May 31, 2020 : What Types Of Hydrocarbons Or Organics Could We Encounter On The Opaque Moon Titan ?

The giant moon of Saturn Titan is known to contain a thick and deep atmosphere dominated by nitrogen like our own atmosphere. However, Titan's atmosphere is devoid of any significant concentration of oxygen but it contains a relatively significant concentration of methane. Methane whose chemical formula is CH4 is a simple molecule composed of one carbon atom and four hydrogen atoms. A parallel between the behavior of methane on Saturn's largest moon and the behavior of water on Earth can be drawn. Methane can be present as a gas or as a liquid on Titan and water can also be present as a gas or as a liquid on Earth. Water can also be present in the form of snow or ice on Earth if the environmental temperature is sufficiently low. In an environment where the atmospheric pressure is around the mean atmospheric pressure of the Earth at sea level, water can appear as ice or snow if the environmental temperature is below or equal to 0 degree Celsius (about 273.15 Kelvin or 32 degrees Fahrenheit). Can methane appear as ice or snow on the surface of Titan ? The harsh environment of the Opaque Moon has the right conditions for the presence of stable liquid methane on its surface but it may not be extreme enough for the presence of large amounts of solid methane on the surface.

The radar images obtained from the Radar Mapper of the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 have clearly revealed the presence of humid areas dominated by methane or ethane. The humid areas turn out to be mostly found in the high latitudes of the northern hemisphere. The low or mid-latitudes of the Orange Moon seem relatively dry whereas the polar areas seem less dry or more humid. The first pool of stable liquids identified on Titan was found in the high latitudes of the southern hemisphere. Prior to that major finding, we had suspected the potential presence of lakes, seas, rivers or even oceans in the dark areas of Titan. The infrared or near-infrared views as well as the radar images acquired from the Cassini orbiter have clearly shown that the suspected ocean of methane or ethane is absent but that there are lakes, seas and rivers in the polar regions of the giant moon. How can we explain that the lakes, seas and rivers are mostly concentrated in the high latitudes or in the polar regions ? How can we explain the fact that the north polar region is by far the most humid area on Titan today ?

The eyes of the Cassini orbiter and of the Huygens probe have clearly shown that seasonal factors must play a major role in the dynamics or in the chemistry of lakes, seas and rivers on the Orange Moon. The mean environmental temperature at the level of the surface on Titan is around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. In an environment where the atmospheric pressure is around 1467 hPa and where the ambient temperature is around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin, methane can be potentially encountered in its liquid form on the surface. Yet, on January 14, 2005, the Huygens probe had landed in a dry area at a relatively low latitude. However, the aerial views obtained from the parachuted probe had clearly shown the presence of potential drainage channels in the bright hills. Therefore, the aerial views suggest that there are rainfall events in the area from time to time. There must be seasonal phenomena such as monsoon events or heavy rainfall events feeding the rivers. The Huygens probe may have landed onto an ancient brook or river.

The impact of the Huygens probe onto the soil had generated a relatively significant release of methane. Therefore, the soil of the landing site may be relatively rich in methane. That exotic soil may be comparable to a Terrestrial soil which has undergone rainfall. The color image obtained from Titan's surface clearly reveals eroded stones or pebbles implying the durable or regular action of liquid methane. Some infrared or near-infrared images acquired from the Cassini orbiter in 2010, at the start of the Spring season in the northern hemisphere or at the start of the Autumn season in the southern hemisphere have clearly revealed the presence of a stormy weather in the low or mid-latitudes. Thus, seasonal phenomena implying relatively strong rainfall events can occur from time to time in the area where the Huygens probe had landed. Does the soil absorb a lot when there are rainfall events in the area ? Are there caves sculpted by liquid methane beneath that dark or brown soil ? The exact composition of the soil has not been determined but the mean density of the moon implies that the crust may be composed of relatively light elements or molecules.

