Titan's Lake Shorelines Are Formed by Methane Waves

Map of Titan's northern region with hydrocarbons "seas" from methane and ethane, created on the basis of radar images "Cassini". New research shows that wind-driven waves are eroding the satellite's coastlines. — A map of Titan's northern region, with hydrocarbon “seas” of methane and ethane, created from Cassini radar images. New research shows that wind-driven waves are eroding the moon's shorelines.

Distant Titan is a curiosity in the solar system. Saturn's largest moon—and the second largest in the entire solar system—has an atmosphere denser than Earth's. In addition, on its surface there are stable lakes and seas of liquid hydrocarbons.

New research shows that waves in these seas are eroding Titan's coastlines.

The study entitled “Signs of Wave Erosion on Titan's Coast” published in the journal Science Advances. Lead author is Rose Palermo, an MIT graduate and research geologist with the USGS.

In 2007, the Cassini spacecraft discovered lakes and seas of liquid hydrocarbons, mostly methane and ethane, on Saturn's moon Titan. Titan and Earth are the only two bodies in the solar system with liquid hydrocarbons on their surfaces. Scientists have only the Cassini data from Titan, and they are studying it carefully to try to understand this strange world.

Titan's seas are one of the most intriguing features in the entire solar system. But they are difficult to observe because of the dense atmosphere. Researchers have wondered whether the waves form the coastlines. The problem is that there is conflicting evidence about the nature of the seas. They can be rough, or they can be smooth. work 2014, it was suggested that transitional features in Titan's northern sea, the Ligean Sea, may be waves.

But there is no certainty about this.

“Some people who tried to see evidence of waves didn't see them and said, 'These seas are mirror-smooth,'” lead author Palermo said in a press release accompanying the study. “Others said they could see some roughness on the surface of the liquid, but weren't sure it was caused by the waves.”

It seems likely that Titan could have waves. To study this question, scientists from the Massachusetts Institute of Technology compared Titan's coastlines with coastlines on Earth to see if they matched up.

Titan's seas and lakes are very similar to those on Earth. They look like flooded valleys and depressions. But scientists are not sure that these bodies of water erode the coastline as they do on Earth. “Spacecraft observations and theoretical models suggest that wind can cause waves in Titan's seas, potentially leading to coastal erosion, but wave observations are indirect, and the processes that influence the evolution of Titan's coastline remain unknown,” the authors write in their paper.

The problem is that even on Earth, there’s no reliable way to directly link shoreline morphology to the mechanisms that shape it. To try to understand how erosion affects Titan’s coastlines, the researchers started on Earth. They studied how different coastal erosion mechanisms shape Earth’s coastlines, and then applied that framework to Titan.

There are basically two types of coastal erosion: wave and uniform. Each type results in different coastlines.

Wave erosion is caused by wind and results in changes proportional to the strength of the waves. Waves are generally stronger the farther they travel before hitting the shore. Wave erosion creates long, smooth stretches of coastline where the shore is completely exposed, and bays in sheltered areas where erosion is less. The distance that wind can generate waves on a particular body of water before hitting the shore is called the “surge length.”

“Wave erosion depends on the height and angle of the wave,” Palermo explained. “We used wave surge to approximate wave height because the larger the wave surge, the greater the distance over which the wind can blow and the wave grow.”

Uniform erosion is something else entirely. It does not depend on the mechanical action of waves. The difference in composition between Earth and Titan is obvious when it comes to uniform erosion. “Titan's crust is composed primarily of water ice, but solid particles on its surface may also include molecules of heavy hydrocarbons such as benzene, which are soluble in liquid methane and ethane, so liquid lakes and seas can slowly dissolve the solid shores of the north polar landforms,” the authors explain in their study.

Over a sufficiently long period of time, uniform erosion occurs at the same rate in all places, creating distinct morphological features: coastlines are generally smooth, even in bays with sharp headlands that pierce them.”

“Here we test the hypothesis that coastal erosion shaped Titan's seas by examining whether shoreline shapes correspond to wave erosion, uniform erosion, or the absence of coastal erosion,” the authors write.

This figure from the study illustrates how two types of erosion would shape coastlines. The images are based on modeled landforms and coastlines on Titan. A shows the initial state of Titan's water bodies, where rivers carved out channels and rising seas flooded them. B shows the morphology that would result from wave erosion, where the rate of erosion depends on wind strength. C shows the morphology that would be created by erosion that was uniform in all locations. Dark blue indicates greater depth, and light yellow indicates higher elevation.

