The Most Common Habitable Planets II -- Salty Oceans in Low Mass Habitable Planets and Global Climate Evolution

Abstract: Global climate evolution models for habitable earthlike planets do not consider the effect of ocean salinity on land ice formation through the hydrological cycle. We consider two categories of such planets: planets with deep oceans, but intrinsically high salinities due to the weaker salt removal process by hydrothermal vents; and planets with shallow oceans, where the increase in salt content and decrease in ocean area during the onset of glaciation cause a negative feedback, helping delay the spread of land ice. We developed a toy climate model of a habitable planet on the verge of an ice age, using a range of initial salt concentrations. Planets with deep oceans and high salinity show considerable increase in the time necessary to fill arctic land with ice sheets, up to 23% considering the maximum salinity range. For planets with shallow oceans, the effect of intrinsic high salinity is reinforced by the negative feedback, counteracting positive feedbacks like the ice-albedo and Croll-Milankovitch perturbations, to the point of effectively terminating land ice sheet growth rate during the simulated timescale. We also apply this model to the putative ocean of early Mars, finding intermediate results: salinity probably did not play a role in the evolution of Mars' climate, considering the timescale of its ice ages. We conclude that this phenomenon is essentially an abiotic self-regulation mechanism against ice ages and should be regarded in the context of habitable planets smaller and drier than the Earth, which may well represent the bulk of habitable planets.