August 19, 2019 : Titan Is A Giant Puzzle Which Might Tell Us A Lot Regarding The Secrets Of Nature

The largest moon of Saturn has all the ingredients to draw our whole attention. Almost all the moons of the Solar System are devoid of any atmosphere. That's not the case for Titan which is covered with a remarkable atmosphere. Titan's atmosphere is completely opaque from outer space in the visible spectrum and is much thicker than our own atmosphere. That's surprising because the Earth is much more massive than Titan. For instance, Ganymede, the largest moon of Jupiter and the largest moon in the Solar System, is devoid of any atmosphere. How can we explain that apparent paradox ? Is the atmosphere of Titan stable over the geological time scale ? Are there internal sources to the gas layer covering the globe ? The atmosphere of the Orange Moon is reminiscent of the atmosphere of Venus which is also completely opaque and which is extremely thick and dense. However, the composition of Titan's atmosphere is radically different from the composition of the Venusian atmosphere. In fact, to a certain extent, the Titanian atmosphere looks like our atmosphere in terms of composition.

Titan's atmosphere is mostly composed of molecular nitrogen like our atmosphere but the second most abundant gas in Titan's atmosphere is not oxygen like in the atmosphere of the Blue Planet. The second most abundant gas in Titan's atmosphere is methane. Water can't appear in its liquid form on Titan's surface because the environmental temperature is extremely low at the level of the ground. The mean environmental temperature on Titan's surface is around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. There are the right combinations of atmospheric pressure and environmental temperature at the level of Titan's surface for the presence of pools of methane, ethane or even propane. Water can only appear in its solid form on the surface of the Orange Moon. A parallel can be drawn between the meteorological cycle of the Earth based on water and the meteorological cycle of Titan based on methane. Clouds of methane or ethane have been clearly observed in the Titanian atmosphere during the Cassini-Huygens mission.

Some planetologists advance that the atmosphere of Titan looks like the atmosphere of the Early Earth. A complex haze of organics or hydrocarbons is generated in the deep and thick atmosphere of Saturn's largest moon thanks to photochemical processes occurring in the upper atmosphere of the giant moon. UV light from the Sun interacts with particles, molecules, atoms or ions present in the upper atmosphere of Titan to engender new molecules or compounds. The larger or heavier molecules or particles tend to go down toward the surface and can form tholins or a type of mud. Remarkably large molecules or relatively complex molecules have been identified in Titan's atmosphere thanks to data from the Cassini orbiter. Titan's atmosphere is remarkably dynamic since polar cyclones or vortices as well as dynamic cloud systems can take shape. There are clearly seasonal phenomena on the Opaque Moon. A giant polar cyclone or vortex of ethane had been observed from the eye of the Cassini spacecraft during the Winter season in the northern hemisphere. A polar vortex had been observed from the eye of the Cassini spacecraft, as well, during the Autumn season in the southern hemisphere.

A Titanian year is particularly long since it represents almost 30 Terrestrial years and each Titanian season lasts about 7 Terrestrial years. The Cassini orbiter has gathered data regarding Titan's atmosphere, Titan's surface and Titan's interior from the start of the mission in the Saturn System in 2004 to the end of the mission with the crash against the Ringed Planet in 2017. Thus, the long mission has represented much less than a Titanian year or about half a Titanian year. The dynamics or the composition of Titan's atmosphere can change due to seasonal factors. That's why we need to continue to gather data or clues regarding the dynamics or the composition of the exotic atmosphere of Saturn's largest moon. Planetologists are particularly interested in the methane cycle of Titan because that meteorological cycle is likely to account for the strange distribution of lakes, seas and rivers on Titan. How to explain that the pools of hydrocarbons are mostly concentrated in the high latitudes of each hemisphere or in the polar areas ? The radar data obtained from the Radar Mapper of the Cassini orbiter had clearly shown that the north polar region is the most humid area on Titan.

