July 11, 2020 : What Types Of Exotic Phenomena Might We See On The Surface Of Titan ?
On Earth, one can observe, from time to time, remarkable phenomena such as lightning, magnetic auroras, rainbows, tornadoes or twisters. Could those phenomena be observed on Saturn's largest moon Titan ? Planetologists often say that Titan and the Earth look alike to a certain extent. Titan is covered in a relatively thick atmosphere like our planet. Most moons in the Solar System are devoid of any atmosphere and Titan represents an exception. Titan is much lighter than the Earth but it contains a deeper, thicker or denser atmosphere than the Blue Planet. The study of Titan's atmosphere can tell us a lot regarding the dynamics or the evolution of our atmosphere. The chemistry of Titan's atmosphere draws the whole attention of planetologists or exobiologists. Titan's atmosphere is dominated by molecular nitrogen like our atmosphere. However, oxygen is absent or almost absent in the atmosphere of Saturn's largest moon. Methane appears to be the second most abundant compound in that exotic atmosphere. There is a methane cycle between the atmosphere, the surface and probably the interior of the giant moon. In fact, typical phenomena regularly encountered in Earth's atmosphere can be observed in Titan's atmosphere.
Thanks to the Cassini-Huygens mission, we have been in a position to collect a huge amount of information regarding the dynamics or the chemistry of Titan's atmosphere from 2004 to 2017. Exotic clouds can take shape in the harsh environment of the Opaque Moon. Those clouds are not dominated by water ice or water vapor unlike the typical clouds observed on Earth. The clouds of Titan tend to be dominated by hydrocarbons. On Titan, clouds of methane or ethane can form or develop. Transient and dynamic clouds have been observed in the low or mid-latitudes of Titan during the Cassini-Huygens mission in the Saturn System. In the south polar region or in the high latitudes of the southern hemisphere, numerous dynamic clouds or storms have also been clearly observed during the Summer season in the southern hemisphere. During the Autumn season in the southern hemisphere, a yellowish vortex or cyclone evolving and developing at a high altitude over the south polar region has also been identified thanks to data acquired from the Cassini orbiter. The clouds of hydrocarbons can generate rainfall and fuel the lakes, seas or rivers. The system of clouds spotted in the high latitudes of the southern hemisphere can be found in the area of Ontario Lacus, the first pool of liquids identified on Titan.
Titan appears to be the only extraterrestrial world in the Solar System containing stable pools of liquids on its surface. The lakes, seas or rivers of methane or ethane on Titan can engender complex chemical reactions involving organics or carbon. That's why exobiologists are particularly interested in the intriguing chemistry of Titan's atmosphere and Titan's surface. In an environment where the gravity is relatively low and where the atmospheric pressure is relatively high, will there be stronger waves in the Titanian lakes or seas than in the Terrestrial lakes or seas ? The analysis of Ontario Lacus which has been observed with the infrared or near-infrared eyes and with the Radar Mapper of the Cassini orbiter suggests that the pool may represent a type of pond. Therefore, the pool may be particularly flat most of the time. Ontario Lacus may look like a mountain lake in terms of dynamics to a certain extent. The size and the level of Ontario Lacus may change during the Titanian year which represents almost 30 Earth years. If condensation processes are higher than evaporation processes, the size and the level of the lake or sea will tend to grow and if condensation processes are lower than evaporation processes, the size and the level of the lake or sea will tend to decrease.
In fact, seasonal factors must play a key role in the dynamics or in the chemistry of Titan's atmosphere. During the Cassini-Huygens mission, a relatively strong dichotomy in the distribution of lakes, seas and rivers had been observed between the southern hemisphere and the northern hemisphere. The eyes of the Cassini orbiter have clearly shown that the north polar region or the high latitudes of the northern hemisphere represent the most humid area on Titan. How can we explain the relatively strong asymmetry in the concentration of lakes, seas and rivers between the south polar region and the north polar region ? Is it related to physical or orbital factors or is it related to seasonal factors ? The Cassini orbiter has had the opportunity to study Titan's atmosphere and Titan's surface during the Winter season, the Spring season and the Summer season in the northern hemisphere. But the Cassini researchers or engineers have captured data during a relatively small portion of the Summer season in the northern hemisphere. A season on Titan represents about 7 Terrestrial years and the Summer season in Titan's northern hemisphere had begun in May of the year 2017.
