The surface of Titan. Simulations by MIT geologists indicate that the lakes and seas on Titan, Saturn’s largest moon, were formed by wave-induced erosion. Credit: NASA/JPL-Caltech
Researchers find that wave activity on Saturn’s largest moon could be strong enough to erode the shorelines of lakes and seas.
MIT researchers have used simulations to suggest that Titan’s coastlines, Saturn‘s largest moon, are formed by waves. This finding builds on images from NASA‘S Cassini spacecraft, which first confirmed the existence of Titan’s methane and ethane bodies. Understanding how these waves can erode coastlines could provide insight into Titan’s climate and future marine evolution.
Titan’s unique alien ‘waters’
Titan, Saturn’s largest moon, is the only other planetary body in the solar system that currently hosts active rivers, lakes and seas. These extraterrestrial river systems are thought to be filled with liquid methane and ethane that flow into broad lakes and seas, some as large as Earth’s Great Lakes.
The existence of Titan’s large seas and smaller lakes was confirmed in 2007 with images taken by NASA’s Cassini spacecraft. Since then, scientists have studied this and other images for clues about the moon’s mysterious liquid environment.
Now MIT geologists have studied Titan’s coastlines and shown through simulations that the moon’s large seas were likely formed by waves. So far, scientists have found indirect and conflicting signs of wave activity based on remote images of Titan’s surface.
The Lakes of Titan. Saturn’s largest moon is home to active rivers, lakes and seas, likely formed by waves according to MIT researchers who used simulations to study the erosion of Titan’s coastlines. Credit: NASA
Waves as erosive forces on Titan
The MIT team took a different approach to investigate the presence of waves on Titan, first modeling the ways in which a lake on Earth might erode. They then applied their modeling to Titan’s seas to determine what form of erosion might have caused the coastlines in Cassini’s images. Waves, they found, were the most likely explanation.
The researchers emphasize that their results are not definitive; to confirm that there are waves on Titan, direct observations of wave activity on the moon’s surface are needed.
“We can say from our results that if the coastlines of Titan’s seas are eroded, waves are the most likely culprit,” said Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT. “If we stood at the edge of one of Titan’s seas, we could see waves of liquid methane and ethane lapping and crashing onto the shores during storms. And they would be able to remove the material that makes up the coast.” exists to erode.”
Example model landscapes starting with a coastline with flooded river valleys (left) and eroded by waves (top right) or uniform erosion (bottom right). Credit: Courtesy of the researchers
Perron and his colleagues, including first author Rose Palermo, a former graduate student at the MIT-WHOI Joint Program and a research geologist at the U.S. Geological Survey, will publish their research in an upcoming issue of Scientific progress. Their co-authors include MIT researcher Jason Soderblom, former MIT postdoc Sam Birch, now an assistant professor at Brown University, Andrew Ashton of the Woods Hole Oceanographic Institution, and Alexander Hayes of Cornell University.
Controversies and insights about Titan’s wave activity
The presence of waves on Titan has been a controversial topic ever since Cassini discovered liquid masses on the moon’s surface.
“Some people who tried to see evidence of waves didn’t see any and said, ‘These seas are like glass,’” Palermo says. “Others said they did see some roughness on the surface of the liquid, but weren’t sure if waves were causing it.”
Knowing whether Titan’s seas host wave activity can give scientists information about the moon’s climate, such as the strength of the winds that might create such waves. Wave information can also help scientists predict how the shape of Titan’s seas might evolve over time.
Rather than looking for direct signs of wave-like features in images of Titan, Perron says the team “had to take a different approach and see, just by looking at the shape of the coastline, whether we could determine what the coasts has affected.”
Simulation techniques and erosion scenarios
Titan’s seas are thought to have formed as rising fluid levels flooded a landscape intersected by river valleys. The researchers focused on three scenarios for what could have happened next: no coastal erosion; erosion caused by waves; and “uniform erosion”, caused by “dissolution”, in which fluid passively dissolves the material of a coast, or a mechanism in which the coast gradually crumbles under its own weight.
The researchers simulated how different coastline shapes would evolve under each of the three scenarios. To simulate wave-driven erosion, they took into account a variable known as “fetch,” which describes the physical distance from one point on a coastline to the other side of a lake or sea.
“Wave erosion is driven by the height and angle of the wave,” Palermo explains. “We used fetch to approximate wave height because the larger the fetch, the longer the distance the wind can blow and the waves can grow.”
To test how shoreline shapes would differ between the three scenarios, the researchers started with a simulated sea with flooded river valleys around the edges. For wave-induced erosion, they calculated the fetch distance from every point along the shoreline to every other point and converted these distances to wave heights. They then ran their simulation to see how waves would erode the initial shoreline over time. They compared this to how the same shoreline would evolve under erosion caused by uniform erosion. The team repeated this comparative modeling for hundreds of different initial shoreline shapes.
Comparing erosion types and their effects
They found that the final forms were very different, depending on the underlying mechanism. In particular, uniform erosion produced inflated banks that widened uniformly everywhere, even in the flooded river valleys, while wave erosion mainly smoothed the parts of the banks that were exposed to long fetch distances, making the flooded valleys narrow and rough.
“We had the same initial shorelines and we saw that you get a very different final shape under uniform erosion than under wave erosion,” Perron says. “They all look a bit like the Flying Spaghetti Monster because of the flooded river valleys, but the two types of erosion produce very different end points.”
The team checked their results by comparing their simulations to actual lakes on Earth. They discovered the same shape difference between terrestrial lakes known to have been eroded by waves, and lakes affected by uniform erosion, such as the dissolution of limestone.
Mapping and modeling Titan’s largest seas
Their modeling revealed clear, characteristic shoreline shapes, depending on the mechanism by which they evolved. The team then asked: Where would Titan’s shorelines fit within these characteristic shapes?
They focused in particular on four of Titan’s largest and best-charted seas: Kraken Mare, which is comparable in size to the Caspian Sea; Ligeia Mare, larger than Lake Superior; Punga Mare, which is longer than Lake Victoria; and Ontario Lacus, which is about 20 percent the size of its Earthly namesake.
The team mapped the coastlines of each Titan sea using Cassini’s radar images and then applied their modeling to each sea coastline to see which erosion mechanism best explained their shape. They found that all four seas fit firmly into the wave-driven erosion model, meaning that waves produced coastlines that most resembled Titan’s four seas.
“We found that when coastlines are eroded, their shapes are more consistent with wave erosion than with uniform erosion or no erosion at all,” Perron said.
Future research directions and implications
The researchers are trying to determine how strong the winds on Titan would need to be to generate waves that could repeatedly crash into the coasts. They also hope to use the shape of Titan’s coastline to decipher the main directions the winds are blowing from.
“Titan presents this case of a completely unaffected system,” says Palermo. “It could help us learn more fundamental things about how coasts erode without the influence of humans, and perhaps that could help us better manage our Earth’s coastlines in the future.”
Reference: “Signatures of Wave Erosion in Titan’s Shores” by Rose V. Palermo, Andrew D. Ashton, Jason M. Soderblom, Samuel PD Birch, Alexander G. Hayes, and J. Taylor Perron, June 19, 2024, Scientific progress.
DOI: 10.1126/sciadv.adn4192
This work was supported in part by NASA, the National Science Foundation, the USGS, and the Heising-Simons Foundation.