There are three planets or moons in our solar system known to now have, or once had, surface rivers, lakes, deltas and a hydrologic system. There’s Earth, of course, Mars long ago when it was warmer and wetter, and the so different yet so similar rivers of hydrocarbons on Saturn’s moon, Titan.
Understanding the dynamics of rivers in particular is crucial to understanding the workings of a planet or moon. That is why so much time is spent studying the flow and spread and slopes of rivers on Earth, and why the Mars rovers Curiosity and Perseverance have spent year delving into the fossil riverbeds and fossil lakes and fossil deltas of Mars.
And that’s also why researchers have begun to focus on most unusual rivers and lakes and even seas of Titan. Yes, the liquids are a mix of methane, ethane, water ice and nitrogen, so they are very different from our liquid worlds.
But they apparently have rapids, whirlpools and waterfalls, just like rivers on Earth.
And using research done two decades ago into the predictable behavior of Earth’s flowing water, a team of geologists and planetary scientists at the Massachusetts Institute of Technology and elsewhere has made progress in understanding some basics about the flow of Titan’s surface liquids.
A new Proceedings of the National Academy of Sciences paper describes some of what has been learned, and the flow of methane and ethane rivers seems to have many of the characteristics of flowing water on Earth and that long-ago flow of water on Mars.
But there are also some differences that could be windows into greater, and intriguing disparities. A finding highlighted in the PNAS paper is that almost all Titan rivers flow into lakes and seas without ever creating fan-shaped deltas.
On Earth — and long ago on Mars — almost all major rivers ended the with these distinctive transitions into larger bodies of water.
So what could explain the difference?
We’ll get back to the delta question, but first some background on the characteristics of Titan.
The moon is about 40 percent the size of Earth, making it the 10th largest object in the solar system. Unlike all the other planets and moons except Earth and Venus, has a very thick atmosphere — which makes the running surface liquid possible but also makes observing Titan especially difficult and requires radar observation.
That atmosphere is, like Earth, mostly nitrogen, with some methane and hydrogen. Some of the compounds produced by the splitting and recycling of methane and nitrogen create a kind of smog—and that thick, orange-colored haze makes the moon’s surface difficult to view from space.
A good portion of Titan is desert or near desert, though deserts and dunes with plentiful organic compounds. While these desert dunes are devoid of open liquid, they nonetheless hold more organics than all of Earth’s coal reserves and their “sand” has been likened to coffee grounds.
But that’s just the start of Titan’s hydrocarbons. Estimates have it that that the visible lakes and seas of Titan contain about 300 times the volume of Earth’s proven oil reserves.
Those visible rivers and lakes and even seas are all located around the north and south poles. This is a function of temperatures needed for the hydrocarbon gases to liquify –around -300 F. for methane and ethane. The temperatures at the poles are in that range while the rest of the moon is somewhat warmer.
Titan does have the hydrocarbon equivalent of rainfall that feeds the rivers and lakes. The amount of precipitation is usually quite limited except for one month every Titan year, which is the equivalent of seven Earth years. Then it pours nonstop.
Beneath the hydrocarbon surface and its ice shell, Titan is believed to have a large ocean of water.
The frigid temperatures of the surface would appear to make Titan an unlikely candidate for potential habitability.
But the combination of the subsurface ocean, the moon’s vast store of hydrocarbons — and therefore organics — and the thick atmosphere place it high on the list of solar system locations where life could potentially start.
(NASA)
The new research into the rivers of Titan grew out of puzzlement over images taken by the NASA/European Space Agency Cassini-Huygens spacecraft, which explored the Saturn system between 2004 and 2017. The images that showed largely delta-less rivers were taken during the spacecraft’s 127 flybys of Titan.
More recently that Samuel Birch, then a postdoctoral fellow at MIT, and MIT professor Taylor Perron, decided to dive more deeply into the apparent anomaly. Birch said that his fascination with the question of the delta-less rivers was a major driver in the effort to study Titan’s rivers.
The group they formed built on the work of co-author Gary Parker of the University of Illinois at Urbana-Champaign, who had developed a series of mathematical equations to measure river flow on Earth.
Parker had studied measurements of rivers taken directly in the field by others. From this trove, he found there were certain universal relationships between a river’s physical dimensions — its width, depth, and slope — and the rate at which it flowed and carried sediment.
He drew up equations to describe these relationships mathematically, accounting for other variables such as the gravitational field acting on the river, and the size and density of the sediment being pushed along a river’s bed. Parker’s work on the workings and characteristics of rivers is voluminous, but is focused on Earth.
The MIT team saw that Parker had discovered some universal relationships that define rivers and wanted to see if they held up on Mars and Titan. As Perron said in a release of Parker’s work, it shows that “rivers with different gravity and materials should follow similar relationships…That opened up a possibility to apply this to other planets too.”
