April 23, 2025: A New Study Clearly Shows The Surprising Scarcity Of Deltas In The Rivers Of Titan
A new study entitled "Detectability of Coastal Landforms on Titan With the Cassini RADAR", published in the Journal of Geophysical Research: Planets on March 20, 2025 and proposed by a team of researchers involving Sam Birch reveals or confirms the surprising scarcity of deltas in the rivers of Titan. The Radar Mapper of the Cassini spacecraft had obtained a large amount of data regarding the landscape of Saturn's largest moon unveiling the presence of seas or large lakes of hydrocarbons as well as the presence of rivers connected to the pools during its orbital dance in the Saturn System from 2004 to 2017. The Synthetic Aperture Radar (SAR) views show remarkable details regarding the coastline of the pools and regarding the network of channels connected to those pools. The shape of the rivers or the shape of the coastline are likely to tell us a lot regarding the dynamics or the nature of the liquid in the area. Curiously, deltas appear particularly rare on Titan compared to the configuration of the Earth. Planetologists must take into account the limitation of the instruments related to the observation of the surface, the soil or the pools in their analyses. However, they were in a position to unveil or to deduce surprising features on the surface of that world.
The analysis of landscape features on Titan is far from being obvious due to the limited resolution of the Radar Mapper and due to the limited resolution of the infrared or near-infrared eye of the Cassini orbiter. Researchers must also take into account, in their studies, the fact that the liquids or the fluids of the giant moon of the Ringed Planet are transparent to microwave radiation. In the harsh environment of Titan, water can't appear in its liquid form on the surface. The only molecules that can appear in their liquid form in a stable way on the surface of that world are methane, ethane and propane. The exact composition of the liquids found in the seas, lakes or rivers of Titan is a major topic of research. What type of chemistry takes shape and develop in those environments ? What is the dynamics of those pools or rivers of hydrocarbons ? The study of the shape of the rivers or the study of the coastline can tell us a lot regarding the nature or the dynamics of those fluids likely dominated by methane and ethane. The team of Sam Birch performed a numerical model to simulate the coastline of the Earth with the same eye as the eye of the SAR instrument of the Cassini orbiter. The model allows the planetologists to better understand the landscape features of Titan.
The researchers of the study demonstrate that multi-kilometer-scale landforms can be identified on Titan if the contrast between the land surface and the sea floor is strong enough. The observations of the rivers of the Opaque Moon, on the basis of the data acquired with the Radar Mapper, clearly show a very limited fraction of deltas in the large coastal rivers. That's a configuration which contrasts with the configuration of the Earth where the same types of rivers generally unveil a delta in the area of the coastline. In its research work, the team of Sam Birch was in a position to identify two probable deltas near the south pole of the giant moon. The other channels or rivers of that world appeared devoid of any delta which is surprising because the deltas are nearly systematically present in the significant rivers of the Earth. The planetologists determined that only about 1.3% of the large rivers that are connected to a pool on Saturn's largest moon contain deltas. If the model of the team is correct, how can we explain that natural contrast between the rivers of methane and ethane on Titan and the rivers of water on Earth ? Is the dynamics of the liquids present on Titan's surface fundamentally different from the dynamics of the liquids present on the surface of the Earth ?
The hydrological system of Titan at the level of the surface is relatively small compared to the major hydrological system of the Earth in the upper layer of the crust. Thanks to the Cassini spacecraft in particular, we have been in a position to determine that the seas, lakes and rivers are mostly concentrated in the high latitudes of the giant moon and that the low or middle latitudes are relatively dry with the presence of dune fields extending over long distances in the relatively dark areas of the low or middle latitudes in particular. How can we explain that active lakes, seas and rivers are mostly concentrated in the high latitudes and in the north polar region in particular ? Are there internal sources to the pools dominated by methane or ethane for instance ? Methane and ethane can interact and can form liquid layers within the same pools on Titan. Planetologists must take into account the fact that several types of liquid can be present within the same pool, lake or sea. On Earth, in the ocean, in the sea, in the lake or in the pool, we have water from the surface to the bottom of the liquid body even if the concentration of elements or molecules like oxygen or sodium chloride (NaCl) can vary depending on the area or depending on the depth.