The dark areas found in the low or mid-latitudes of Titan appear to be dominated by Seif dunes or linear and parallel dunes extending over long distances. Those dunes clearly identified in the radar images obtained from the Cassini spacecraft during its long mission in the Saturn System are reminiscent of the dunes located in the Namib Desert on Earth. Some planetologists advance that those dunes may be rich in hydrocarbons or organics. Are those dunes composed of tholins ? Is the sand of Titan's dunes composed of molecules coming from the haze which makes the atmosphere opaque from outer space ? That's a strong hypothesis that has been often advanced. A complex chemistry can be observed in the upper atmosphere of the giant moon where ultraviolet light from the Sun interacts with the particles, elements, ions or molecules present in the environment to engender new molecules or particles. The new molecules, elements, ions or particles can interact and recombine to form heavier molecules or more complex molecules. Thus, some molecules can go down toward the surface if they are heavy enough. In other words, the haze can generate a type of snow rich in hydrocarbons or organics.

Particularly complex molecules composed of carbon and hydrogen have already been identified or detected in the exotic atmosphere of Saturn's largest moon thanks to the data obtained from the Cassini orbiter. Various types of hydrocarbons, organics or molecules rich in carbon, hydrogen and nitrogen can take shape in the upper atmosphere or in the haze of Titan from methane to ethane, propane, acetylene, ethylene, benzene or hydrogen cyanide. Some natural plastics can take shape in that harsh environment. Titan represents a natural laboratory for the study of the chemistry of organics and hydrocarbons. In fact, the exotic chemistry of Titan may look like the chemistry of the Early Earth because Titan's atmosphere looks like the atmosphere of the Early Earth in terms of composition. Chemical reactions may be extremely slow on Titan due to the low level of energy received from the Sun. However, that exotic chemistry may potentially lead to the formation or development of prebiotic compounds such as proteins, lipids or amino acids. Some amino acids have already been generated in laboratory experiments simulating the chemistry of the Early Earth.

The Cassini-Huygens mission has allowed us to gather new clues regarding the secrets of nature or the potential worlds one can encounter beyond the Solar System. We know that, beyond Earth, there are meteorological cycles involving other molecules than water. Titan looks like the Earth because the moon unveils a meteorological cycle but the meteorological cycle of that world is based on a completely different molecule. Yet, that molecule is present on Earth in the form of natural gas. On Earth, methane is closely related to biology. Any living organism, from cows to dogs or cats, can generate methane from time to time. Our planet is rich in hydrocarbons but the hydrocarbons of the Earth are generally related to ancient forests. A fortiori, the oil of Titan is not related to life or biological entities. The lakes, seas and rivers found on the giant moon of the Ringed Planet may be composed of a mixture of relatively simple hydrocarbons. Methane and ethane may be the two major compounds of the pools of liquids. In fact, the composition of Titan's lakes, seas and rivers may depend on geographical factors and on seasonal factors. Planetologists imagine that the typical lake or sea of that world is dominated by methane and also contains ethane and dissolved nitrogen.

The stable presence of relatively significant concentrations of methane in Titan's atmosphere appears surprising because methane tends to be destroyed by UV light from the Sun in the long run. Therefore, are there internal sources to the methane observed in the thick and opaque atmosphere ? Is there a cryovolcanism involving methane ? Some clear signs of cryovolcanism have already been observed on worlds like Neptune's largest moon Triton and the Dwarf Planet Pluto. The tiny moon Enceladus also unveils cryovolcanoes or geysers in its fractures. However, the cryovolcanoes or geysers of that icy world mainly spew water. The landscape of Titan is much more varied than the landscape of Enceladus. So, the parallel has its limits. Titan may contain huge reservoirs dominated by light hydrocarbons beneath its external crust. Some clues suggest that there is a subsurface ocean rich in water beneath the crust of the Opaque Moon. But at a lower depth, there may also be a layer rich in liquid methane. That configuration may explain the sustained presence of methane in Titan's atmosphere. Liquid methane on Titan is likely to stimulate complex chemical reactions. Titan may allow us to learn a lot regarding the chemistry of organics and hydrocarbons.

The image in the upper part of the table reveals a raw image of a portion of Titan obtained on May 31, 2013 from the Cassini orbiter. The view whose file name is N00210429.jpg was acquired 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 view 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. A sharp contrast between surface features can be clearly identified. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process: Marc Lafferre, 2020.

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