October 5, 2020 : A Layer Relatively Rich In Ethane And Relatively Poor In Nitrogen Above A Layer Relatively Rich In Methane And Nitrogen In The Shallow Pools Of Titan ?
A new study entitled « Stratification Dynamics of Titan's Lakes via Methane Evaporation », recently published in The Planetary Science Journal and proposed by a team led by Jordan K. Steckloff who is a researcher at the Planetary Science Institute, suggests that the shallow pools of liquids on Titan may paradoxically contain a layer dominated by ethane above a layer dominated by methane. That stratification phenomenon may be related to a difference in the density of the layers. The layer whose density is higher will be found beneath the layer whose density is lower. The stratification phenomenon can also be encountered on Earth where temperature variations play a key role in the formation of the layers. Planetologists believe that the lakes or seas of Titan are dominated by a mixture of methane, ethane and dissolved nitrogen. The concentration of those compounds in the lakes or seas can vary depending on the area, on the season or on the depth. The nitrogen molecules or elements present in Titan's atmosphere can interact with the lakes or seas and engender stratification phenomena. If the layer dominated by methane is enriched in nitrogen, the density of that layer can become higher than that of the layer dominated by ethane so that the layer dominated by methane evolves beneath the layer dominated by ethane.
Methane, ethane or even propane can appear in their liquid form on the surface of Saturn's largest moon due to the extremely low environmental temperature encountered at the level of the surface. Methane whose chemical formula is CH4 is lighter than ethane whose chemical formula is C2H6. So, we could imagine a layer of pure methane above a layer of pure ethane. But reality may be much more complicated on the Opaque Moon due to the complex interactions between the hydrocarbons of Titan and, in particular, between compounds like methane, ethane and nitrogen. On Earth, during the Summer season, the higher environmental temperature engenders an expansion of the hot water so that its density decreases and becomes significantly lower than the density of the cold water. In that configuration, a stratification phenomenon between the hot water and the cold water takes shape. The hot water whose density is lower will form a layer that will float above a layer of cold water. The typical stratification phenomenon on our planet is related to temperature variations in the environment. On Titan, the stratification phenomenon is rather intimately linked to chemical interactions between various compounds.
If the lake or sea on Titan absorbs more nitrogen from the dense atmosphere, the layer dominated by methane can go down due to the increase in its density and the layer dominated by ethane can go up because its density becomes lower than that of the layer dominated by methane. Jordan K. Steckloff pointed out : « Lakes on Titan, more than mere puddles of liquefied natural gas, are dynamic places that experience complex physical processes. They can stratify, overturn, and possibly erupt. » Several types of liquid based on carbon and hydrogen can potentially take shape on Titan's surface. The interactions between the various hydrocarbons and the various organics can result in a particularly complex mixture of compounds within the lakes, seas or rivers. Is the liquid of the pools viscous like the dark crude oil found on Earth ? Are the liquids of Titan's lakes, seas or rivers transparent or opaque ? What is the color of the lakes or seas ? Are there several types of lakes or seas on Titan ? Are the lakes or seas of the Hazy Moon less dynamic than the lakes or seas on Earth ? Are there significant differences between the composition of the pools of liquids in the high latitudes of Titan's northern hemisphere and the composition of the pools of liquids in the high latitudes of its southern hemisphere ?
The team of Jordan K. Steckloff believes, on the basis of numerical simulations, that the layer dominated by methane in Titan's lakes or seas can become sufficiently enriched in nitrogen to make the layer go down due to an increased density. Nitrogen can't appear in its liquid form on the surface of the giant moon because the environmental temperature is too high for that configuration. However, nitrogen can be dissolved in the mixture of methane and ethane. The oceans of our planet contain liquid water but they also contain a relatively high concentration of sodium chloride. Water can only appear in its solid form on the surface of Saturn's largest moon. However, water molecules can potentially be encountered in the form of ice within the lakes or seas of the Opaque Moon. Mysterious phenomena in Ligeia Mare or Kraken Mare have been identified thanks to the Radar Mapper of the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017. Bright patches had emerged within the relatively dark and uniform Ligeia Mare close to the coastline. Those bright patches had rapidly vanished. Were those bright patches new islands or exotic icebergs ? Did they represent a field of bubbles of nitrogen ? Did they represent an area of relatively strong waves ?
