During its flyby of T85 Titan on July 24, 2012, the Cassini probe registered an unexpectedly bright reflection on the surface of Lake Kivu Lacus. Data from its Visual and Infrared Mapping Spectrometer (VIMS) were interpreted as roughness on the lake of methane and ethane, which could be a sign of mud, emerging bubbles or waves.
“Our landscape evolution models show that shores on Titan are most consistent with terrestrial lakes that have been eroded by waves,” says Rose Palermo, a coastal geomorphologist at the St. Petersburg Coastal and Marine Science Center, who led a study examining signs of erosion waves on Titan. The evidence for waves is still inconclusive, but future manned missions to Titan should probably have some surfboards just in case.
Rough seas
While waves have for some time been considered the most likely explanation for the reflections seen in the Cassini VIMS images, other studies aimed at confirming their presence have found no wave activity at all. “Other observations show that liquid surfaces were very calm, very flat in the past,” 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 that time.” To confirm the waves, we would have to have better resolution data,” he adds.
The problem is that this higher resolution data is not coming to us anytime soon. Dragonfly, the next mission to Titan, is not expected to arrive until 2034, even if all goes according to plan.
To get a better idea of possible waves on Titan a little earlier, Palermo’s team decided to infer their presence from indirect clues. Scientists hypothesized that the coastline on Titan could have been formed in one of three possible scenarios. At first they assumed that there was no erosion at all; the second modeled uniform erosion caused by bedrock dissolution by ethane-methane fluid; and the third assumed erosion by wave action. “We did a random topography of the rivers and filled in the river valleys flooding the basins all around the lake. We then used a computer model of landscape development to erode the coastline to 50 percent of its original size,” explains Palermo.
Sizing waves
Palermo’s simulations showed that wave erosion led to shoreline shapes that closely resembled 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 like. With greater than 77.5 percent confidence, we were able to predict these known processes using our modeling,” says Palermo.
But even the study that claimed waves were visible in the Cassini VIMS images concluded that they were roughly 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 the same as on Earth, with some notable differences. “There is a difference in viscosity between water on Earth and methane-ethane on Titan compared to the atmosphere,” says Palermo. Gravity is also much weaker, reaching only one-seventh the gravity on Earth. “Gravity, along with differences in material properties, contribute to waves being higher and steeper than those on Earth at the same wind speed,” says Palermo.
But despite these increases in size and strength, could the waves on Titan actually be good for surfing?
You surf
“There are certainly many open questions that our work leads to. What is the direction of the dominant waves? Knowing this can tell us about the winds and thus the climate on Titan. How big are the waves? In the future, we might be able to say, by modeling, how much erosion is occurring in one part of the lake versus another over estimated time scales. There’s a lot more we could learn,” says Palermo. As for surfing, she said that assuming a minimum height for a surfable wave of around 15 centimeters, surfing on Titan should most likely be feasible.
A key limit to 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 the Caspian Sea on Earth. 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 raise waves, is limited to tens of kilometers instead of more than 1,500 kilometers on Earth. “However, some models show that waves on Titan are up to one meter high.” I would say it’s a surfable wave,” Palermo concluded.