AAS Nova

Water World or Land Planet: What Determines Ocean Coverage on Rocky Exoplanets?

By on Features
Share:
artist's impression of an earth-like planet orbiting a star
This illustration depicts Kepler-186f, the first confirmed Earth-size planet orbiting in the habitable zone of its host star. How can we predict whether planets like Kepler-186f have continents or a global ocean? [NASA Ames/JPL-Caltech/T. Pyle]

What determines whether a planet is dotted with continents or has oceans as far as the eye can see? Astronomers turn to models to understand how bulk planetary properties affect surface oceans on rocky, water-rich exoplanets.

Land Ho?

If a planet’s surface is perfectly smooth, water will wash across it to form a global ocean of constant depth. If instead the surface is roughened with mountains and valleys, the planet can host more water before its surface is fully covered. But what factors control whether a planet’s surface is flat and featureless or textured with topography, and thus how much water it can hold before becoming an ocean world?

cartoon of dynamic topography

A schematic showing the generation of dynamic topography [Adapted from Guimond et al. 2022]

A new article led by Claire Marie Guimond (University of Cambridge, UK) explores how the distribution of dry land and oceans on exoplanetary surfaces might be impacted by dynamic topography. Dynamic topography arises when a planet’s core is hotter than its surface, causing the material in between — the mantle — to undergo a slow churning on million-year timescales. This convection applies pressure to the planet’s crust, inducing upwellings and depressions up to a kilometer in size. Unlike plate tectonics, which is largely responsible for the diversity of topography on Earth but doesn’t occur on all planets, dynamic topography is expected to be a nearly universal feature of rocky planets.

plots of topography in meters as a function of planet age, mass, core mass fraction, and uranium and thorium abundances relative to solar

Plots of the change in a planet’s root-mean-square topography as a function of four potentially observable quantities. The quantities held fixed in each set of simulations are listed in the upper right corner of each panel. Click to enlarge. [Adapted from Guimond et al. 2022]

Modeling Mantle Convection

Guimond and collaborators first used complex two-dimensional numerical models to understand how a planet’s dynamic topography scales with factors like the thickness and composition of the mantle. These trends formed the backbone of less computationally expensive models, with which the team explored how a planet’s mass, age, radioactive element abundance, and core mass impact the size of its surface features.

The team found that the typical height of surface features increases with increasing planetary age and core mass and decreases with increasing planetary mass and radioactive element abundance. Planetary mass and radioactive element abundance had the strongest effect, suggesting that these factors are key to predicting a planet’s topography. The modeled surface features tended to be in the hundred-meter range, though the most massive planets had subtle hills and valleys of only a few tens of meters.

Weighty Water Worlds and Low-Mass Land Planets

plot of basin capacity, planet mass, and surface water fraction

Basin capacity as a function of planet mass. If a given surface water mass fraction exceeds the basin capacity, the planet will have a global ocean (labeled “water planets”). If the water mass fraction is less than the basin capacity, the planet will have dry land (labeled “land planets”). [Adapted from Guimond et al. 2022]

How much water can a planet hold before all its surface features are submerged beneath a global ocean? This quantity depends on the maximum height of the planet’s surface features, since a planet with an Everest-sized mountain will require more water to be fully covered than one with small rolling hills. Intriguingly, the authors found that the most massive planets can support the least water before turning into water worlds since they have the shallowest surface features.

Aside from dynamic topography, planets are likely to have many other surface processes at play, like impacts from asteroids and comets, volcanism, and plate tectonics, all of which might boost the amount of high-and-dry land available on a planet. The authors hope that further modeling as well as observations of exoplanets and Earth itself can increase our ability to predict the surface conditions of distant planets.

Citation

“Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?,” Claire Marie Guimond et al 2022 Planet. Sci. J. 3 66. doi:10.3847/PSJ/ac562e