September 15, 2022: The Discoveries On Titan Made Possible By The Cassini-Huygens Spacecraft Allowed Us To Identify A New Dimension Of Nature In The Universe

The long exploration campaign of the Cassini-Huygens spacecraft in the Saturn System from 2004 to 2017 has allowed us to perform major discoveries regarding Titan, the largest moon of the Ringed Planet Saturn, in particular. The opaque atmosphere of that intriguing world prevents us from discerning surface features in the visible spectrum from outer space. The Pioneer 11 spacecraft, the Voyager 1 spacecraft and the Voyager 2 spacecraft had not been in a position to identify surface features during their flyby due to that opaque atmosphere. Planetologists realized that Titan is a really particular moon that deserves an ambitious mission to study it from the atmosphere to the surface or the underground. They suspected the presence of oceans or seas rich in methane or ethane on the surface of the Orange Moon Titan. Thus, NASA and ESA in particular collaborated to realize a complex mission to study Saturn, Titan and the other moons of the Gas Giant. The engineers and the researchers of the mission developed the Cassini-Huygens spacecraft. That spacecraft lifted off in 1997 for a long trip to the Saturn System. The Saturn Orbit Insertion (SOI) occurred in 2004.

The spacecraft used the gravitational forces of Saturn and the numerous moons to perform a multitude of flybys of the Ringed Planet, Titan and the other neighboring moons. The Cassini-Huygens spacecraft performed a multitude of elliptical orbits in the Saturn System during its long mission. The Cassini-Huygens spacecraft contained a module to explore the surface of Titan. That module was the Huygens probe. The Huygens probe performed a parachuted descent and landed on that intriguing world on January 14, 2005. The aerial views acquired during the atmospheric descent revealed surface features resembling Terrestrial landscapes. One could notice bright hills containing a network of dark channels as well as a brown or dark plain that looked like a dried-up sea. The dark channels are likely drainage channels related to meteorological phenomena. They seemed to be dry during the parachuted descent. Some researchers and some individuals in the general public had drawn a parallel between the images of the landscape of Titan acquired during the parachuted descent and the images of the French Riviera from a plane approaching the airport of Nice.

The parachuted descent had been performed at a low latitude in the southern hemisphere of the giant moon. The brown or dark plain could be compared to an evaporated sea that remained relatively wet or humid. Can there be flooding events in the area from time to time during the long Titanian year ? Can the brown or dark plain become an active sea from time to time ? Are there very dry periods and very wet periods in the area during the long Titanian year ? The observations of the atmosphere of the Opaque Moon acquired from the Cassini spacecraft have demonstrated that the meteorology of Titan is complex and can be surprising. The low or middle latitudes of the giant moon can undergo heavy rainfall events related to seasonal factors. The area of the relatively bright Adiri and of the relatively dark Shangri-La that is found at a particularly low latitude can potentially receive strong rainfall events. The atmosphere of Titan appears to be dominated by molecular nitrogen like the atmosphere of the Earth and the second most abundant gas of the atmosphere of that intriguing moon is methane. That's why most clouds encountered in the hazy atmosphere of that world must be dominated by methane.

A parallel between water on Earth and methane on Titan can be drawn. Water on Earth can be present in its icy form if the environmental temperature is sufficiently low, can be present in the form of a liquid if the environmental temperature is sufficiently high and can be present in the form of vapor due to evaporation processes. On Titan, water can only be present in its solid form on the surface due to the extremely low environmental temperatures. However, in the harsh environment of Titan, methane can condense to form a liquid and methane can be present as a gas in the atmosphere. One must imagine that the environmental temperature at the level of the surface of the Opaque Moon is around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. The physical conditions and the chemical conditions are completely different from the physical and chemical conditions on our planet. Prior to the Cassini-Huygens mission in the Saturn System, many planetologists had anticipated that there could be oceans, seas or rivers of methane or ethane on the surface of Titan. Unfortunately, the Huygens probe landed in an area devoid of any active sea, lake or river. But there were clearly signs of erosional processes related to lakes, seas or rivers.