Differences in the morphological features created by wave and uniform erosion are obvious. Wave erosion tends to flatten exposed shorelines where the surf is high and to keep the shoreline where the surf is low within bays.

Uniform erosion is different from this. It widens the ledges and smoothes out small irregularities in the shoreline, regardless of the surf. The exception is the capes, which sharpen, turning into thick-necked points sticking out into the main basin.

“We had the same initial shorelines, and we saw that the final shape of uniform erosion and wave erosion was very different,” said study co-author Taylor Perron, a professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences. “They all look like the Flying Spaghetti Monster because of the flooded river valleys, but the two types of erosion lead to very different end points.”

Ligeia Mare is the second largest liquid body on Titan. Researchers say its shoreline appears to have been altered by wave erosion.

“We found that when a shoreline is eroded, its shape is more consistent with wave erosion than with uniform erosion or no erosion at all,” Perron says.

But this is just a simulation, and it needs to be tested carefully.” The team's next step was to quantify these differences in the real world. The researchers explain that to understand and quantify the differences, they “developed a methodology that focused on local relationships between shoreline roughness and specifically, they assessed what they call “roughness” to distinguish wave-induced erosion from uniform erosion. “Simply put, less roughness means a smoother section of shoreline compared to the rest of the lake, and more roughness refers to a relatively uneven portion of the shoreline,” they write.

This figure from the study shows the roughness and surge area for two of Titan's seas: Mare Kraken and Mare Ligeia. B and D show the roughness for each sea. E and F show the normalized surge area. Surge-limited means that the waves continue to grow as long as the surge length increases. G and H show the normalized surge area assuming a saturated surge length of 20 km. This means that the waves only grow up to a certain surge length and then saturate. In this case, the system is saturation-limited, and "the length of the wave surge in all directions is truncated to a maximum value". — This figure from the study shows the roughness and surge area for two of Titan's seas: the Kraken Sea and the Ligeia Sea. B and D show the roughness for each sea. E and F show the normalized wave surge area. Limited wave surge means that the waves continue to grow as long as the wave surge length increases. G and H show the normalized wave surge area, assuming a saturated wave surge length of 20 km. This means that the waves only grow up to a certain surge length and then saturate. In this case, the system is saturation-limited and “the wave surge length in all directions is truncated to a maximum value.”

The researchers claim that “…shoreline roughness and normalized wave surge area can be used to identify wave and uniform erosion and distinguish them from shorelines consisting only of flooded river valleys,” as shown in the first image.

So what does this all come down to?

“Our results suggest that the shorelines of Titan's largest bodies of water most closely correspond to shorelines that have been modified by wave erosion and river incision,” the researchers write in their paper. They analyzed four shorelines and found that the probability of uniform erosion in the saturation-limited scenario was less than 5% and in the surge-limited scenario was less than 20%. This leaves wind erosion as the most likely cause of erosion, which seems to confirm that waves are observed in Titan's lakes and seas. “Our results thus suggest that the largest seas and lakes are not the result of erosion by homogeneous processes (i.e., dissolution), as previously assumed for some of Titan's landscapes,” the authors conclude.

This is a scientific way of presenting results, and their article is part of a long conversation with other scientists. In a press release, they present their findings in a more understandable way for others.

“Based on our results, if the coastlines of Titan's seas are eroding, the most likely culprit is waves,” said Perron, a professor in the MIT Department of Earth, Atmospheric, and Planetary Sciences. “If we could stand on the edge of one of Titan's seas, we would see waves of liquid methane and ethane running up the coast and crashing on the shore during storms. And they could be eroding the material that makes up the coast.”

“Waves are ubiquitous in Earth's oceans. If there are waves on Titan, they would likely dominate the surface of lakes,” says Juan Felipe Paniagua-Arroyave, an associate professor at the School of Applied Science and Engineering at EAFIT University in Colombia who was not involved in the study. “It would be interesting to see Titan's winds creating waves, not of water, but of exotic liquid hydrocarbons.”

The next step is to determine how strong Titan's winds would have to be to cause coastal erosion. The researchers also hope to decipher which directions the wind blows predominantly from.

“Titan is an example of a completely pristine system,” Palermo said. “It can help us learn more fundamental things about how coastal erosion occurs without human influence, and perhaps that will help us better manage Earth's coastlines in the future.”