April 26, 2022: A New Research Work Reveals The Potential Sedimentary Cycle Of Titan

A new study "entitled "The Role of Seasonal Sediment Transport and Sintering in Shaping Titan's Landscapes: A Hypothesis", published on April 1st, 2022 in Geophysical Research Letters and proposed by a team of researchers involving Mathieu Lapôtre from Stanford University brings us key information regarding the various potential formation processes of the landscape of Saturn's largest moon Titan. The research work unveils a model that can help us better understand the sedimentary cycle of the giant moon. There are clearly some similarities between the landscape of Titan and the landscape of the Earth even if the molecules involved at the level of the surface of Titan and at the level of the surface of the Earth can be radically different. The atmosphere of Titan, dominated by molecular nitrogen like the atmosphere of the Earth, contains a relatively significant fraction of methane. In the harsh environment of Titan, methane can form rivers, lakes and seas in the high latitudes of each hemisphere. Methane can also form clouds that can engender rainfall events on that giant moon. The exotic meteorology of Titan can appear familiar to us to a certain extent. Water can be present on Titan in its solid form due to the extremely low environmental temperature at the level of the surface.

The relatively dark areas found in the low or middle latitudes of Titan tend to be dominated by Seif Dunes or linear and parallel dunes extending over long distances. The shape of those dunes is closely linked to prevailing winds. The composition of the Titanian dunes can appear really exotic since they must be dominated by hydrocarbons or organics. The chemistry and the dynamics of the Titanian dunes must be quite different from the typical dunes we regularly encounter on Earth. The mean density of the dunes found on Earth must be much higher than the mean density of the dunes found on Titan. On the Blue Planet, the dunes will tend to be rich in silicate-based substances whereas they will tend to be rich in carbon-based substances on the Opaque Moon. The team of Mathieu Lapôtre has managed to determine the processes that can lead to the formation of the dunes, the formation of the sand grains or the formation of the bedrock one can find on Titan. Winds, rainfall events and streams flow play a key role in the various potential processes leading to the landscape one can see today. Several types of landscape features can be identified on the Orange Moon, from dunes to plains, lakes, seas or labyrinth terrains.

The new model reveals the influence of the seasonal movement of liquid hydrocarbons upon the formation, the development or the dynamics of sand grains largely found in the dune fields located in the low or middle latitudes of that opaque world. Mathieu Lapôtre who is an assistant professor of geological sciences at Stanford's School of Earth, Energy & Environmental Sciences pointed out: "Our model adds a unifying framework that allows us to understand how all of these sedimentary environments work together". He added: "If we understand how the different pieces of the puzzle fit together and their mechanics, then we can start using the landforms left behind by those sedimentary processes to say something about the climate or the geological history of Titan - and how they could impact the prospect for life on Titan." The chemistry of hydrocarbons and organics on the surface of the giant moon is clearly a major topic of research because it can be relatively dynamic and because it can lead to the development of prebiotic molecules such as proteins, lipids or amino acids. In fact, the environment of Titan can potentially tell us a lot regarding our own biosphere.

The group of researchers had to determine how the organic compounds of Titan, supposed to be much more fragile than silicon-based molecules or grains on Earth, can form sand grains or even heavier molecules that will remain relatively stable over time. The carbon-based molecules of Titan can form dust like the typical dust one can encounter on Earth and can form heavier molecules or compounds that will tend to enrich the soil or the sedimentary layer of Saturn's largest moon. The chemistry and the dynamics of the sedimentary layer of the Earth represent a foundation in our level of understanding of the potential sedimentary layer of Titan. On the Blue Planet, the silicate rocks or the minerals found on the surface will tend to generate sediment grains in the long run. Those sedimentary grains will then move and interact under the influence of winds, rainfall events, streams or rivers. The sedimentary grains can merge to form new rocks in the long term under the influence of pressure, groundwater or heat. Thus, there is a cycle between the sedimentary grains and the rocks with erosion processes or aggregation or fusion processes. Planetologists imagine similar types of sedimentary processes on Titan.

