The Importance of the Ocean Size on the Climate and the Water Loss of Mars-size planets

The size of Mars past ocean may have played a relevant role on its climate. The climate of terrestrial planets is very sensitive to feedbacks produced by the water cycle. Water phase changes can affect the surface albedo and the temperature of the planet. I use a global climate model to explore the climate of Mars-size planets with different surface water fractions (SWF) and an Earth-like atmosphere. The results show that Mars-size planets are warmer than Earth-size analogs, due to their lower gravity and extended atmospheres. Only Mars-size planets with large SWFs reach the moist greenhouse threshold (MGT) and they reach the runaway greenhouse threshold (RGT) at shorter orbital distances than drier planets. At both the MGT and the RGT, the planets lose water gradually. Since these climatic states depend on the amount of surface water, I define the Water Loss Threshold (WLT) as the state at which the water mixing ratio is 1 g/kg at the photodissociation level, independently of the climatic state. A Mars-size aquaplanet reaches the WLT in a moist greenhouse state at a distance of about 1.07 AU in the solar system, while a Mars-size planet with a SWF of 1% reaches the WLT at 0.96 AU in a runaway greenhouse state. I calculate the water loss including the increase in the stellar radiation and the decrease in the ocean level, until the planets get totally dry. Planets with large SWFs remain habitable closer to their host star than drier planets. The results show how the extension of the seas, the surface albedo, and the planetary size play a crucial role in the habitability of terrestrial planets.