic events. Researchers have not observed many cloud systems in the low or mid-latitudes of Saturn's largest moon during the journey of the Cassini spacecraft in the Saturn System. However, large cloud systems can rapidly develop in the low or mid-latitudes due to seasonal factors. Can those clouds engender strong rainfall events ?

The radar views of the low or mid-latitudes of Titan, acquired from the Cassini orbiter, unveil dark areas which tend to be dominated by linear and parallel dunes extending over long distances. Those dunes are reminiscent of the Seif Dunes found in the Namib Desert on Earth for instance. How can we explain the presence of large systems of dunes in the low or mid-latitudes of the Opaque Moon ? Do the dark areas of the low or mid-latitudes represent ancient seas or oceans ? The presence of large concentrations of sand potentially implies strong erosional processes in the dark areas of the low or mid-latitudes. If there are regular monsoon events related to seasonal factors in the area, one can imagine that dunes can take shape and develop over time. The shape of the dunes brings clues regarding the potential strength of prevailing winds in the area. Prevailing winds can also play a strong role in the formation and in the development of the dunes. The haze of Titan's atmosphere can produce a type of snow that can fall to the surface and engender the dunes we have observed from the Cassini spacecraft. Some researchers believe that those dunes are rich in hydrocarbons and organics like benzene (C6H6).

Lakes, seas and rivers are mostly concentrated in the high latitudes of the northern hemisphere of the giant moon. Their exact composition is unknown but they may be dominated by methane and ethane. The composition of the pools of stable liquids may in fact vary due to seasonal factors or geographical factors. Methane is the second most abundant gas in Titan's atmosphere. Therefore, one can assume that the lakes, seas or rivers may be dominated by methane. The way methane, ethane or nitrogen interact within the lakes, seas or rivers is a major topic of research. The molecule of ethane (C2H6) is more complex and heavier than the molecule of methane (CH4) so that it may generate a liquid layer beneath the upper layer dominated by methane. In the harsh environment of Saturn's largest moon, methane, ethane and propane can appear in their liquid form thanks to the right combination of atmospheric pressure and ambient temperature. Cloud systems rich in methane as well as cloud systems rich in ethane have been observed from the Cassini spacecraft during the orbital dance of the Cassini spacecraft in the Saturn System. Thus, methane and ethane can combine within the lakes or seas to form different liquids.

The hydrological cycle of Titan is clearly complex. The meteorology of Titan is likely to tell us a lot regarding our own meteorological system. There are similarities between the meteorology of Saturn's largest moon and the meteorology of the Blue Planet but there are also major differences. The lakes, the seas or the oceans of the Earth are dominated by liquid water whereas the pools or the rivers of Titan are dominated by hydrocarbons. On Titan, water can only appear in its solid form on the surface and on the Earth, methane and ethane can only appear in the form of gas due to the relatively high environmental temperatures. The lakes, seas and rivers of Titan represent a new dimension of nature for our researchers. Chemists, biologists, geologists or planetologists are likely to discover new properties of nature by studying those pools or rivers. Complex organics or hydrocarbons are likely to take shape within the stable pools of liquids on the Opaque Moon. Stable liquids can favor the development of complex molecules. But in the harsh environment of the giant moon of Saturn, can prebiotic molecules take shape or develop ? That's a major question we'll be in a position to answer one day !

Titan demonstrates the potential diversity of the worlds found beyond our own star. Many worlds located in extremely cold environments around their star can contain oceans, seas or lakes but those stable pools or those liquid layers will tend to be found beneath their external crust. Mars contains polar caps of water ice and frozen carbon dioxide for instance. Some researchers advance that, beneath those blankets of ice, there may be pools of liquid water. Beyond the Red Planet, in the Solar System, there may also be other worlds containing pools of liquid water. In fact, some worlds like Europa, Enceladus, Triton or Pluto may contain subsurface oceans or seas of liquid water. Titan is also part of the list of the worlds that may contain a subsurface ocean of liquid water in the Solar System. Titan may even contain several liquid layers. One could imagine a layer dominated by liquid methane close to the surface and a layer dominated by liquid water at a higher depth beneath the external crust. If there are cryovolcanic events on the surface of Titan, one will be in a position, one day, to gather major clues regarding the composition of the presumed internal ocean.

The geysers or the cryovolcanoes of Enceladus clearly demonstrate that the small moon of the Ringed Planet is geologically active. Enceladus is dominated by water ice like Mimas, Tethys, Dione or Rhea. Enceladus and Triton are perfect examples which show that an icy world located in an area where the level of energy received from the Sun is relatively low can be active in its interior or on its surface. Tidal forces can have a major impact on the internal activity of a world. That's the case for Io, one of the four major moons of Jupiter. Io is in fact the most active world in the Solar System among the numerous icy worlds or Terrestrial worlds orbiting the Sun. The gravitational influence of Jupiter and the gravitational influence of the other moons of the Gas Giant can engender strong volcanic events on Io. Magnetic phenomena generated by Jupiter also play a major role in the surprising phenomena observed on Io. The remarkable activity of Enceladus is also closely related to the tidal forces engendered by the planet of the system of moons and to the tidal forces generated by the other moons of the Ringed Planet. Titan is located much farther from Saturn than Enceladus but its internal activity may also be remarkable.

The image in the upper part of the table represents a raw view of a portion of Saturn's largest moon Titan. The image whose file name is W00014169.jpg was acquired on March 18, 2006 from the Cassini spacecraft on the basis of the CB3 filter and of the CL2 filter. The view had not been validated or calibrated at the time of the observation and a validated or calibrated version was going to be archived with the Planetary Data System proposed by NASA. The image in the lower part of the table represents a colorized version of the original view. One can notice the sharp contrast between a bright area and a dark area in particular. Are the dark areas found in the low or mid-latitudes ancient seas or oceans ? Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2021.

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