Nitrogen which dominates the atmosphere of Titan or the Earth may play a key role in the dynamics and the structure of the pools found in the high latitudes of the Opaque Moon. The chemical interactions between methane and the atmospheric nitrogen are likely to lead to the potential configuration envisaged by the group of scientists. Clouds of methane or ethane have been identified in Titan's atmosphere. Those clouds clearly demonstrate that the lakes, seas or rivers of the Hazy Moon must be mostly composed of those molecules. Methane and ethane are in fact the lightest hydrocarbons. In the stratification phenomenon of the lakes or seas, relatively heavy hydrocarbons or organics can be found in the abyss of the pools. The lower layer can be compared to layers of sediments on Earth. In the extremely harsh environment of the giant moon, chemical reactions are much slower than on our planet but the liquids found on the surface can favor relatively complex chemical reactions. Liquid methane or liquid ethane can play the same role as liquid water in the development of amino acids, proteins, lipids or sugars.
Can the stratification phenomenon lead to instabilities within the lakes or seas ? Methane and ethane are flammable on Earth due to the presence of oxygen in the air. However, in the harsh environment of Titan, those molecules may be relatively stable even if they are quite volatile. The difficulty is to anticipate the potential chemical reactions within the lakes or seas. Various hypotheses must be put forward in order to study the potential interactions between the different molecules or elements of the pools and of the atmosphere. If the environmental temperature drops to a level below the environmental temperature determined by researchers, the stratification phenomenon can take shape or develop. Jordan K. Steckloff pointed out : « We focused on small, shallow lakes that fill following Titan's rain events, and found that, if the temperature is low, the evaporation of methane from the surface can drive out dissolved nitrogen, which is heavy, resulting in an ethane-enriched (methane-nitrogen poor) layer floating on top of a methane-rich layer. » The huge amount of data obtained from the Cassini spacecraft and the Huygens probe has clearly shown that seasonal factors such as variations in the environmental temperature can play a key role in the dynamics of the lakes or seas.
The Huygens probe which had landed at a relatively low latitude in the southern hemisphere on January 14, 2005 had recorded a surface temperature of about minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin and an atmospheric pressure on the surface of 1,467 hPa. The greenhouse effect generated by Titan's atmosphere, at the level of the surface, appears relatively limited despite the relatively high atmospheric pressure on the surface. In the high latitudes or in the polar regions, environmental temperatures are lower than in the lower latitudes at sea level like on Earth but temperature differences are more limited. However, the concentration of the lakes, seas and rivers in the high latitudes of the giant moon may be closely related to environmental temperatures. A limited increase in the environmental temperature of the north polar region can have a strong impact on the level or on the size of the lakes or seas. At the end of the Cassini-Huygens mission in the Saturn System, the Summer season in the northern hemisphere of Titan was starting and many clouds, probably related to relatively strong evaporation processes, were observed in the humid area of the northern hemisphere.
Methane is known to be very volatile and in the harsh environment of Saturn's largest moon, a slight increase in the environmental temperature can lead to a relatively significant drop in the level or in the size of the lake or sea. Titan is governed by meteorological processes like our own planet but the key molecule involved in the meteorology of the Opaque Moon is methane. A parallel can be drawn between the meteorological system of Titan and the meteorological system of the Earth but there are also major differences between the hydrology of Titan dominated by methane and the hydrology of the Earth dominated by water. Titan undergoes rainfall events like the Earth for instance but the concentration of clouds on Titan is much more limited than that on Earth. Jordan K. Steckloff advanced : « Earth is the most Titan-like planet known. Like Titan, Earth has dynamic lakes. Similar processes are active on both, showing that the complicated behaviors of surface liquids can be controlled by a few simple rules and processes. » The study was financed in part by a grant from the Cassini Data Analysis Program proposed by NASA. We probably have a lot to learn from the exotic lakes, seas and rivers of Titan.
The image above unveils two radar views of a portion of Kraken Mare, the largest pool of liquids on Titan. The radar views correspond to Synthetic Aperture Radar (SAR) images obtained from the Cassini orbiter during its orbital dance in the Saturn System. The view at left was captured on May 23, 2013 at an incidence angle of 56 degrees whereas the view at right was acquired on August 21, 2014 at an incidence angle of 5 degrees. A new group of islands seems to have emerged in the view of August 21, 2014. Was the new group of bright features really a group of islands ? Was it a group of icebergs ? Was it a field of waves or bubbles ? Image credit: NASA/JPL-Caltech/ASI/Cornell.
- To get further information on that news, go to: https://psi.edu/news/titanlakesstratify and https://iopscience.iop.org/article/10.3847/PSJ/ab974e.