January 1, 2021 : A New Study Involving Scot C. R. Rafkin Reveals The High Complexity Of Air-Sea Interactions On Titan
A new research work proposed by Scot C. R. Rafkin and Alejandro Soto, recently published in the journal Icarus and entitled « Air-sea interactions on Titan: Lake evaporation, atmospheric circulation, and cloud formation » reveals the high complexity of the various interactions between the lakes or seas and the air on Saturn's largest moon Titan. Evaporation processes, condensation processes and precipitation processes can be encountered on the Opaque Moon like on Earth. A parallel can be drawn between the meteorology of Titan and the meteorology of the Earth even if the key molecule of the meteorological system is different. The meteorology of the Earth is dominated by water (H2O) whereas the meteorology of Titan is dominated by methane (CH4). The mean environmental temperature of the giant moon of the Gas Giant Saturn is much lower than the mean environmental temperature of the Earth so that water can't appear in its liquid form on the surface of Titan in the absence of hot spot. However, the mean environmental temperature of Titan allows the stable presence of liquid methane, liquid ethane or liquid propane on the surface. Thanks to the Cassini-Huygens mission, we know, now, that there are lakes, seas and rivers on Titan.
The lakes, seas or rivers of Titan tend to appear in the high latitudes of each hemisphere. Therefore, the clouds tend to be mostly found at a relatively high latitude of each hemisphere or in the polar areas. Planetologists try to determine the potential interactions between the lakes or seas and the exotic atmosphere. Titan's atmosphere is dominated by molecular nitrogen like our own atmosphere. However, the second most abundant gas of Titan's atmosphere is methane. Oxygen is absent or almost absent in the opaque atmosphere of Saturn's largest moon. Methane represents the key molecule of Titan's hydrology whereas water represents the key molecule of Earth's hydrology. Let's point out that the abundance of liquid methane on Titan's surface is relatively low compared to the abundance of liquid water on the surface of the Blue Planet. The group of researchers revealed the complex mechanisms of the evaporation process between the lakes and the air of the giant moon of the Ringed Planet. Evaporation is in fact governed by a complex process of thermodynamics and dynamics. The scientists also showed that a cold, stable marine layer regularly takes shape in the environment of the pools. The development of clouds on Titan implies special conditions. There may also be very small turbulent fluxes and relatively quiet winds.
The study of the various interactions between the lakes, seas or rivers and the dense and hazy atmosphere of Titan represents the study of air-sea interaction and implies the study of the movement of methane molecules and the study of transfers of energy between the pools and the air. Liquid methane can become methane vapor via the well-known evaporation process. At a certain altitude, methane molecules can condense to form clouds. Once the compounds become too heavy, they will tend to fall in the form of rain. On Earth, we are used to rain of water or to snowfall events. On Titan, the rain can be composed of methane molecules or even ethane molecules. The planetologists studied the turbulent interactions between the pools of liquid and the atmosphere on the basis of an atmospheric mesoscale model combined with a slab model representation of an underlying pool of liquids. The air-sea interactions are analyzed on the basis of several factors through dozens of two-dimensional simulations. The factors taken into account are the impact of lake size, the effective pool mixed layer depth, the background wind speed, the temperature difference between the air and the lake and the atmospheric humidity.
The relatively stable solution or the general solution appears to be a non-linear superposition of a very limited background plume circulation fuelled by the buoyancy of evaporated methane with a more powerful opposing thermally direct (sea breeze) circulation generated by the contrast in temperature or energy between the cold marine blanket over the pool and the warmer inland air. The specific solution will be based on the value of the parameters related to the atmosphere analyzed and to the properties of the pool. By contrast, the general solution of the superposition of the two circulation phenomena is stable or persistent. The sensible heat flux and the latent heat flux tend toward opposite and equal values so that their ratio, the Bowen ratio, almost reaches -1.0 in most, but not all, of the relatively stable state solutions. That configuration is in line with the analytical work of other researchers. Let's insist on the fact that in almost all scenarios, the absolute magnitude of the fluxes tends toward tiny values so that the equilibrium solution appears to be insignificant because the air-sea energy exchange is around 3 W m-2 or less.