That was done for both Mars and Titan.
Using Parker’s math and their own testing of the equations on 491 Earth rivers, the group applied them to the Martian data collected by Mars orbiters and the Curiosity rover in Gale Crater and the Perseverance rover in Jezero Crater.
Their conclusion was that the equations worked and that details of the long-ago flow of Martian rivers could be determined by the equations they had refined. Using them, they concluded that rivers likely flowed for 100,000 years in Gale Crater and 1 million years in Jezero. They also found that the size of sediments flowing in Martian rivers predicted by their theories were in fact the size found by the rovers.
Titan’s hydrocarbon rivers– with their puzzling lack of deltas — came next.
The team focused on two locations where river slopes could be measured by Cassini instruments, including a river that flows into a lake the size of Lake Ontario. This river appears to form a delta as it feeds into the lake. However, the delta is one of only a few thought to exist on the moon — nearly every viewable river flowing into a lake mysteriously lacks a delta.
The team then applied their method to one of these other delta-less rivers. They calculated both rivers’ flows and found that they may be comparable to some of the biggest rivers on Earth, with endpoints estimated to have a flow rate as large as the Mississippi.
Both rivers should move enough sediment to build up deltas. Yet, most rivers on Titan lack the fan-shaped deposits. Something else must be at work to explain this lack of river deposits.
Both Perron and Birch said there are no firm answers yet to the question of the missing Titan delta, and that they plan a substantial research campaign to investigate the moon and its characteristics.
Deltas, or the absence of deltas, are important guides to piecing together the past and present of a landscape, to mapping where coastlines once existed and to understanding climates and the geology of a region.
“On Earth, rivers slow when they meet the coastlines, causing sediment to fall out and form a deposit. This is the norm, which we initially expected to be the case on Titan,” Birch wrote in an email.
“On Titan, it is the opposite, the rivers may be naturally denser than the seas, and even more so if they carry suspended sediment.”
Birch continued: “One of our hypotheses then is that when Titan’s rivers interface with the seas at the shoreline, they plunge along the seafloor, never depositing their sediment load until they are deep into the interior of the seas. Such a sedimentary deposit would not be observable, as it would be buried under 100’s of meters of fluid, unlike a delta which is above the fluid and right at the coastline.”
“We have a lot of hypotheses now as to why sediment may not fall out and/or be stable at the terminus of Titan’s rivers, all of which link to different aspects of Titan’s present and slowly varying climate.”
In a 45-minute radio interview with the Planetary Society, Birch offered a number of other possibilities for the dearth of Titan deltas.
In the north, he said, there appears to be some largescale flooding which could — or could not — be limiting delta formation. The two deltas the team did find were in the south, where the sea levels were found to be dropping rather than rising.
He said that wind and tides also can move the sediments that form deltas and in that way make them much more difficult to find, particularly in a difficult to observe locale like Titan, which is 10 times further from the Sun than Earth is.
And all the images, as well as measurements of the land or a river’s slope, were taken as the spacecraft zipped by Titan at 5,401 meters (almost 18,000 feet) per second .
While Cassini opened to door to studying and understanding Titan, it never got terribly close to the moon — the closest pass was 60,000 kilometers or about 37,000 miles. Clearly, a spacecraft will have to get closer to substantially open that door of understanding.
In 2019, NASA selected the Dragonfly Titan mission as an astrobiology and planet exploring New Frontiers effort. That means Dragonfly will be searching for signs of past or present life, and for indications the moon is, or is not, potentially habitable.
The spacecraft is being designed to land on Titan and then fly off — a robotic “rotorcraft” — to different locales on the moon, an advance on the similar craft used on Mars with the Perseverance rover.
To Perron and Birch, Dragonfly will also provide an opportunity to test their hypotheses about Titan rivers and deltas.
Perron said, for instance, that the hydrocarbon liquids (mostly methane and ethane) on Titan are less dense and less viscous (more “runny”) than water on Earth, and the icy or organic sediment that rivers carry on Titan is less dense than rock on Earth.
“We account for these differences in our calculations, and the consequences can be important,” he wrote in an email. “For example: You know how you weigh less when you’re swimming in water? The same is true for sand or gravel in rivers, and this effect is especially strong for sediment in Titan’s rivers because of the different materials. As a result, a river on Titan should be wider and less steep than rivers carrying the same rate of flow on Earth or Mars.”
These predictions, and the dearth of deltas on Titan, are the kind of insights that lay the groundwork for truly understanding a planet or moon.
Everybody wants to know whether a planet or moon is habitable or inhabited. But those are remarkably difficult conclusions to reach.
So before scientists get to anything that grand and momentous, they have to learn the myriad forces at work on that body. And it’s not obvious what will turn out to be a guiding and essential finding.
Many Worlds 
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