Habitable Zone of Land Planets; 1-D EBM with t Transportation of Surface Liquid Water

Abe et al. (2011) investigated climates on planets with a very small amount of water on their surface (called ‘land planets’). The notable feature of land planets is local balance between precipitation and evaporation. They, using GCM (General Circulation Model), have shown that liquid water is localized on high latitudes, while low latitudes are dried up. As a result, the value for incoming radiation by which liquid water on a land planet is completely evaporated (called the ‘complete evaporation limit’) corresponds to 170% of the incident solar flux that the present earth receives, which is substantially larger than that of a planet that is globally covered with ocean (122%, Nakajima et al. (1992)).

The key for land planets to have large value for complete evaporation limit is the localization of liquid water on high latitudes. Their GCM, however, does not include transportation of liquid water on surface, which may disturb water localization.

In order to investigate the influence of transportation of surface water on the complete evaporation limit for land planets, we develop the 1-D EBM (Energy Balance Model) including transportation of surface liquid water, which is given in forms of diffusion. As a result, we find that the complete evaporation limit is 122% of the incident solar flux when liquid water on planetary surface is transported very efficiently (one tenth as efficient as water vapor transportation in the atmosphere). This value is the same one as the complete evaporation limit for planets globally covered with ocean. On the other hand, when liquid water on the surface is not transported so efficiently (one five hundreds as efficient as water vapor transportation), water localization takes place and the complete evaporation limit of our model is about 140-150% of the incident solar flux, depending on the water content of the planet.