The curious morphology of the significant drainage channels identified on Titan can tell us something about the nature of the soil, about the composition of the soil, about the composition of the liquids or about the dynamics of the liquids. The liquids of the drainage channels tend to erode the surface and to bring sedimentary materials to the pools connected to those rivers. The deltas are interesting places for geologists or planetologists because they are rich in terms of materials or sediments transported by the stream. Those areas are likely to contain complex organics, complex hydrocarbons or materials related to a cryovolcanic activity. However, the radar views unveil a particularly limited number of deltas on the Orange Moon. The liquid hydrocarbons of the rivers may not have a very strong power to sculpt or erode the landscape. What happens if a stream of methane or a stream of ethane evolves on an icy surface dominated by water ice for instance ? Will the liquid methane or the liquid ethane sculpt or erode the icy soil ? On Earth, we can observe rivers or drainage channels that have strongly eroded or sculpted the landscape. But the composition of the soil of our planet is radically different compared to the composition of the soil on Titan. And the composition of the liquids on Titan has nothing to do with the composition of the stable surface liquids on Earth as well !
Previous studies had shown that the pools of Titan can be almost as smooth as a mirror. If those pools are extremely flat and if those pools are almost devoid of any waves, one can imagine that the erosional processes will be much more limited than in a configuration in which the pools, the seas or the lakes contain significant waves. One can easily imagine that a faster stream will engender more destructions or stronger erosional processes of the soil. Like on Earth, the activity of the pools, of the lakes, of the seas or of the rivers of Titan must vary depending on the latitude, depending on the area or depending on the season. There is a meteorology involving cloud formation and precipitation processes on Titan like on Earth. But the Cassini spacecraft has clearly revealed that clouds are far from being widespread and are mainly concentrated in the high latitudes of the giant moon. Let's recall that the surface of our planet is dominated by oceans of liquid water. On Titan, the fraction of the surface dominated by stable liquids is particularly small. Thus, the processes of cloud formation on Titan are much more limited than on Earth. The clouds of Titan can be composed of methane or can be composed of ethane.
The infrared or near-infrared images acquired from the Cassini spacecraft during its long mission in the Saturn System have clearly shown that dynamic cloud systems can form and develop in the polar areas of the giant moon of Saturn. Those clouds can bring rain like on Earth and rain can fuel the rivers that are connected with the pools. What is the size of the raindrops of methane or ethane when they hit the ground or the surface of the river ? What is the speed of those raindrops when they hit the ground or the surface of the river ? Those parameters are fundamental if we want to understand the interactions between the liquids found on the surface of Titan and the soil whose composition can vary depending on the area. Let's keep in mind that the Titanian air is denser than the air on Earth at sea level and that the gravity is much lower than on Earth so that raindrops can fall much slowly than on Earth. The physical configuration on Titan is very different from the physical configuration of the Earth. One has to imagine an environment in which rainfall events seem to be in slow motion. A slower impact on the surface implies a lower level of erosion or destruction of the soil.
The analysis of radar data in the areas where liquids are present can be difficult because the fluids appear transparent in the views obtained thanks to the Synthetic Aperture Radar instrument. The shape of the coastline can be hard to correctly identify on the basis of the radar data due to that property of liquid methane or liquid ethane. However, the planetologists were in a position to discern deep features of the pools, of the seas or of the lakes thanks to the level of transparency of the liquid. The researchers have been in a position to also identify surprising features in the pools. They found submerged pits as well as river valleys inside some pools. How can we explain that surprising configuration ? Are there internal sources to the pools ? Are the pools connected to an underground network of liquid hydrocarbons ? The planetologists advance the hypothesis that any delta could be regularly erased by rapid movements in the level of the pool from a low level to a high level and from a high level to a low level. In that case, any delta could not have time to take shape or develop. But how could we explain the speed at which the level of those pools can change ? Can it be related to strong rainfall events ? Can it be related to a sudden rise of liquid from a pit of the pool for instance ? Winds and tidal currents are also likely to influence the level of the pool.
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The image above reveals a portion of a radar swath of Titan obtained from the Cassini spacecraft during the T28 flyby performed on April 10, 2007. The file name of the original view is BIFQI52N010_D126_T028S01_V03.jpg. Each side of the view represents approximately 100 kilometers (around 62 miles). A major drainage channel connected to a pool can be clearly discerned. The area where the major river is connected to the pool is reminiscent of the deltas we can find on Earth. Deltas may be scarce on Saturn's largest moon. Credit for the original image: PDS Image Atlas. Montage credit: Marc Lafferre, 2025. |
- To get further information on that news, go to: https://www.brown.edu/news/2025-04-18/titan and https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JE008737 .