During its flyby of Titan T85 on July 24, 2012, the Cassini spacecraft detected an unexpectedly bright reflection on the surface of Lake Kivu Lacus. Data from its Visual and Infrared Mapping Spectrometer (VIMS) were interpreted as bumps on the methane-ethane lake that could have been a sign of tidal flats, rising bubbles, or waves.
“Our landscape evolution models show that the shorelines on Titan are most consistent with Earth’s lakes eroded by waves,” says Rose Palermo, a coastal geomorphologist at the St. Petersburg Coastal and Marine Science Center who led the study examining wave erosion on Titan. The evidence for waves is not yet conclusive, but future manned missions to Titan should pack a few surfboards just in case.
Troubled sea
While waves have been considered the most plausible explanation for the reflections seen in Cassini’s VIMS images for some time, other studies attempting to confirm their presence have failed to detect any wave activity at all. “Other observations show that the liquid surfaces were very calm and very flat in the past,” says Palermo. “One possible explanation is that at the time we observed Titan, the winds were quite weak, so there were not many waves at that time. To confirm waves, we would need better resolution data,” she adds.
The problem is that we won’t get this higher resolution data any time soon. Dragonfly, the next mission to Titan, isn’t scheduled to arrive until 2034, even if everything goes according to plan.
To get a better idea of possible waves on Titan earlier, Palermo’s team inferred their presence from indirect evidence. The researchers hypothesized that Titan’s shorelines could have been formed by one of three possible scenarios. The first assumed no erosion at all; the second modeled uniform erosion caused by the dissolution of bedrock by the ethane-methane fluid; and the third assumed erosion by wave activity. “We took a random topography with rivers and filled in the basin-flooded river valleys around the lake. Then we used a computer model of landscape evolution to erode the coast to 50 percent of its original size,” Palermo explains.
Determine the size of the waves
The Palermo simulations showed that wave erosion created coastal features very similar to those on Titan.
The team validated its model using data from the surrounding area. “We compared the results with lakes on Earth where we know the erosion processes, using the same statistical analysis. With a certainty of over 77.5 percent, we were able to predict these known processes with our modeling,” says Palermo.
But even the study that claimed ripples were visible in Cassini’s VIMS images concluded that they were about 2 centimeters high at best. So even if there are ripples on Titan, the question is how high and strong they are.
According to Palermo, the wave-generating mechanisms on Titan should work the same as on Earth, but with some notable differences. “There is a difference in viscosity between water on Earth and methane-ethane liquid on Titan compared to the atmosphere,” says Palermo. Gravity is also much weaker, only one-seventh of that on Earth. “Gravity, together with the differences in material properties, contributes to the waves being higher and steeper than on Earth for the same wind speed,” says Palermo.
But despite these increases in size and strength, could the waves on Titan actually be good for surfing?
The waves are here
“Our work definitely raises a lot of open questions. In which direction are the prevailing waves? Knowing that will help us learn about the winds and therefore the climate on Titan. How big do the waves get? In the future, we might be able to tell that by modeling how much erosion occurs in one part of the lake compared to another over estimated time periods. We could learn a lot more,” Palermo says. As for surfing, she said that assuming a minimum height for a surfable wave of about 15 centimeters, surfing on Titan should most likely be feasible.
The biggest limitation on the size and strength of waves on Titan is that most of its seas are about the size of the Great Lakes in the United States. The largest of them, the Kraken Mare, is about the size of the Caspian Sea on Earth. There is no global ocean on Titan, and that means the range, the distance over which the wind can blow and make the waves grow, is limited to tens of kilometers, instead of over 1,500 kilometers on Earth. “Still, some models show that the waves on Titan can be up to a meter high. I would say that’s a surfable wave,” Palermo concluded.