During its T85 Titan flight on July 24, 2012, the Cassini spacecraft recorded an unexpectedly bright reflection on the surface of Lake Kivu Lacus. The Visual and Infrared Mapping Spectrometer (VIMS) data was interpreted as a roughness on the methane-ethane lake, which could have been a sign of tidal flats, rising bubbles or waves.
“Our landscape evolution models show that Titan’s coastlines are most similar to terrestrial lakes eroded by waves,” said Rose Palermo, a coastal geomorphologist at the St. Petersburg Coastal and Marine Science Center, who led the investigation of the characteristics of wave waves. erosion on Titan. The evidence of waves is still inconclusive, but future crewed missions to Titan should probably take some surfboards with them just in case.
Troubled seas
Although waves have long been considered the most plausible explanation for reflections visible in Cassini’s VIMS images, other studies to confirm their presence found no wave activity at all. “Other observations show that the liquid surfaces have been very still and very flat in the past,” says Palermo. “One possible explanation for this is that at the time we observed Titan, the wind was quite low, so there were not many waves at that time. To confirm waves, we need better resolution data,” she adds.
The problem is that this higher resolution data isn’t coming our way anytime soon. Dragonfly, the next mission to Titan, shouldn’t arrive until 2034, even if all goes according to plan.
To get a better idea of possible waves on Titan a little earlier, the Palermo team decided to infer their presence from indirect signals. The researchers assumed that the coastlines on Titan could have been formed by one of three possible scenarios. They first assumed there was no erosion at all; the second modeled uniform erosion caused by the dissolution of the rock by the ethane-methane fluid; and the third assumed erosion by wave activity. “We took a random topography of rivers and filled in the watersheds around the lake. We then used the landscape evolution computer model to erode the coast to 50 percent of its original size,” Palermo explains.
Sizing the waves
The Palermo simulations showed that wave erosion resulted in coastline shapes that closely matched those actually observed on Titan.
The team validated its model using data from closer to home. “We used the same statistical analysis to compare it with lakes on Earth, where we know what the erosion processes are. We were able to predict these known processes with our models with a certainty of more than 77.5 percent,” says Palermo.
But even the study that claimed waves were visible in Cassini’s VIMS images concluded that they were about 2 centimeters high at best. So even if there are waves on Titan, the question is how high and strong are they?
According to Palermo, the wave generation mechanisms on Titan should work just like they do on Earth, with some notable differences. “There is a difference in viscosity between water on Earth and methane-ethane fluid on Titan compared to the atmosphere,” Palermo says. Gravity is also a lot weaker, amounting to only one-seventh of Earth’s gravity. “Gravity, together with the differences in material properties, contributes to the waves being larger and steeper than those on Earth at the same wind speed,” says Palermo.
But even with that increase in size and power, could the waves on Titan actually be good for surfing?
Surf’s Up
“There are certainly many open questions that our work leads to. What is the direction of the dominant waves? If we know this, we can tell something about the wind and therefore about the climate on Titan. How big will the waves get? In the future, modeling may allow us to see how much erosion is occurring in one part of the lake versus another on estimated time scales. There is much more we can learn,” says Palermo. As for surfing, she said that, assuming a minimum height for a surfable wave of about 6 inches, surfing Titan should most likely be feasible.
The main limitation on the size and strength of Titan’s waves is that most of its seas are about the size of the US Great Lakes. The largest of these, the Kraken Mare, is about the size of Earth’s Caspian Sea. There is no such thing as a global ocean on Titan, and this means that the fetch, the distance over which the wind can blow and make the waves grow, is limited to tens of kilometers instead of more than 1,500 kilometers on Earth. “Yet some models show that the waves on Titan can be up to a meter high. I would say this is a surfable wave,” Palermo concluded.