Water versus land on temperate rocky planets

Abstract

Water and land surfaces on a planet interact with gases in the atmosphere and with radiation from the star. These interactions define the environments that prevail on the planet, some of which may be more amenable to prebiotic chemistry, some to the evolution of more complex life. This review article covers (i) the physical conditions that determine the ratio of land to sea on a rocky planet, (ii) how this ratio would affect climatic and biologic processes, and (iii) whether future astronomical observations might constrain this ratio on exoplanets. Water can be delivered in multiple ways to a growing rocky planet -- and although we may not agree on the contribution of different mechanism(s) to Earth's bulk water, hydrated building blocks and nebular ingassing could at least in principle supply several oceans' worth. The water that planets sequester over eons in their solid deep mantles is limited by the water concentration at water saturation of nominally anhydrous mantle minerals, likely less than 2000 ppm of the planet mass. Water is cycled between mantle and surface through outgassing and ingassing mechanisms that, while tightly linked to tectonics, do not necessarily require plate tectonics in every case. The actual water/land ratio at a given time emerges from the balance between the volume of surface water on the one hand, and on the other hand, the shape of the planet (its ocean basin volume) that is carved out by dynamic topography, the petrologic evolution of continents, impact cratering, and other surface-sculpting processes. By leveraging the contrast in reflectance properties of water and land surfaces, spatially resolved 2D maps of Earth-as-an-exoplanet have been retrieved from models using real Earth observations, demonstrating that water/land ratios of rocky exoplanets may be determined from data delivered by large-aperture, high-contrast imaging telescopes in the future.

Figures (7)

• Figure 1: Composite image of the Earth's surface compiled from NASA’s Terra satellite MODIS images, with clouds removed. Credit: NASA Goddard Space Flight Center Image by Reto Stöckli (land surface, shallow water, clouds). Enhancements by Robert Simmon (ocean color, compositing, 3D globes, animation). Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean Group, Public Domain, https://commons.wikimedia.org/w/index.php?curid=50497070
• Figure 2: The dynamically-supported component of Earth's topography as calculated by straume_impact_2024, based on the SMEAN2 seismic tomography model. Data are available at https://doi.org/10.5281/zenodo.8262689. Note topography supported by compositional isostasy (e.g., continents), which is excluded from this map, makes up an additional component to the total topography.
• Figure 3: (a): Models of the growth of the continental crustal volume, normalised to the present-day continental crust volume, compared with the present-day cumulative surface age distribution of goodwin_precambrian_1996. Figure adapted from cawood_secular_2022. Note that a few more recent curves rosas_rapid_2018guo2020 follow armstrong_radiogenic_1981. (b): Continental crust growth from two models of honing_land_2023 (pink lines), compared with the same models' cumulative age distributions at the present day (black lines). Dash-dotted lines show models where 2/3 of the crustal production rate is proportional to the mantle flow velocity and 1/3 is proportional to the continent surface area; dotted lines show the reverse. All models consider thermal blanketing by continents and partitioning of heat-producing radiogenic elements into the crust. The cumulative surface age distributions are similar despite the substantially different growth curves, due to continuous production and recycling of the crust korenaga2021land.
• Figure 4: Elements of the deep water cycle on rocky planets that are discussed in this chapter. Reservoirs are labelled in sans serif font. The dominant form of 'water' in certain reservoirs is noted in serif font. The major fluxes potentially possible on rocky planets in general are marked in red arrows, whereas the fluxes operating only under plate tectonics are marked in yellow arrows. The profile in the bottom left corner indicates the relative water concentration at water saturation in nominally anhydrous minerals for an Earth-like mantle composition. Note geometry is not to scale.
• Figure 5: Phase plane spanned by the surface ocean mass (expressed as the difference from one Earth ocean mass) and the coverage of the Earth's surface by continental crust, calculated by honing_continental_2016 for present-day mantle temperatures. Local arrows are shown pointing in the direction of time evolution. Also shown are lines along which the rates of change of continental coverage (red) and ocean mass (blue) are zero. These lines intersect at three points (triangle markers) where both rates of change are zero, indicating equilibrium states. At the top fixed point (brown triangle; 'land planet'), the planet is at a stable equilibrium, mostly covered by continents; net $\sim$0.1 ocean masses have been ingassed to the mantle. At the lower fixed point (blue triangle; 'ocean planet'), the planet is at another stable equilibrium, almost completely covered by oceans; net $\sim$0.4 surface ocean masses have been gained from mantle degassing. The middle fixed point (grey triangle; 'Earth') is a saddle point, stable with respect to ocean mass but unstable with respect to continental coverage. The black circular markers indicate the endpoints of the 4.5 Ga evolution calculations, which do not necessarily reach equilibrium states. About 80% of the endpoints cluster around the 'land planet' fixed point; the lower fixed point is never reached because the mantle becomes too dry, cold, and viscous. Only a few percent of models end close to the 'Earth' saddle point. The models are initialised with randomly chosen temperatures between 1800 and 2100 K and with an inventory of one ocean mass in the mantle.