One of the most dramatic and memorable scenes in Interstellar takes place on the Mirror Planet. If you don’t want spoilers for an 11-year-old movie, skip to the next paragraph. When the crew arrives at what could become humanity’s new home, they are faced with a literal 1.2km-high wall of water rapidly surging towards them. This is a good illustration of how waves on other planets can behave differently than on Earth. Indeed, according to Kip Thorne, the film’s scientific advisor, these waves are actually caused by Earth’s proximity to a local black hole, rather than the wind that forms the waves here.
However, winds are not the same on every planet. Oceans and lakes are not made of water either. Also, the atmospheric pressure is not the same. In other words, there are many variables that determine how waves interact with the rest of the world. A new paper published in the Journal of Geophysical Research: Planets by MIT’s Una Schneck and colleagues shows how different types of planets generate different waves.
To explore this idea, they developed a software model called Planet Waves. It takes into account the energy input from the wind and subtracts it based on breakage, turbulence, bottom friction, and fluid viscosity. With this model, they discovered two “universal” rules for waves. First, the minimum wind speed required to create waves is lower for liquids with low surface tension, objects with high atmospheric pressure, and low gravity. Second, when the liquid is less dense and under a thicker atmosphere, the lower the gravity, the higher the waves.
This video explains more about Interstellar’s water planet. Don’t watch if you don’t want spoilers. Credit – Salom YouTube Channel
These may seem obvious truths, but throughout this paper they are applied to a wide range of practical (and theoretical) settings. One obvious candidate was Mars. There is currently no liquid water on the surface of Mars, but our current understanding of its history suggests that it may have had liquid water in the past. On Mars, the waves would have started at lower wind speeds because of lower gravity. Their size also would have varied widely based on the density of Mars’ atmosphere, which ranged from 200 kPa (about twice that of Earth) to 50 kPa, which is expected before surface water dries up.
Liquid lakes still exist on Titan, but they are not made of water. This moon of Saturn is dominated by liquid methane and ethane, has very low gravity, and a very dense atmosphere. These characteristics combine to allow waves to form at only 0.6 m/s and grow up to 3 meters in height with relative ease. Also, because gravity is lower on the moon’s surface, it may move in slow motion compared to Earth. However, the model here encounters some observational difficulties. Cassini reflected radar pulses off Titan’s lakes, and the signals returned suggested that its surface was glass-smooth and may contain “magical islands” of localized wave patches. So if Titan has winds (which it probably does), why didn’t the Cassini data reveal any wave patterns? Perhaps we’ll have to wait until Dragonfly solves the mystery.
We will have to wait longer to test the other three models analyzed by the authors. The first one was Kepler-1649b. This planet is real, comparable to Venus, and they modeled it with sulfuric acid lakes and a thick carbon dioxide atmosphere, but mind you, we don’t know if there’s actually liquid on its surface, and we don’t entirely know what its atmosphere is made of. In this scenario, stronger winds (5.3 m/s) are required for the waves to form, but once they are underway, similar gravity forces them to reach Earth’s wave height.
Diagram from the paper showing the different forces used to calculate wave height. Credit – UG Schneck et al.
The second theoretical test case is LHS 1140-b, which was recently the subject of rigorous investigation by the James Webb Space Telescope. It has been modeled as a super-Earth water world, and current data from JWST suggests that this is indeed the case. Because it is more massive than Earth, the wind speed threshold for generating waves is higher (2.7 m/s) and the resulting waves are much shorter.
But that’s nothing compared to the last theoretical thing they modeled. 55 Cancri is a “lava world”, with lakes of molten rock that are naturally very viscous. Combining this with the planet’s massive gravity, the researchers found that it would take hurricane-force wind speeds of 37.1 m/s to even create ripples on the lava’s surface. In fact, JWST recently found evidence of a thin atmosphere of either carbon monoxide or carbon dioxide on Earth, so the existence of lava waves in 55 Cancri is a real possibility.
As it turns out, this research does more than just inform future science fiction film advisors. As we begin to study the atmospheres and surfaces of exoplanets in detail, the waves will begin to reflect what we expect. In other words, understanding the dynamics of otherworldly waves will be reflected in how astronomers study wave mechanics, and may one day help us definitively prove the existence of otherworldly oceans. It may be a long way off at this point, but having a model to compare is a great starting point.
learn more:
MIT/EurekaAlert – Waves strike differently on other planets
U.G. Schneck et al. – Modeling wind waves on other planets: Applications to Mars, Titan, and exoplanets
Utah – Titan Lake’s shoreline is shaped by waves of methane
UT – Surf’s Up on Titan! Cassini may have discovered Titan’s ocean waves
#Alien #wave #mechanics