Planetologists need to study Titan's atmosphere in the long run in order to understand it correctly. There are similarities between the environment of Titan and the environment of the Earth but there are also major differences. The mean temperature on the surface of our planet is around 15 degrees Celsius whereas the mean temperature on the surface of the Opaque Moon is around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. Therefore, in the harsh environment of Titan, water can only appear in its solid form on the surface and methane which represents a gas on Earth can appear in its liquid form on the surface. That's also the case for ethane and propane which also represent hydrocarbons. Methane is particularly volatile so that a limited increase in the level of energy received from the Sun can engender relatively strong evaporation processes in the lakes or seas found in the high latitudes or in the polar areas of the giant moon. There may be net evaporation processes in the high latitudes of the northern hemisphere during the Summer season in the area and net condensation processes in the same area during the Winter season in the northern hemisphere.
On Earth or on Mars, the polar areas contain a cap of ice that can represent water ice or carbon dioxide ice but on Titan, the polar areas contain lakes or seas. That's surprising and that shows that nature can be particularly exotic. Radar data obtained from the Cassini spacecraft have unveiled the mysterious phenomenon of the « Magic Island ». Bright patches close to the coast of Ligeia Mare, which had not been observed in previous radar images of the same area, had been observed with the Radar Mapper of the Cassini orbiter. The images generated from the Radar Mapper of that spacecraft have shown that those bright patches represented dynamic and transient features. Were the new bright patches within the pool of liquids new islands or icebergs ? Were those bright patches fields of bubbles related to a cryovolcanic activity beneath the lake or sea ? Were they related to turbulent winds in the area ? Did they represent an area of relatively strong waves ? Did they represent a type of exobiological bloom unveiling a particular radar signature ? Even if chemical reactions are slower on Titan than on Earth, there can be surprises on the surface of the Orange Moon.
Prior to the plunge of the Huygens probe into Titan's atmosphere, some researchers had envisaged that the relatively dark areas which mark a sharp contrast with bright areas in the low or mid-latitudes might represent seas or oceans of methane or ethane. However, the aerial views as well as the views from the surface, taken from the Huygens module, have clearly shown that the brown or dark areas don't represent seas or oceans. There are bright hills containing a network of dark channels which may represent drainage channels but on January 14, 2005, the area appeared relatively dry. There may be strong rainfall events or monsoon events at the low or mid-latitudes from time to time. It's not unlikely that in the past, the relatively dark areas of the low or mid-latitudes have represented seas or oceans of methane or ethane. The radar images generated from the Radar Mapper of the Cassini spacecraft during its long mission in the Saturn System have clearly shown that the dark areas found at low or mid-latitudes are dominated by Seif dunes or linear and parallel dunes extending over long distances. Erosional processes must be strong in the area and prevailing winds must play a key role in the shape of the exotic dunes.
There may be snowfall events related to some clouds or the haze found in the dense atmosphere of Titan. Heavier molecules of the haze may fall toward the surface and form a type of sludge or mud called tholin. The composition of the dunes may be intimately related to the photochemical haze of the giant moon. Is lightning possible in the haze or in the atmosphere of Saturn's largest moon ? We have not observed any clear signs of lightning during the Cassini-Huygens mission but that possibility is not ruled out. We have not observed any phenomenon of magnetic aurora on Titan. Yet, magnetic auroras have been clearly observed in the atmosphere of the Gas Giant Saturn. If we were standing on the surface of Titan, a fortiori, we could not see Saturn in the visible spectrum due to the organic haze. The disk of the Sun would appear particularly small due to the relatively high distance between the Saturn System and the Sun. During the phase of sunset, the color of the Sun would change from yellow to orange and brown. During the phase of sunrise, that would be the opposite, if we manage to identify the Sun from the ground obviously.
The image in the upper part of this table represents a raw view of Titan's disk acquired on September 27, 2010 on the basis of the CL1 filter and of the CB3 filter of the Cassini spacecraft. The view whose file name is N00163811.jpg had not been validated or calibrated at the time of the release and a validated or calibrated version was going to be archived with the Planetary Data System proposed by NASA. The disk clearly reveals an unusual bright feature resembling an arrow and corresponding to a cloud system. The image in the lower part of this table represents a colorized version of the original view. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2020.