The importance of continents for life

Water is a vital component of life on Earth. It is therefore not surprising that, in the search for habitable exoplanets, one of the first parameters of habitability sought is that of the presence of a sufficient amount of liquid water. But it's not just how much water counts, but how it's distributed on the planet. This essentially leads us to look for a right ratio of surface emerged (called mainland) and submerged surface (like oceans). In the case of our planet the ratio is 29% for the mainland and 71% for the oceans the question to be asked is: what is the optimal ratio?

The article by Dennis Höning and Tilman Spohn, in a review of Astrobiology, entitled "Land Fraction Diversity on Earth-like Planets and Implications for their Habitability" tries to clarify the issue at a theoretical level. The article highlights how the continental part is richer in nutrients and therefore in biomass than the deep ocean. This is due to an effect of atmospheric events that easily carry nutrients along the surface. The second is determined by tectonical motions. These movements, first of all if consistent, keep the relationship between the oceans and the continents "unchanged", but these movements of the earth’s crust give several advantages. The first is heat: The heat produced by the radioactive materials in the mantle is retained by the environmental component. This is mediated because, as these materials are brought to the surface by tectonic movements, they "exhaust" their radioactive charge. This forms a unique heat cycle in the continental land. This heat flow is also helped by the water component in the mantle that changes the viscosity.

So a surface too unbalanced towards the oceans would lack the nutrients, derived from the atmospheric agents, necessary for the growth of biomass, on the other hand, too many continents would make the planet too dry and desert, thus reducing the total biomass. The paper, unfortunately, has no quantitative result but aims to better understand the parameters that allow life to search for it in exoplanets. 

Sources: Universe today; Land Fraction Diversity on Earth-like Planets and Implications for their Habitability