Published in the Journal of Geophysical Research: Planets, the study introduces a model the team has coined "PlanetWaves." The researchers apply it to predict how waves behave on planetary bodies that might host liquid lakes and oceans, including Titan, ancient Mars, and three planets beyond the solar system.
The model predicts that a gentle wind would be enough to stir up huge waves on Titan, where lakes are filled with light liquid hydrocarbons. In contrast, it would take hurricane-force winds to barely move the surface of a lake on the exoplanet 55-Cancri e, thought to be a lava world covered in hot, dense liquid rock.
"On Earth, we get accustomed to certain wave dynamics," said study author Andrew Ashton, associate scientist at the Woods Hole Oceanographic Institution and faculty member of the MIT-WHOI Joint Program. "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."
The team is particularly focused on understanding wave formation on Titan, the only other planetary body in the solar system besides Earth known to currently host liquid lakes. NASA's Cassini mission previously captured radar images of lake formations suspected to be filled with liquid methane and ethane.
"Anywhere there's a liquid surface with wind moving over it, there's potential to make waves," said Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT. "For Titan, the tantalizing thing is that we don't have any direct observation of what these lakes look like. Now this model gives us an idea."
Lead author Una Schneck, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences, noted that a future probe sent to Titan's lakes would need to be designed to withstand the wave environment predicted by the model.
Previous wave models for other planets accounted only for gravity. The new PlanetWaves model also incorporates properties of the surface liquid itself - density, viscosity, and surface tension - as well as atmospheric pressure. The team first validated the model using 20 years of wave measurements collected by buoys across Lake Superior, finding it accurately predicted wave heights for given wind speeds.
Applied to Titan, the model found it is surprisingly easy to generate waves there. The relatively light liquid methane-ethane mix, combined with low gravity and atmospheric pressure, means even a gentle breeze can produce enormous slow-moving waves. "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," Schneck said.
For ancient Mars, the team showed that as the planet's atmosphere gradually thinned over time, reducing atmospheric pressure, progressively stronger winds would have been needed to generate the same wave heights. One of the basins modeled is Jezero Crater, now being explored by NASA's Perseverance rover.
Beyond the solar system, the model was applied to three exoplanets. On LHS1140b, a cool super-Earth with stronger gravity than our planet, the same wind that generates waves on Earth would produce much smaller water waves. On Kepler 1649b, a Venus-like planet with lakes of sulfuric acid roughly twice as dense as water, strong winds would be needed to produce even a ripple. The effect is even more extreme on 55-Cancri e, where the model predicts that hurricane-force winds of about 80 miles per hour would generate only centimeter-scale waves on its liquefied rock surface.
Perron hopes the model will also help resolve longstanding questions about planetary landscape formation. On Titan, for example, there are very few river delta formations despite the presence of rivers and coasts - a pattern that could be explained by the wave dynamics now being quantified.
"These are the kinds of mysteries that this model will help us solve," Perron said.
Research Report:Modeling Wind-Driven Waves on Other Planets: Applications to Mars, Titan, and Exoplanets
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