November 25, 2021: Exploring The Lakes Or Seas Of Titan With A Boat Or A Submarine Will Really Be A Major Challenge
There are two known worlds in the Solar System where stable lakes or seas can be found on the surface. Those worlds are the Earth and Titan. The lakes, seas or rivers of Saturn's largest moon Titan have never been observed from any in situ probe. Those lakes, seas or rivers have only been observed from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017. Thanks to that remarkable spacecraft, we have obtained radar data as well as infrared or near-infrared views of those pools or rivers. Curiously, the lakes, seas and rivers of the Opaque Moon are concentrated in its high latitudes. The lakes, seas or rivers of the giant moon of the Ringed Planet Saturn may appear familiar to us at first sight but they are in fact fundamentally different from the typical lakes, seas or rivers we regularly encounter on Earth. In the harsh environment of Titan where the ambient temperature at sea level evolves around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin, water can't appear in its liquid form on the surface. Water will tend to be completely frozen on the surface. Pebbles dominated by water may be present at the bottom of the lakes, seas or rivers of that enigmatic world.
What is the exact composition of the lakes, seas and rivers of the Orange Moon ? That's a major question we must try to answer in order to develop a probe, a boat, a submarine or a drone that will be in a position to explore those exotic environments in the right way. We know that Titan's atmosphere is dominated by molecular nitrogen like the atmosphere of the Earth. We also know that the second most abundant compound present in Titan's atmosphere is methane or CH4. That's a major difference compared to the atmosphere of the Blue Planet that is extremely poor in methane. Oxygen is absent or almost absent in Titan's atmosphere. During the Cassini-Huygens mission in the Saturn System, we have clearly identified various cloud formations in the high latitudes or in the low or mid-latitudes. One can advance that the composition of the clouds and the composition of the lakes, seas or rivers are closely related. In other words, the composition of the clouds is likely to bring us clues regarding the potential composition of the lakes, seas or rivers of the giant moon. Titan is a world rich in hydrocarbons and organics. In the extremely cold environment encountered at sea level, several types of hydrocarbons can appear in their liquid form.
Methane, a molecule whose chemical formula is CH4 and which represents the simplest stable hydrocarbon, can be present in its liquid form on Titan's surface. That's also the case for ethane whose chemical formula is C2H6 and for propane whose chemical formula is C3H8. The specialists of Titan believe that the composition of the lakes, seas and rivers of the Opaque Moon will depend on the season and on the area. The lakes, seas and rivers probably contain a mixture of methane, ethane and dissolved nitrogen. On Earth, the oceans are dominated by water but they also contain sodium chloride or NaCl. The exact composition of the oceans is relatively rich. The concentration of salt in our oceans or seas can vary. For instance, the concentration of salt in the Dead Sea is remarkably high compared to that of the Mediterranean Sea. Our biosphere also adds some complexity to the composition of our oceans, seas or lakes. The complexity of our hydrosphere can tell us a lot regarding the potential complexity of Titan's hydrosphere. The lakes, seas and rivers of the Hazy Moon are full of mysteries and may surprise us in the coming years or decades when we explore them with a probe, a boat, a submarine or a drone.
During the Cassini-Huygens mission in the Saturn System, we have identified several major pools of liquid hydrocarbons as well as a multitude of small lakes or drainage channels. The first stable extraterrestrial lake or sea ever identified is Ontario Lacus, a sea or lake located in the high latitudes of Titan's southern hemisphere. Ontario Lacus was first identified from the Cassini spacecraft in the infrared or near-infrared spectrum during the Summer season in the southern hemisphere. During the Cassini-Huygens mission in the Saturn System, Ontario Lacus was also imaged thanks to data acquired from the Radar Mapper of the Cassini orbiter. The shape of Ontario Lacus is reminiscent of the shape of a foot. Ontario Lacus appears remarkably flat. Due to the low gravity and due to the dense air of the giant moon, we could have imagined higher waves than on Earth but that was not the case at the time of the observations. How can we explain that surprising configuration ? Is the environment very quiet in the high latitudes of the southern hemisphere ? Are winds very weak in the area ? What is the exact composition of the pool ? What are the properties of the liquid ? What is the mean depth of the pool ?
