Land/Ocean Surface Diversity on Earth-like (Exo)planets: Implications for Habitability

A balanced ratio of oceans to land is thought to be essential for the evolution of an Earth-like biosphere. Emerged continents provide direct access to solar energy while large oceans enhance rainfall and prevent an all-to-dry climate. When assessing the habitability of Earth-like planets, one may be tempted to assume similar geological properties. After all, considering e.g., the volume of the continental crust, the latter is determined by an equilibrium between continental production, by subduction-zone related volcanism and continental erosion. Assuming the interior thermal state of Earth-sized exoplanets to be similar to the Earth’s is a straightforward and reasonable assumption if a similar composition can be assumed and if the temperature- and volatile-dependence of mantle viscosity governs the heat transfer in their mantles as it does in Earth’s. In that case, one might expect a similar equilibrium between continental production and erosion to establish and, hence, a similar continental land fraction. We will show that this conjecture is not likely to be true and that the present-day Earth may rather be an exceptional planet: Positive feedback associated with the coupled mantle water - continental crust cycle enhanced by the role of sediments may lead to a bifurcation of possible outcomes of the evolution. One of these is a land planet with about 80% of its surface covered with continental crust, or about 70% land surface if continental shelves covered with water are accounted for. The other extreme is a planet covered by about 20% with continents or a land fraction of only about 10%, again accounting for shelve areas. Both equilibrium planets minimize their lengths of subduction zones in equilibrium. Of the two, the land planet has a substantially larger zone of attraction in the space of reasonable initial conditions. About 80% of randomly chosen sets of initial conditions evolve to end there. The ocean planet attracts about 20% of the cases. Only around a percent of the evolution models result in an Earth-like configuration for which the continental coverage is about 40% but for which the length of the subduction zones is maximized, suggesting that the equilibrium is unstable. We find this equilibrium fixed point to be a saddle point, stable with respect to mantle water but unstable with respect to continental coverage. Still, because the rates of change are small after some billion years of evolution, the unstable equilibrium – if attained - can be occupied for a long time.  It is interesting to note that the bifurcation develops only after the planet has cooled to an interior temperature within some tens of K from the Earth’s present mantle temperature. This suggests that the dependence of the viscosity on the water concentration in the mantle becomes competitive with the dependence on temperature and the sediments as carriers of water in subduction zones become increasingly important for the bifurcation to occur. On Earth, this occurs roughly near the end of the Archean, about 2 billion years ago. We further studied the role of thermal blanketing by continents enriched in radioactive elements and included the effect of transfer of these elements from the mantle. We found that blanketing may enhance the positive feedback leading to the bifurcation although most of the blanketing effect is compensated by the effective cooling of the mantle depletion in radiogenic elements.

Including CO₂ outgassing in the model and the long-term carbonate-silicate cycle, we found that the land planet and the ocean planet differed by only about 5K in average surface temperature. Still, we would expect that the land planet has a substantially dryer, colder and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. These planets would all be considered habitable but their fauna and flora may be quite different. The Earth in its geologic history has experienced climates that could resemble the one expected for the land planet (e.g., the Pleistocene) and for the ocean planet (e.g., the Paleocene).