If you’re looking for the best waves in the solar system, Saturn’s moon Titan could be your ideal extraterrestrial surfing spot, where a gentle breeze that merely raises a ripple on Earth could drive waves 10 feet (3 meters) tall on the frigid world.
A new model called “PlanetWaves” has been developed by researchers to accurately describe what waves in bodies of liquid on other worlds may look like. Previous attempts at doing so focused only on the gravity of a planet, but PlanetWaves also applies atmospheric pressure and the nature of the liquid being blown — its density, viscosity and surface tension, which quantifies the liquid’s resistance to rippling.
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Schneck’s team calibrated their PlanetWaves model on 20 years’ worth of data collected by buoys on Lake Superior, which is Earth’s largest freshwater lake and is situated on the border between Canada and the United States. The model was able to replicate the measurements precisely, giving the researchers confidence in then applying it to other worlds.
“On Earth, we get accustomed to certain wave dynamics,” Andrew Ashton of MIT and the Woods Hole Oceanographic Institution said in the statement. “But with this model, we can see how waves behave on planets with different liquids, atmospheres and gravity, which can kind of challenge our intuition.”
Saturn‘s largest moon Titan was their primary focus, simply because it is the only other world that we know for sure has bodies of liquid on its surface, with its rivers, lakes and seas being mapped by the Cassini–Huygens mission.
However, Titan’s liquid is not water, but rather oily hydrocarbons such as methane and ethane, rendered liquid only due to the exceedingly cold temperatures of –179 degrees Celsius (–290 degrees Fahrenheit).
“For Titan, the tantalizing thing is that we don’t have any direct observations of what these lakes look like,” said MIT’s Taylor Perron said in the statement. “So we don’t know for sure what kind of waves might exist there. Now this model gives us an idea.”
The team found that a light wind could raise waves 10 feet high in Titan’s lakes, thanks to Titan’s low gravity, which is 14% the strength of gravity on Earth, and the relatively light nature of the liquid which makes it easier to move.
“It kind of looks like tall waves moving in slow motion,” said Schneck. “If you were standing on the shore of this lake, you might feel only a soft breeze but you would see these enormous waves flowing toward you, which is not what we would expect on Earth.”
As waves batter coasts they are the source of considerable erosion. Could Titan’s giant waves potentially answer a puzzle that has persisted about the nature of Titan’s lakes and shorelines?
“Unlike on Earth where there is often a delta where a river meets the coast, on Titan there are very few things that look like deltas even though there are plenty of rivers and coasts,” said Taylor Perron, also of MIT. “Could waves be responsible for this?”
Understanding the size of Titan’s waves will also be important should one of our space agencies ever decide to send a probe to float on one of Titan’s lakes or seas.
“You would want to build something that can withstand the energy of the waves, so it’s important to know what kind of waves these instruments would be up against,” said Schneck.
The team also applied their PlanetWaves model to a variety of other worlds. Mars doesn’t seem to have liquid water anymore, but billions of years ago it did. Over time Mars lost much of its atmosphere, and the air pressure and temperature dropped. As this happened, it would have required stronger winds to raise waves whereas weaker winds would have sufficed before.
Beyond the solar system, there are a variety of worlds that could support liquids of some kind, though so far none have been confirmed to actually have liquids. Nevertheless, the habitable zone planet LHS 1140b has a density that suggests up to 19% of its mass is water in some form. As a super-Earth with stronger gravity than our planet, waves on any hypothetical ocean would be much smaller than on Earth for the same wind speed.
More exotic is the exoplanet Kepler-1649b, which is a hot Venus-like world with similar gravity to Earth. Venus has copious amounts of sulfuric acid in its atmosphere, and Kepler-1649b might have too. If that sulfuric acid can exist on the planet’s surface, then it would require strong winds just to cause the acid lake to ripple because sulfuric acid is twice as dense as liquid water.
The last exoplanet they employed PlanetWaves on was 55 Cancri e, which is a hot world quite possibly covered with lakes of lava. Since lava is generally quite thick and viscous, and 55 Cancri e’s gravity is stronger than Earth’s, it would take hurricane force winds in the region of 80 miles per hour just to cause a ripple on the lava ocean.
The findings were published on April 3rd in the Journal of Geophysical Research: Planets.
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