The model generates, in all configurations, a cool, moist and statically stable shallow marine layer with almost quiet winds and small turbulent flux interactions with a colder underlying pool. The temperature of the pool, the marine properties of the local air as well as the strength of the sea breeze are related to the initial conditions and to a lesser extent, to the boundary conditions. The composition of the lakes, seas or rivers on Titan can depend on seasonal factors or geographical factors. In the high latitudes of the northern hemisphere, environmental temperatures can be as low as minus 183 degrees Celsius, minus 297 degrees Fahrenheit or 90 Kelvin. The environmental temperatures in the north polar regions will tend to be lower than in the low or mid-latitudes but the temperature difference is relatively limited compared to the configuration of the Earth. The Huygens probe which had landed at a particularly low latitude in the southern hemisphere of the Opaque Moon had recorded a surface temperature of about minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. During the Cassini-Huygens mission in the Saturn System, researchers have had the opportunity to determine that the humid areas are mostly found in the high latitudes of each hemisphere.
The Cassini-Huygens spacecraft started its mission in the Saturn System during the Summer season in the southern hemisphere of Titan and Saturn and during the Winter season in the northern hemisphere of Titan and Saturn. The first pool of liquids clearly identified on Titan was Ontario Lacus, a lake or sea found at a high latitude in the southern hemisphere and a lake or sea whose shape is reminiscent of a foot. The lake was first identified thanks to infrared or near-infrared data acquired from the Cassini orbiter. Later, we obtained radar data of the lake or sea thanks to the Radar Mapper of the Cassini probe. That lake or sea may be comparable to a pond. Several systems of dynamic clouds had been observed in the area of Ontario Lacus during the Summer season in the southern hemisphere. One could guess the potential interactions between Ontario Lacus and the atmosphere. Planetologists have tried to study the seasonal dynamics of Ontario Lacus during the Cassini-Huygens mission in the Saturn System. During the Summer season in the southern hemisphere, the depth and the size of the lake may be lower than during the Winter season in the southern hemisphere.
Planetologists have tried to determine whether there are net evaporation processes or net condensation processes in the area of Ontario Lacus. During the Summer season, there may be stronger evaporation processes than during the Winter season simply because the level of energy received from the Sun is higher or because the environmental temperature is higher during the Summer season. The Cassini spacecraft has collected data regarding Titan during three different Titanian seasons, from 2004 to 2017. The end of the Cassini mission in the Saturn System corresponded to the beginning of the Winter season in the southern hemisphere of Titan and Saturn. However, it would have been scientifically interesting to observe the evolution in the shape and in the level of Ontario Lacus or the dynamics of that pool of liquids during the Winter period in the southern hemisphere. At the end of the Cassini-Huygens mission in the Saturn System, a dichotomy in the distribution of lakes and seas clearly appeared between the high latitudes of the southern hemisphere and the high latitudes of the northern hemisphere.
The most humid area on Titan at the end of the Cassini-Huygens mission in the Saturn System was clearly the north polar region or the area found at a high latitude in the northern hemisphere. Do the size and the level of the lakes, seas and rivers found in the high latitudes of the northern hemisphere decrease as the Summer season develops in the area ? That's a question we won't be in a position to answer in the near future because the Cassini spacecraft plunged into Saturn's atmosphere at the start of the Summer season in the northern hemisphere in 2017. Many linear clouds had been observed at a relatively high latitude around the north polar region at the end of the journey implying relatively strong evaporation processes in the area. Some planetologists believe that there may be subsurface layers or pools of liquid methane beneath the external crust of the north polar region. Therefore, the potential level of evaporation may be particularly strong during the Summer season in the northern hemisphere. Methane appears particularly volatile so that some lakes can rapidly disappear on Titan. The exact composition of the lakes, seas and rivers of the high latitudes of the northern hemisphere is not known. The pools may contain mixtures of methane, ethane and molecular nitrogen whose concentration can vary seasonally or geographically.
The image in the upper part of this table represents a raw view of Saturn's largest moon Titan obtained during the Cassini mission in the Saturn System on August 5, 2016 from the eye of the Cassini spacecraft. The view whose file name is N00264434.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 notice, in particular, the relatively dark regions of the low or mid-latitudes as well as dark areas in the lower part of the disk representing seas or lakes dominated by hydrocarbons. The view in the lower part of this table represents a colorized version of the original image. Credit for the original image: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process: Marc Lafferre, 2021.
- To get further information on that news, go to: https://www.sciencedirect.com/science/article/abs/pii/S0019103520302827 .