During the T85 Titan flyby on July 24, 2012, the Cassini spacecraft recorded an unexpectedly bright reflection on the surface of Lake Kivu Lacus. Data from its Visual and Infrared Mapping Spectrometer (VIMS) were interpreted as roughness of the methane-ethane lake, which could be a sign of mud layers, bubbles or ripples.
“Our models of landscape evolution show that Titan’s shores are most consistent with terrestrial lakes that have been eroded by waves,” said Rose Palermo, a coastal geomorphologist at the Center for Coastal and Marine Sciences in St. Petersburg, who leads the study investigating the traces of erosion waves on Titan. The evidence for waves is still inconclusive, but future crewed missions to Titan should probably pack surfboards just in case.
Rough seas
While waves have long been considered the most plausible explanation for the reflections seen in Cassini’s VIMS images, other studies aiming to confirm their presence have not found any wave activity. “Other observations show that liquid surfaces were very still in the past, very flat,” says Palermo. “A possible explanation for this is that at the time we observed Titan, the winds were quite weak, so there weren’t many waves at the time. To confirm the waves, we will need to have better resolution data,” she adds.
The problem is, that higher-resolution data isn’t coming anytime soon. Dragonfly, the next mission to Titan, is not due to arrive until 2034, even if all goes according to plan.
To get a better idea of Titan’s possible waves a little earlier, the Palermo team inferred their presence from indirect signs. The researchers suggest that Titan’s coastline may have been shaped by one of three candidate scenarios. They first assumed that there was no erosion at all; the second modeled uniform erosion caused by bedrock dissolution by the ethane-methane fluid; and the third suggests erosion by wave activity. “We took a random topography with rivers, filled in the flood basins river valleys around the lake. We then used a landscape evolution computer model to erode the bank to 50 percent of its original size,” explains Palermo.
Sizing waves
Palermo’s simulations showed that wave erosion produced shoreline shapes that matched those actually observed on Titan.
The team validated their model using data from closer to home. “We compared, using the same statistical analysis, with lakes on Earth, where we know what the erosion processes are. With a certainty greater than 77.5 percent, we were able to predict these known processes with our modeling,” says Palermo.
But even the study that claims there are ripples visible in Cassini’s VIMS images concludes that they are at best about 2 centimeters high. So even if there are waves on Titan, the question is how high and strong are they?
According to Palermo, Titan’s wave-generating mechanisms should work just like they do on Earth, with some notable differences. “There is a difference in viscosity between Earth’s water and Titan’s methane-ethane liquid compared to the atmosphere,” says Palermo. The gravity is also much weaker, standing at only one-seventh that of Earth. “Gravity, along with differences in material properties, contribute to the waves being higher and steeper than those on Earth at the same wind speed,” says Palermo.
But even with these increases in size and strength, could Titan’s waves really be useful for surfing?
The surf is ready
“There are definitely a lot of open questions that our work leads to. What is the direction of the dominant waves? Knowing this can tell us about the winds and therefore the climate of Titan. How big do the waves get? In the future, we may be able to understand this by modeling how much erosion is occurring in one part of the lake versus another over the estimated time scales. There’s a lot more we can learn,” says Palermo. As for surfing, she said that if we assume a minimum height for a surfing wave of about 15 centimeters, surfing on Titan should probably be possible.
The main limitation on the size and strength of any waves on Titan is that most of its seas are roughly the size of the US Great Lakes. The largest of these, the Kraken Mare, is roughly the size of Earth’s Caspian Sea. There is no such thing as a global ocean on Titan, and this means that fetch, the distance over which the wind can blow and increase waves, is limited to tens of kilometers instead of over 1,500 kilometers on Earth. “However, some models show that Titan’s waves are up to one meter high. I would say it’s a surfing wave,” Palermo concluded.
