Titan, Saturn’s largest moon, stands out as the only celestial body in our solar system besides Earth known to possess active rivers, lakes, and seas. These extraterrestrial waterways are believed to be filled with liquid methane and ethane, flowing into vast lakes and seas, some rivaling the size of Earth’s Great Lakes.
The existence of these impressive bodies of liquid on Titan was confirmed in 2007 thanks to images captured by NASA’s Cassini spacecraft. Since then, scientists have been meticulously analyzing these and subsequent images, searching for clues about the moon’s enigmatic liquid environment.
Recently, a team of MIT geologists focused their attention on Titan’s shorelines. Through simulations, they have presented compelling evidence suggesting that the moon’s large seas have likely been sculpted by the action of waves. This finding is significant because previous evidence for wave activity on Titan has been indirect and sometimes contradictory, based primarily on remote images of the moon’s surface.
The MIT team took a novel approach to investigate the presence of waves on Titan. They began by modeling the erosion processes that shape lakes here on Earth. Then, they applied their model to Titan’s seas to determine what type of erosion could have created the shorelines observed in Cassini’s images. Their conclusion? Waves emerged as the most plausible explanation.
However, the researchers emphasize that their findings are not definitive. Confirming the presence of waves on Titan would require direct observation of wave activity on the moon’s surface. “We can say, based on our results, that if the coastlines of Titan’s seas have eroded, waves are the most likely culprit,” explains Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT. “If we could stand at the edge of one of Titan’s seas, we might see waves of liquid methane and ethane lapping on the shore and crashing on the coasts during storms. And they would be capable of eroding the material that the coast is made of.”
The study, authored by Perron and his colleagues, including lead author Rose Palermo, a former MIT-WHOI Joint Program graduate student and current research geologist at the U.S. Geological Survey, has been published in Science Advances. The existence of waves on Titan has been a subject of debate ever since Cassini first revealed the presence of liquid bodies on the moon. “Some people who tried to see evidence for waves didn’t see any, and said, ‘These seas are mirror-smooth,’” Palermo notes. “Others said they did see some roughness on the liquid surface but weren’t sure if waves caused it.”
Understanding whether waves exist on Titan could provide scientists with valuable insights into the moon’s climate. For instance, it could reveal information about the strength of winds capable of generating such waves. Additionally, knowledge of wave activity could help scientists predict how the shape of Titan’s seas might change over time.
Instead of searching for direct visual evidence of wave-like features in images of Titan, Perron explains that the team decided to “take a different tack, and see, just by looking at the shape of the shoreline, if we could tell what’s been eroding the coasts.”
Titan’s seas are believed to have formed as rising liquid levels inundated a landscape already etched with river valleys. The researchers focused on three possible scenarios for how these shorelines might have evolved: no coastal erosion, erosion driven by waves, and “uniform erosion.” Uniform erosion could be caused by “dissolution,” where the liquid gradually dissolves the coast’s material, or by a mechanism where the coast slowly collapses under its own weight.
The team then ran simulations to see how various shoreline shapes would develop under each scenario. To simulate wave-driven erosion, they factored in a variable known as “fetch,” which represents the distance across a lake or sea from one point on the shoreline to the opposite side.
“Wave erosion is driven by the height and angle of the wave,” Palermo clarifies. “We used fetch to approximate wave height because the bigger the fetch, the longer the distance over which wind can blow and waves can grow.”
To compare how shoreline shapes would differ under each scenario, the researchers began with a simulated sea featuring flooded river valleys along its edges. For wave-driven erosion, they meticulously calculated the fetch distance from every point on the shoreline to every other point and translated these distances into wave heights. They then ran their simulation to observe how waves would erode the initial shoreline over time. This was compared to how the same shoreline would evolve under uniform erosion. The team repeated this comparative modeling process for hundreds of different starting shoreline shapes.
The results revealed distinct differences in the final shapes depending on the erosion mechanism. Uniform erosion typically produced smoother, more rounded shorelines that expanded evenly, even within the flooded river valleys. In contrast, wave erosion primarily smoothed out sections of the shorelines exposed to longer fetch distances, leaving the flooded valleys narrower and more irregular.
“We had the same starting shorelines, and we saw that you get a really different final shape under uniform erosion versus wave erosion,” Perron observes. “They all kind of look like the Flying Spaghetti Monster because of the flooded river valleys, but the two types of erosion produce very different endpoints.”
To validate their findings, the team compared their simulations to actual lakes on Earth. They discovered the same shape distinctions between Earth lakes known to be shaped by waves and those affected by uniform erosion, such as lakes formed in dissolving limestone.
With these characteristic shoreline shapes established, the team turned their attention back to Titan. They wanted to see where Titan’s shorelines would fit within these categories.
They focused on four of Titan’s largest and most comprehensively mapped seas: Kraken Mare, comparable in size to the Caspian Sea; Ligeia Mare, larger than Lake Superior; Punga Mare, longer than Lake Victoria; and Ontario Lacus, roughly 20 percent the size of its namesake on Earth.
Using Cassini’s radar images, the team mapped the shorelines of each Titan sea. They then applied their model to each shoreline to determine which erosion mechanism best explained its shape. The results were consistent: all four seas aligned more closely with the wave-driven erosion model, indicating that wave action was the most likely sculptor of their shorelines.
“We found that if the coastlines have eroded, their shapes are more consistent with erosion by waves than by uniform erosion or no erosion at all,” Perron states. Juan Felipe Paniagua-Arroyave, associate professor in the School of Applied Sciences and Engineering at EAFIT University in Colombia, who was not involved in the study, believes the team’s findings are “unlocking new avenues of understanding.”
“Waves are ubiquitous on Earth’s oceans. If Titan has waves, they would likely dominate the surface of lakes,” Paniagua-Arroyave remarks. “It would be fascinating to see how Titan’s winds create waves, not of water, but of exotic liquid hydrocarbons.”
The researchers are now working to determine the wind strength required on Titan to generate waves capable of eroding the coasts. They also hope to decipher the predominant wind directions on Titan by analyzing the shapes of its shorelines.
“Titan presents this case of a completely untouched system,” Palermo points out. “It could help us learn more fundamental things about how coasts erode without the influence of people, and maybe that can help us better manage our coastlines on Earth in the future.”
This research was supported by NASA, the National Science Foundation, the U.S. Geological Survey, and the Heising-Simons Foundation.
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