June 17, 2020 : A New Study Proposed By Jason D. Hofgartner Reveals That Some Bright Patches Of Titan May Represent Ancient Lakes

A new research work entitled « The root of anomalously specular reflections from solid surfaces on Saturn's moon Titan », proposed by a team involving Jason D. Hofgartner and published in Nature Communications on June 16, 2020 reveals that some bright patches of Saturn's largest moon Titan may represent ancient lakes or empty lakes. The Arecibo Observatory (AO) as well as the Green Bank Telescope (GBT) had allowed us to identify Anomalously Specular Radar Reflections (ASRR) in the tropical region of the Opaque Moon. The researchers had interpreted those ASRR as evidence for liquid surfaces or filled lakes. However, during the long mission of the Cassini-Huygens spacecraft in the Saturn System from 2004 to 2017, no clear signs of filled lakes or seas have been obtained from the low or mid-latitudes of Titan. In infrared or near-infrared views, one can clearly notice a sharp contrast between dark areas and bright areas in the low or mid-latitudes. The dark areas found in the tropical or equatorial regions tend to be dominated by Seif dunes or linear and parallel dunes extending over long distances. Prior to the plunge of the Huygens probe toward Titan's surface, some astronomers had envisaged that those dark areas may represent seas of methane or ethane.

The aerial views as well as the famous color view on the ground obtained from the Huygens probe on January 14, 2005 have clearly shown that the dark or brown area which marks a sharp contrast with the bright hills is not a sea or ocean. However, the aerial views had clearly shown that the bright hills are composed of a network of dark channels which may correspond to drainage channels. From time to time, at the low latitude where the Huygens probe had landed, it must rain and rivers of hydrocarbons must take shape in the bright hills. The dark or brown area can become saturated and become a transient sea. The color image acquired from the surface clearly shows eroded stones or pebbles. In fact, researchers believe that the parachuted module may have landed onto an ancient river or brook. The infrared or near-infrared views captured from the Cassini orbiter during its long mission in the Saturn System have not shown any clear sign of stable lakes or rivers in the low latitudes of Titan. In a bright region like Xanadu, bright channels can be seen but those channels must be empty most of the time. The sinuous channels can be related to meteorological phenomena such as monsoon events or storms. They can also be related to a cryovolcanic activity.

In the locations where the Anomalously Specular Radar Reflections (ASRR) have been observed via the Arecibo Observatory and the Green Bank Telescope (GBT), no filled lakes or seas have been identified from the eyes of the Cassini orbiter. Can the bright patches identified from our planet represent ancient lakes or are they related to the particular nature of the surface in the area. For more than a decade, planetologists have failed in their analyses of the surprising phenomenon. The new study reveals that the ASRR is related to one terrain unit. In fact, that terrain unit may correspond to a paleolake or a paleosea. In other words, the ASRR may be the signal of the presence of an empty lake or sea. Does the lake or sea reappear from time to time due to seasonal factors ? Does the bright patch represent an ancient lake or sea that took shape or developed in a different meteorological environment or climate ? The Cassini orbiter spent less than half a Titan year or Saturn year in the Saturn System and collected a huge amount of data regarding Saturn's largest moon from the Winter season in the northern hemisphere to the start of the Summer season in the same hemisphere. Therefore, we don't have a full view of Titan's meteorological cycle over a Titanian year which represents almost 30 Earth years.

If monsoon events or heavy rainfalls occur in the low or mid-latitudes of the Hazy Moon, lakes or seas can take shape and rapidly disappear depending on the level of volatility of the liquid. Curiously, the pools of liquids tend to be found in the high latitudes of Titan. The maps of Titan generated on the basis of infrared or near-infrared data as well as radar data clearly show that the most humid areas are found in the high latitudes of the northern hemisphere. The south polar region appeared less wet than the north polar region during the Cassini-Huygens mission. Are there seasonal migrations of methane or ethane from one polar area to the other polar area during a full Titanian year ? Is the geographical asymmetry in the distribution of lakes, seas and rivers related to orbital factors, physical factors or seasonal phenomena ? The meteorological cycle of Titan, dominated by methane, is not clearly understood or is incomplete in our level of understanding. We don't know whether there are internal sources to the methane found in the deep, dense, hazy and opaque atmosphere of the giant moon. Researchers advance that methane tends to disappear over time. Methane can also be engendered in the photochemical haze of Saturn's largest moon.