One can imagine that Ontario Lacus looks like a typical pond encountered on Earth. One could draw a parallel between Ontario Lacus and the ponds in Palavas-les-Flots near Montpellier in France for instance. However, the composition of Ontario Lacus appears clearly exotical because that extraterrestrial pool must be composed of a mixture of methane, ethane and dissolved nitrogen. That composition may vary depending on the season. Seasons are very long on Titan. Let's recall that one Titanian year represents almost 30 Earth years. Therefore, each Titanian season represents about 7 Earth years. During the Winter season in the southern hemisphere, the level or the size of Ontario Lacus may be higher than during the Summer season in the southern hemisphere. During the Summer season in the southern hemisphere, the level of evaporation may be higher than the level of condensation in the area of Ontario Lacus. During the Winter season in the southern hemisphere, that may be the opposite. Some systems of dynamic clouds had been observed in the area of Ontario Lacus during the Summer season in the southern hemisphere. Those clouds demonstrate that relatively strong evaporation processes can take shape in the area.
The clouds of Titan are much rarer than on Earth. The clouds of Saturn's largest moon are mostly concentrated in the high latitudes of each hemisphere. But from time to time during the long Titan year, large cloud systems can also emerge and develop in the low or mid-latitudes. The infrared or near-infrared eyes of the Cassini orbiter had revealed remarkable systems of dynamic clouds in the low or mid-latitudes 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. Those clouds are likely to generate heavy rainfall events. One can imagine heavy rainfall events involving liquid methane in which the droplets fall at a relatively small speed compared to the typical droplets encountered on Earth. The heavy rainfall events of Titan can engender transient lakes or seas in the low or mid-latitudes. The Huygens probe had unveiled a brown or dark plain as well as bright hills composed of a network of dark channels connected to the brown or dark plain during its parachuted descent on January 14, 2005. The landing site of the probe was found at a relatively low latitude in the southern hemisphere. The channels identified from the probe are likely drainage channels.
The famous color view obtained from the Huygens probe from the surface clearly shows pebbles or eroded stones which implies that the probe may have landed onto an ancient brook, stream or river. The area may experience heavy rainfall events from time to time during the long Titanian year. Seasonal factors must play a key role. Prior to the parachuted descent of the Huygens probe, some researchers or planetologists had imagined that the relatively dark areas which mark a sharp contrast with relatively bright areas in the low or mid-latitudes were in fact seas or oceans of methane or ethane. However, the radar views captured from the Cassini spacecraft have clearly revealed that those relatively dark areas are in fact dominated by linear and parallel dunes extending over long distances. Those dunes are reminiscent of the Seif Dunes encountered in the Namib Desert on Earth. The dunes of Titan may be rich in hydrocarbons or organics. They may be fed by relatively heavy molecules generated in the global haze. In the haze, the heavier molecules will tend to fall toward the surface like snow on Earth. The Dragonfly rotorcraft will have the opportunity to study those dunes in the 2030s.
The Cassini-Huygens mission in the Saturn System has clearly shown that the most humid area on Titan is found in the high latitudes of the northern hemisphere. There is a dichotomy in the distribution of lakes, seas and rivers between the southern hemisphere and the northern hemisphere. That's quite curious ! Are the lakes or seas found in the high latitudes of the northern hemisphere connected to subsurface reservoirs of methane or ethane ? Are they connected to a subsurface layer of methane or ethane ? Is the size or the level of the lakes and seas found in the northern hemisphere higher during the Winter season in the northern hemisphere ? Planetologists are particularly interested in the properties of the liquid found in Kraken Mare, Ligeia Mare or Punga Mare. We must try to anticipate the potential interactions between those exotic pools and the probe, boat, submarine or drone that will explore them in order to avoid a potential failure. If the probe, boat, submarine or drone is too warm, it is likely to engender strong evaporation processes or a strong fog where it evolves. Will the environment of the probe, boat or drone be foggy ? Will the liquid of the lake or sea be transparent or opaque ? Will it be viscous ? What is the mean density of the lake or sea ? One can expect some surprises or discoveries !
The image above reveals a simulated view of Punga Mare from the surface of Saturn's largest moon Titan. Punga Mare appears to be the third largest pool of liquids in the high latitudes of Titan's northern hemisphere. The lakes, seas and rivers found in the high latitudes of each hemisphere may be composed of a mixture of methane, ethane and dissolved nitrogen. The simulated view which reveals a portion of Saturn's disk on the horizon, through the haze of the opaque atmosphere, is comparable to a view obtained with a typical smartphone whose field of view is around 60 degrees from left to right. The disk of Saturn from the surface of Titan would appear much bigger than the disk of our moon, The Moon, observed from the surface of the Blue Planet. Credit for the simulated view: Marc Lafferre, 2021.
- To get further information on that news, go to: https://en.wikipedia.org/wiki/Lakes_of_Titan .