July 12, 2023: A New Study Reveals A Potential Weak Intensity Of Most Rivers On Titan

A new research work entitled "Reconstructing river flows remotely on Earth, Titan, and Mars", published in the Proceedings of the National Academy of Sciences on July 10, 2023 and proposed by a group involving Samuel P. D. Birch, Gary Parker, Paul Corlies and J. Taylor Perron reveals that the intensity of most rivers on Titan may be relatively weak. The radar views obtained from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 have clearly shown a multitude of lakes, seas or rivers in the high latitudes of each hemisphere on Titan. Curiously, the researchers observe that there is apparently a lack of fan-shaped deltas at the mouths or at the connections of most rivers on the Opaque Moon. How can we interpret that configuration ? Is the flow of those particular rivers relatively weak ? Is the concentration of sediments in the liquid relatively limited ? The lakes, seas and rivers of that world orbiting around the Gas Giant Saturn must be dominated by methane or ethane and must also contain dissolved nitrogen. Therefore, that composition has nothing to do with the composition of the lakes, seas, rivers or oceans on Earth. We know very well the erosional processes associated with liquid water but our knowledge regarding the erosional processes associated with liquid methane or liquid ethane is obviously limited.

The researchers of the study focused their attention on three worlds of the Solar System that may contain lakes, seas or rivers or that may have contained lakes, seas or rivers. The list is small ! The Red Planet Mars is part of those worlds that have contained lakes, seas or rivers a long time ago. Currently, the Earth and Titan are the only worlds of the Solar System that contain stable pools of liquid on their surface. The drainage channels of Mars are completely dry today. The topography of that dry world can tell us a lot regarding its past. The team of geologists at MIT developed a new technique to determine how intensely rivers used to flow on the Red Planet and how strong the flow of most rivers is on Saturn's largest moon Titan at the present time. The method mobilizes satellite data to evaluate the rate at which rivers move the liquid and the potential sediments downstream. The geologists were in a position to calculate how fast and deep rivers were in some areas of Mars more than one billion years ago. They also managed to evaluate how fast and deep active rivers are on Titan today even if the quality of the data they have at their disposal regarding the topography of Titan is much more limited than the quality of the data they have at their disposal regarding the topography of Mars.

The landscape and the lakes or seas on Titan can be discerned in the infrared or near-infrared spectrum from outer space. They can also be identified with a radar mapper from outer space. The atmosphere of Titan is in fact completely opaque from outer space in the visible spectrum. The radar images acquired with the Radar Mapper of the Cassini orbiter have clearly revealed a multitude of rivers in the high latitudes of the northern hemisphere of the Opaque Moon in particular. The north polar region of Titan seems particularly active in terms of hydrology at first sight. Taylor Perron, the Cecil and Ida Green Professor in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS) pointed out: "What's exciting about Titan is that it's active. With this technique, we have a method to make real predictions for a place where we won't get more data for a long time." He added: "And on Mars, it gives us a time machine, to take the rivers that are dead now and get a sense of what they were like when they were actively flowing." The shape of the rivers on Titan brings us major clues regarding their dynamics or regarding the strength or the composition of the fluid.

The research work took shape following the questioning of J. Taylor Perron and Samuel P. D. Birch regarding the rivers of Titan. How can we explain the relative scarcity of deltas in the major rivers of that world ? The study benefited from a previous work performed by the collaborator Gary Parker in the 2000s. Gary Parker had developed a series of mathematical equations to characterize the river flow on our planet. He took into account river data directly obtained in the field by other researchers. He came to the conclusion that there were some universal relationships between the physical dimensions of the stream or river (its width, its depth and its slope) and the rate at which it flowed. He developed equations to determine those relationships mathematically on the basis of those variables or parameters as well as on the basis of other variables or parameters such as the gravitational field influencing the dynamics of the river and such as the size and density of the sediment circulating along the bed of the stream or river. J. Taylor Perron pointed out: "This means that rivers with different gravity and materials should follow similar relationships." He added: "That opened up a possibility to apply this to other planets too."