The ultraviolet light from the Sun plays a key role in the soup of molecules, elements, ions or particles found in the upper atmosphere. The dunes found at low or mid-latitudes may be dominated by organics or hydrocarbons resulting from snowfall related to the complex chemistry occurring in the photochemical haze of the giant moon. Heavier molecules tend to fall to the surface to form a type of mud or sludge called tholin. That type of mud or sludge which appears brown or dark can be encountered on a world like Pluto for instance. The linear and parallel dunes of Titan which are shaped by prevailing winds may also be found on an ancient sea or ocean. The Sahara Desert on Earth may also represent an ancient sea. Over time, a field of sand can develop via erosional processes. The observations from the Cassini orbiter during its long mission in the Saturn System have clearly shown that systems of transient clouds can take shape in the low or mid-latitudes during the period around the Equinox. The infrared or near-infrared data had demonstrated that the surface can become saturated with liquid hydrocarbons due to heavy rainfalls. Can the dark areas of the low or mid-latitudes become transient seas during the Titanian year ?

Most of Titan's surface is relatively dry today. However, during the landing phase of the Huygens probe, a relatively significant increase in the amount of methane had been detected. Therefore, one can advance that some areas of Saturn's largest moon are likely to absorb a lot of methane vapor or liquid methane. The nature of the bright areas which mark a sharp contrast with the dark areas may be radically different. If the soil is dominated by water ice, will it absorb any liquid methane like a sponge or will it be impermeable ? One can bet that the bright areas are rich in water ice. But they could also be rich in frozen carbon dioxide as well. Researchers need to study the interactions between methane, ethane, propane or molecular nitrogen and various substances in order to anticipate what we could encounter on that exotic world evolving around Saturn. There can be several types of terrain on Titan in the same way that there are several types of terrain on Earth. In an environment dominated by limestone, the action of water can engender caves or a network of subsurface rivers or subsurface lakes for instance.

During the Cassini-Huygens mission, the eyes of the Cassini orbiter have allowed us to identify more than 500 lakes or seas. The Radar Mapper of the Cassini spacecraft has been very useful to clearly see the shape of the pools of liquids as well as the drainage channels that were connected to those lakes or seas. The infrared or near-infrared eyes of the spacecraft have also allowed us to see through the opaque atmosphere of the giant moon and to observe those lakes or seas found in the north polar region in particular. The level or the size of the lakes or seas can change over time. The potential level of evaporation seems relatively high. Some lakes or depressions are partly filled with the exotic liquid. The composition of the lakes or seas can be different depending on the area or the period of the Titanian year. A mixture of methane, ethane and dissolved nitrogen must be found in the lakes or seas. The infrared or near-infrared eyes of the Cassini orbiter have also allowed us to identify cloud systems, vortices or cyclones in the areas where we had spotted lakes, seas or rivers. That's the demonstration that the precipitation phenomenon must occur from time to time.

Titan is governed by a methane cycle like the Earth is governed by a water cycle. Methane represents a relatively significant fraction of the composition of Titan's atmosphere. However, the fraction of the globe that is covered with stable liquids is much lower on Titan than on Earth. The fraction of clouds across the globe is also much lower on Titan than the fraction of clouds across the globe on Earth. The liquid dominated by hydrocarbons on Titan seems to be more volatile than the liquid dominated by liquid water on Earth. The bright patches observed from the Arecibo Observatory and from the Green Bank Telescope from 2000 to 2008 seem to be ancient pools of liquids. The Anomalously Specular Radar Reflections of those topographic features teach us a lot regarding the way to interpret surface features on other worlds in the Solar System or beyond. In the prospect of finding oceans on exoplanets or planets resembling the Earth, we have to be in a position to correctly interpret the features or the patches we see. That's why a clear definition of specular must be developed. A particularly smooth terrain resulting from an ancient lake or sea can generate a particular reflection.

The image above, obtained from the Narrow-Angle Camera of the Cassini spacecraft on June 25, 2009, reveals the disk of Titan. The view was taken on the basis of a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers. The eyes of the Cassini orbiter can see through the opaque atmosphere in the infrared or near-infrared spectrum. Thus, surface features can be discerned. The view clearly unveils the dark regions of Fensal and Aztlan in particular. Image credit: NASA/JPL/Space Science Institute.

- To get further information on that news, go to: https://www.nature.com/articles/s41467-020-16663-1.



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