Titan is a world which looks like the Earth to a certain extent. Evaporation processes, condensation processes and precipitation processes occur on the Orange Moon like on Earth. Planetologists try to gather clues regarding potential internal sources to the lakes, seas or rivers found in the high latitudes of the northern hemisphere. Is there a layer of liquid methane beneath the presumed icy crust ? Are there geysers or cryovolcanoes spewing methane in the exotic environment of Saturn's largest moon ? There may be a subsurface ocean beneath the external crust but its composition is unknown. Is the presumed subsurface ocean composed of liquid methane or liquid water ? Are there several layers of liquid beneath the presumed icy crust ? One can easily imagine a layer of liquid methane beneath the surface at a relatively low depth and a layer of liquid water at a higher depth. Obviously, liquid water can't be encountered on Titan's surface. However, at a relatively high depth, if there is the right combination of temperature and pressure, water can be present in its liquid form. The internal structure of Titan is a major topic for planetologists.

Over a geological time scale, methane should have disappeared if there are no replenishment sources from the interior of the giant moon. In the harsh environment of the Saturn System, Titan's atmosphere unveils a remarkable dynamics and like on Venus, the winds can be particularly strong. The global winds can reach speeds of 780 miles per hour or 1,255 kilometers per hour. That's a surprising phenomenon known as  Super-Rotation . The level of energy from the Sun reaching the Titanian atmosphere is relatively weak compared to the level of energy received by our atmosphere. That's why one can be surprised to observe extremely strong winds so far away from the Sun. The landscape of Titan can be eroded or shaped by prevailing winds. The radar data obtained from the Radar Mapper of the Cassini spacecraft have clearly shown the presence of linear and parallel dunes extending over long distances in the relatively dark areas of the low or mid-latitudes. Those exotic dunes can be up to a few kilometers wide and 100 kilometers or 62 miles long.

The dunes of Titan may be composed of molecules related to the haze of organics and hydrocarbons. The heavier molecules of the haze may fall to the surface to form tholins or to form the sand contained in those dunes. The Titanian dunes may be rich in organics or hydrocarbons like benzene. Thanks to the Dragonfly mission, we will have the opportunity to study the dunes for a long period of time. A drone will fly in an environment of dunes. It will do much more than the Huygens probe had done on January 14, 2005. Prevailing winds apparently play a key role in the shape of those exotic dunes. Rainfall can also occur at low or mid-latitudes from time to time. Therefore, the landscape at low or mid-latitudes is likely to be strongly eroded by methane rain. The Cassini orbiter had revealed the presence of dynamic and transient cloud systems at low or mid-latitudes in 2010. Those large clouds must have brought a lot of rain in the land of dunes. One can't rule out that the land of dunes in the dark areas is the outcome of an ancient sea or ocean like the Sahara on Earth.

Craters are quite rare on the surface of Titan due to erosional processes related to winds or rain in particular. The level of porosity of the Titanian soil is a major topic for researchers. If the soil is particularly porous, one can imagine the presence of caves containing lakes or rivers of methane or ethane for instance. Planetologists try to anticipate the types of complex organics that we could encounter on the surface of Titan, beneath the external crust of Titan or in the haze of the giant moon. A lifeform based on liquid water can't be envisaged on the surface of Titan simply because water can't appear in its liquid form at the level of the ground and CO2 and O2 are absent or almost absent in Titan's atmosphere. So, the typical lifeform based on liquid water and carbon can't be envisaged at the level of the ground. However, beneath the external crust, there may be a layer dominated by liquid water where a lifeform based on liquid water and the other compounds of the local environment could potentially exist. The lakes, seas and rivers of hydrocarbons observed in the high latitudes of the Opaque Moon might host an exotic lifeform based on liquid methane or liquid ethane. Can liquid methane or liquid ethane act as solvents for the development of an exotic lifeform ? That's a big question which deserves our attention. But, can molecules like proteins, lipids, carbohydrates, amino acids, RNA or DNA take shape in the harsh environment of Saturn's largest moon ? Can we imagine other types of dynamic molecules which are more adapted to the harsh environment of Titan ?

The image above reveals the Titanian disk in natural colors. The image was generated on the basis of data obtained with the Wide-Angle Camera of the Cassini spacecraft on August 21, 2005. The views were taken using red, green and blue spectral filters at a distance of about 213,000 kilometers or 132,000 miles from the Opaque Moon and at a Sun-Titan-probe, or phase, angle of 55 degrees. The image clearly shows, in the visible spectrum, that the surface can't be discerned from outer space due to the exotic haze. However, in the infrared or near-infrared spectrum, the surface of the giant moon can be discerned from outer space. Image credit: NASA/JPL/Space Science Institute.

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