The famous color view obtained with the Huygens probe from the surface of Titan shows strongly eroded stones or pebbles. The Huygens probe seems to have landed onto an ancient brook, stream or river. That's clearly a sign that it must rain from time to time in the area. The soil seems humid. During the touchdown, the probe had recorded a relatively strong release of methane suggesting that the soil must be relatively rich in methane. The eroded stones or pebbles must be rich in water ice. The brown or dark soil must contain hydrocarbons or organics. The hazy atmosphere of the giant moon must engender relatively heavy molecules or grains that can fall to the surface to form dunes for instance. The Radar Mapper of the Cassini spacecraft has clearly shown that the relatively dark areas found in the low or middle latitudes, identified in the infrared or near-infrared spectrum, are dominated by Seif dunes or linear and parallel dunes extending over long distances. The nature of those exotic dunes is quite mysterious but researchers believe that they may be relatively rich in organics and hydrocarbons like benzene. The shape of those dunes is closely related to prevailing winds.

The photochemical haze of Titan can represent a source for the giant dune fields found in the relatively dark areas of the low or middle latitudes. Snowfall events can potentially occur in the low or middle latitudes and fuel the dunes that look like the Seif Dunes found in the Namib Desert on Earth to a certain extent. The rotorcraft of the Dragonfly mission should be in a position to study those exotic dunes when it explores Titan in the 2030s. The chemistry of the organics and of the hydrocarbons found on Titan may tell us a lot regarding the chemistry of organics on Earth. Can the chemistry of organics and hydrocarbons represent a prebiotic chemistry or a chemistry that can lead to life ? The carbon of Titan can interact with liquid methane, liquid ethane or liquid propane to form relatively complex molecules. What types of molecules can those types of interactions engender ? Can a lifeform based on liquid methane take shape in the harsh environment of Saturn's largest moon ? We know that our biosphere is based on carbon and liquid water. Liquid water represents a solvent for life. Can liquid methane or liquid ethane engender prebiotic molecules or biological entities as well ?

Thanks to the Cassini orbiter, we know, now, that Titan contains lakes, seas and rivers on its surface. But the distribution of those lakes, seas and rivers is surprising. The lakes, seas and rivers are mostly concentrated in the high latitudes of each hemisphere or in the polar regions. Therefore, the Huygens probe had not had the opportunity to float or to perform a splash. The Cassini spacecraft has shown that the high latitudes of the northern hemisphere were the most humid areas of the Opaque Moon. Three major lakes or seas can be identified in the high latitudes of the northern hemisphere. The largest pool is Kraken Mare. The two other major pools of the northern hemisphere are Ligeia Mare and Punga Mare. The first lake or sea identified on Titan was Ontario Lacus, a dark pool identified in the high latitudes of the southern hemisphere thanks to the infrared or near-infrared eye of the Cassini orbiter. Ontario Lacus whose shape is close to the shape of a foot may look like a pond. Ontario Lacus was identified in the first half of the mission in the Saturn System during the Summer season in the southern hemisphere of Titan and during the Winter season in its northern hemisphere.

The surprising dichotomy in the distribution of lakes, seas and rivers on Titan may be closely related to seasonal, physical or orbital factors. The level and the size of the lakes or seas in the southern hemisphere may be higher during the Winter season due to net condensation processes. The eye of the Cassini probe had revealed a multitude of transient and dynamic clouds in the area of Ontario Lacus during the Summer season in the southern hemisphere. At the end of the Cassini mission in the Saturn System, we had also clearly identified clouds in the area of the lakes, seas and rivers in the high latitudes of the northern hemisphere. The Summer season was starting in the northern hemisphere and the illumination from the Sun was progressively increasing so that evaporation processes could increase engendering cloud formations. Those clouds dominated by methane or ethane can produce rain like the typical clouds on Earth. During the long Titanian year which lasts almost 30 Earth years, there can be migration phenomena of methane from one polar area to the other polar area. From the Winter season to the Summer season, a polar area can lose a lot of methane due to net evaporation processes so that the size or the level of the lakes, seas or rivers found in that polar area tends to decrease. Titan clearly shows the remarkable diversity of worlds one can potentially find in the Universe.

The image in the upper part of the table reveals a raw image of a portion of the disk of Titan acquired on September 13, 2017 from the Cassini spacecraft. The view whose file name is N00289293.jpg was captured on the basis of the CL1 filter and of the CB3 filter. The image 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. One can clearly discern the shape of a pool or sea in the lower part of the disk in particular. The image in the lower part of the table represents a colorized version of the original image. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2022.

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