The appearance of the dunes, the plains or the labyrinth terrains that were identified on Titan from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 can be explained by the dynamics and the chemistry of sand grains likely dominated by carbon elements. The sediments of Titan appear radically different from the sediments of Venus, the Earth or Mars which are relatively rich in silicon. The soil of Titan must be rich in organics and hydrocarbons. The researchers must try to demonstrate how the organic compounds can grow to form heavier molecules that will remain relatively stable over geologic time. Mathieu Lapôtre explained: "As winds transport grains, the grains collide with each other and with the surface. These collisions tend to decrease grain size through time. What we were missing was the growth mechanism that could counterbalance that and enable sand grains to maintain a stable size through time." The elements and molecules mobilized in the geology of Titan are clearly different from the elements and molecules mobilized in the geology of our own planet. The geology of Titan must be rich in water ice, methane, ethane or benzene.

In order to anticipate the nature of the potential grains found on Titan, the team of Mathieu Lapôtre studied ooids which represent small spherical grains found on Earth. Those types of sediments tend to be mostly encountered in shallow tropical seas. Ooids can be found around the Bahamas for instance. Ooids take shape when calcium carbonate related to a water column forms layers around a grain such as quartz. Ooids are particular because they form thanks to chemical precipitation. That chemical precipitation tends to make ooids grow. However, in parallel, there is a process of erosion closely related to impacts or collisions that tends to slow down the growth of the ooids. The grains tend to interact and to generate collisions under the action of waves or storms. The size of the grains remains relatively stable over time due to the balance between chemical precipitation and erosion phenomena. Can we find that type of grain on Titan ? The dynamics of ooids may represent a good example of what one can encounter on other worlds like Titan or Pluto. The geology of the icy worlds found in the Outer Solar System is clearly poorly understood because it involves lighter molecules than the geology of the Terrestrial worlds found in the Inner Solar System.

Mathieu Lapôtre advanced: "We were able to resolve the paradox of why there could have been sand dunes on Titan for so long even though the materials are very weak." He added: "We hypothesized that sintering - which involves neighboring grains fusing together into one piece - could counterbalance abrasion when winds transport the grains." The heavier molecules will tend to accumulate on the surface whereas the lighter molecules will tend to evolve in the relatively dense air of the giant moon. Mathieu Lapôtre and his collaborators mobilized the huge amount of data regarding the climate, the meteorology and the landscape of Titan to identify three major regions on that intriguing world. The low latitudes of Titan, at the level of the equator, tend to be dominated by Seif Dunes or linear and parallel dunes extending over long distances. Prevailing winds apparently play a key role in the shape of those dunes. Plains tend to dominate the middle latitudes. Labyrinth terrains tend to be found near the poles where the lakes, seas and rivers can be encountered. At the level of the equator, relatively strong winds may limit the sintering phenomena and may fuel the development of dunes composed of fine sand grains.

The team of Mathieu Lapôtre anticipates weaker winds in the middle latitudes so that sintering phenomena can be stronger than in the lower latitudes where winds are stronger. Thus, larger grains can take shape in the middle latitudes where rocks or stones may be more widespread. The plains found in the middle latitudes may be relatively rich in rocks or stones compared to the lower latitudes dominated by Seif Dunes. The labyrinth terrains found in the high latitudes and containing collapsed features may be composed of dissolved organic sandstones. A parallel could be drawn between those types of terrain and the karstic terrains in limestone on Earth. Rainfall events and rivers encountered in the high latitudes are likely to transport large amounts of sediments. The sediments can generate a type of karstic terrain via sintering and abrasion processes. Mathieu Lapôtre pointed out: "We're showing that on Titan - just like on Earth and what used to be the case on Mars - we have an active sedimentary cycle that can explain the latitudinal distribution of landscapes through episodic abrasion and sintering driven by Titan's seasons." He concluded: "It's pretty fascinating to think about how there's this alternative world so far out there, where things are so different, yet so similar." The dynamics and the chemistry of Titan's environment are really intriguing and can potentially tell us a lot regarding the chemistry of life.

The annotated image above represents a raw view of Titan's surface acquired from the Huygens probe on January 14, 2005. The landing site can be found at a low latitude in the southern hemisphere of Saturn's largest moon in the area of the relatively bright Adiri and of the relatively dark Shangri-La. One can observe eroded stones or pebbles which implies that the Huygens probe may have landed onto an ancient stream, brook or river. Image credit: ESA/NASA/JPL/University of Arizona .

- To get further information on that news, go to: https://news.stanford.edu/2022/04/25/scientists-model-landscape-formation-titan and https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL097605 .



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