The equations upon the dynamics of rivers on the Blue Planet bring precise results because the field measurements collected by geologists are precise. We know precisely the width and the slope of the river. We also know the average size of the sediments. Those variables or parameters fuel the equations of Gary Parker that allow him to accurately anticipate the flow rate of the river. He is in a position to determine how much water and how much sediment the river can move downstream. The physical characteristics of the channels are harder to obtain on Mars or Titan. Researchers must rely on observations performed by orbiters or by probes or rovers directly on the surface. Orbiters around Mars have obtained high-resolution images of the surface so that the analytical work of the channels can be relevant. That's far from being the case regarding the Opaque Moon. The resolution of the images acquired from the Cassini orbiter is quite limited. The infrared or near-infrared views obtained from the Cassini spacecraft allow us to discern surface features and the radar views obtained from that orbiter allow us to identify much more clearly lakes or seas. The radar views taken from the Cassini spacecraft allow us to obtain detailed views of the rivers or the islands within the lakes or seas.

Samuel P. D. Birch had to adapt the research work due to the lack of precise data regarding the rivers of Titan or the channels of Mars. Most data regarding those worlds are taken remotely from orbiters. From the orbiters, one can obtain the width of the channel as well as the slope. He integrated those parameters into the research work mobilizing adapted equations based only on the width parameter and on the slope parameter. He then applied the modified equations adapted to those rivers to the data regarding the 491 rivers on our planet. It turns out that the predictions based exclusively on the width and on the slope of the river are accurate. The researchers applied the equations to the Red Planet and in particular to the channels connected to Gale Crater and Jezero Crater which are likely ancient rivers. Gale Crater and Jezero Crater are believed to have been lakes of liquid water billions of years ago. The researcher Samuel P. D. Birch took into account the parameter of the gravity of Mars and an evaluation of the potential width and of the potential slope of the presumed river in order to evaluate the flow rate of each river. The estimates were performed on the basis of images and elevation measurements acquired by orbiting probes.

The predictions of flow rate regarding the potential rivers of Mars allowed the researchers of the study to determine that the potential rivers may have flowed for at least 100,000 years at Gale Crater and for at least 1 million years at Jezero Crater. The scientists speculate that the period of active rivers is long enough to allow the presence of a water-based life on the Ancient Mars. The planetologists were also in a position to compare their evaluation of the average size of sediment on each river's bed with the measurements of Martian grains obtained in-situ by the rovers Curiosity and Perseverance. They could notice that the field measurements of Mars are in line with their evaluation based on their equations. Mathematical models demonstrate their power to predict reality ! The researchers applied their recipe to the rivers of Titan. They focused their attention on two specific locations where river slopes can be measured. They studied a drainage channel connected to a lake whose size is comparable to Lake Ontario. The drainage channel forms a delta at the intersection with the pool. That configuration involving a delta appears in fact scarce on the giant moon of Saturn.

The planetologists observe that almost every viewable river flowing into a lake is devoid of any delta at the level of its limit with the lake. They applied their method and their equations to the other drainage channels on Titan. They were in a position to determine the flow rate of the rivers. The rivers can be as big as some of the biggest rivers on the Blue Planet and the flow rate can be as high as the flow rate of the Mississippi. The intensity of the river is supposed to be high enough to move a large amount of sediments and to engender a delta. In reality, a limited amount of fan-shaped deltas can be observed. The planetologists also calculated that the rivers of Saturn's largest moon should be wider and have a gentler slope than drainage channels of the same intensity on the Blue Planet and on the Red Planet. The parallel between the rivers, the lakes and the seas of the Earth and Titan is really captivating. There must be similarities between the behavior or the dynamics of the lakes, seas and rivers of the Earth and Titan but there must also be major differences due to different materials or due to a different composition or chemistry. Methane and ethane represent gases in the environment of the Earth but on Titan, they can be present in their liquid form. The scenery must be fantastic in the north polar region of Titan undoubtedly !

The image above represents a radar portion of the surface of Titan obtained during the T25 flyby performed by the Cassini orbiter on February 22, 2007. Each side of the view represents approximately 100 kilometers. One can notice, in particular, a pool dominated by liquid hydrocarbons as well as a drainage channel connected to that pool. One can discern a delta at the level of the intersection between the river and the pool. Credit for the original view: NASA/JPL/Cassini Radar Team/Jason Perry. Montage credit: Marc Lafferre, 2023.

- To get further information on that news, go to: https://news.mit.edu/2023/river-flows-beyond-earth-0710 and https://www.pnas.org/doi/10.1073/pnas.2206837120.



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