Titan, Saturn’s largest moon, is the only known planetary body besides Earth on which standing liquids persist. Liquid hydrocarbons, supplied by rainfall from the moon’s thick atmosphere, form rivers, lakes, and seas, most of which are found in the polar regions. In new research, a team of geologists at MIT studied Titan’s shorelines and found that the moon’s large lakes and seas have likely been shaped by waves.
An artist’s rendering of the surface of Saturn’s largest moon, Titan. Image credit: Benjamin de Bivort, debivort.org / CC BY-SA 3.0.
The presence of waves on Titan has been a somewhat controversial topic ever since NASA’s Cassini spacecraft spotted bodies of liquid on the moon’s surface.
“Some people who tried to see evidence for waves didn’t see any, and said, ‘These seas are mirror-smooth.’ Others said they did see some roughness on the liquid surface but weren’t sure if waves caused it,” said Dr. Rose Palermo, a geologist at the U.S. Geological Survey.
“Knowing whether Titan’s seas host wave activity could give scientists information about the moon’s climate, such as the strength of the winds that could whip up such waves.”
“Wave information could also help scientists predict how the shape of Titan’s seas might evolve over time.”
“Rather than look for direct signs of wave-like features in images of Titan, we had 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 thought to have formed as rising levels of liquid flooded a landscape crisscrossed by river valleys.
The researchers zeroed in on three scenarios for what could have happened next: no coastal erosion; erosion driven by waves; and uniform erosion, driven either by dissolution, in which liquid passively dissolves a coast’s material, or a mechanism in which the coast gradually sloughs off under its own weight.
They simulated how various shoreline 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 shoreline to the opposite side of a lake or sea.
“Wave erosion is driven by the height and angle of the wave,” Dr. Palermo said
“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.”
Cassini pinged the surface of Titan with microwaves, finding that some channels are deep canyons filled with liquid hydrocarbons. One such feature is Vid Flumina, the branching network of narrow lines in the upper-left quadrant of the image. Image credit: NASA / JPL-Caltech / ASI.
To test how shoreline shapes would differ between the three scenarios, the scientists started with a simulated sea with flooded river valleys around its edges.
For wave-driven erosion, they calculated the fetch distance from every single point along the shoreline to every other point, and converted these distances to wave heights.
Then, they ran their simulation to see how waves would erode the starting shoreline over time.
They compared this to how the same shoreline would evolve under erosion driven by uniform erosion.
The authors repeated this comparative modeling for hundreds of different starting shoreline shapes.
They found that the end shapes were very different depending on the underlying mechanism.
Most notably, uniform erosion produced inflated shorelines that widened evenly all around, even in the flooded river valleys, whereas wave erosion mainly smoothed the parts of the shorelines exposed to long fetch distances, leaving the flooded valleys narrow and rough.
“We had the same starting shorelines, and we saw that you get a really different final shape under uniform erosion versus wave erosion,” Dr. Perron said.
“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.”
This image is a composite of several images taken during two separate Titan flybys in 2006. The large circular feature near the center of Titan’s disk may be the remnant of a very old impact basin. The mountain ranges to the southeast of the circular feature, and the long dark, linear feature to the northwest of the old impact scar may have resulted from tectonic activity on Titan caused by the energy released when the impact occurred. Image credit: NASA/JPL/University of Arizona.
Dr. Perron and colleagues checked their results by comparing their simulations to actual lakes on Earth.
They found the same difference in shape between Earth lakes known to have been eroded by waves and lakes affected by uniform erosion, such as dissolving limestone.
Their modeling revealed clear, characteristic shoreline shapes, depending on the mechanism by which they evolved.
They then wondered: Where would Titan’s shorelines fit, within these characteristic shapes?
In particular, they focused on four of Titan’s largest, most well-mapped seas: Kraken Mare, which is comparable in size to the Caspian Sea; Ligeia Mare, which is larger than Lake Superior; Punga Mare, which is longer than Lake Victoria; and Ontario Lacus, which is about 20% the size of its terrestrial namesake.
The researchers mapped the shorelines of each Titan sea using Cassini’s radar images, and then applied their modeling to each of the sea’s shorelines to see which erosion mechanism best explained their shape.
They found that all four seas fit solidly in the wave-driven erosion model, meaning that waves produced shorelines that most closely resembled Titan’s four seas.
“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,” Dr. Perron said.
The scientists are working to determine how strong Titan’s winds must be in order to stir up waves that could repeatedly chip away at the coasts.
They also hope to decipher, from the shape of Titan’s shorelines, from which directions the wind is predominantly blowing.
“Titan presents this case of a completely untouched system,” Dr. Palermo said.
“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.”
The findings appear today in the journal Science Advances.
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Rose V. Palermo et al. 2024. Signatures of wave erosion in Titan’s coasts. Science Advances 10 (25); doi: 10.1126/sciadv.adn4192
