4:15 PM - 4:30 PM
[MIS14-16] Theoretic study of atmosphere and hydrological cycle with groundwater transportation on land planets
Keywords:land planets, atmosphere, groundwater
The presence of liquid water is essential for planetary habitability. Knowledge of a variety of hydrological cycles and climate systems developed on terrestrial planets is important to understand a diversity of habitable planets. According to the previous theoretical studies using general circulation models (GCM) (Abe et al., 2011; Kodama et al., 2018), there are two major planetary climate systems depending on the amount of water on the surface. One is an aqua planet with wet climates and global oceans, and the other is a land planet with arid climates and polar ice caps. Although the previous studies showed that water distributed predominantly in polar regions as ice caps on a land planet, groundwater circulation could transport liquid water formed at the basement of ice caps to middle to low latitudes. Global-scale groundwater circulations would have occurred in reality on land planets in the Solar System, i.e., early Mars and Titan (e. g., Carr et al., 2003; Faulk et al., 2020). Here, we numerically investigate the roles of groundwater circulations in a diversity of planetary climate systems and hydrological cycles on land planets.
In the numerical simulations, we used two models: One is the general circulation model (GCM) of agcm5.4g (Kodama et al., 2018), which calculates atmospheric dynamics, precipitations, and radiation processes in three dimensions, and the other is the commercially-available groundwater circulation simulator of GETFLOWS (Tosaka et al., 2000), which calculates groundwater transport in three dimensions. To achieve a consistency between the models, we performed calculations in the following 3 steps. In Step 0, spatial distribution of ice caps and precipitation/evaporation on land planets are calculated for given atmospheric CO2 levels and orbital setting using the GCM. We obtain snow-line latitudes, until which ice caps can exist. In Step 1, we calculated the transport dynamics and distribution of groundwater generated at the basement of the ice caps using the groundwater circulation model. In the calculations, we find that groundwater beneath the ice caps is transported to the middle and low latitudes as the permeability of crustal rocks increases. In particular, if the permeability is higher than 105~106 md, which corresponds to the permeability of gravels and sands on Earth, efficient groundwater transport extends water-existing areas to low latitudes. In Step 2, we investigated the atmospheric circulations and climatic conditions using the GCM for the surface water distributions derived from the groundwater circulation model. We find that when the permeability is high (higher than 105~106 md), the water-existing areas reach the edge of Hadley Cell at ~30 degrees in latitude. This leads to wetting of air in low latitudes due to water vapor transport by the Hadley circulation, and the climate state shifts from an extremely dry land planet to a wet aqua planet.
Our results of a combination of the numerical simulations show that planetary climate systems are largely affected by surface and shallow-subsurface geological conditions (i.e., permeability of rocks), which have not been considered previously. In particular, the present study suggests the possible existence of a planetary climate state, which has the characteristics of a land planet in terms of surface water distribution (ice caps at high latitudes) and the characteristics of an aqua planet in terms of climate (wet low latitudes). Given the fact that air in low latitudes becomes wet for high permeabilities, we suggest that land planets do not necessary have a wide habitable zone, depending on geological conditions of the surface and shallow subsurface.
In the numerical simulations, we used two models: One is the general circulation model (GCM) of agcm5.4g (Kodama et al., 2018), which calculates atmospheric dynamics, precipitations, and radiation processes in three dimensions, and the other is the commercially-available groundwater circulation simulator of GETFLOWS (Tosaka et al., 2000), which calculates groundwater transport in three dimensions. To achieve a consistency between the models, we performed calculations in the following 3 steps. In Step 0, spatial distribution of ice caps and precipitation/evaporation on land planets are calculated for given atmospheric CO2 levels and orbital setting using the GCM. We obtain snow-line latitudes, until which ice caps can exist. In Step 1, we calculated the transport dynamics and distribution of groundwater generated at the basement of the ice caps using the groundwater circulation model. In the calculations, we find that groundwater beneath the ice caps is transported to the middle and low latitudes as the permeability of crustal rocks increases. In particular, if the permeability is higher than 105~106 md, which corresponds to the permeability of gravels and sands on Earth, efficient groundwater transport extends water-existing areas to low latitudes. In Step 2, we investigated the atmospheric circulations and climatic conditions using the GCM for the surface water distributions derived from the groundwater circulation model. We find that when the permeability is high (higher than 105~106 md), the water-existing areas reach the edge of Hadley Cell at ~30 degrees in latitude. This leads to wetting of air in low latitudes due to water vapor transport by the Hadley circulation, and the climate state shifts from an extremely dry land planet to a wet aqua planet.
Our results of a combination of the numerical simulations show that planetary climate systems are largely affected by surface and shallow-subsurface geological conditions (i.e., permeability of rocks), which have not been considered previously. In particular, the present study suggests the possible existence of a planetary climate state, which has the characteristics of a land planet in terms of surface water distribution (ice caps at high latitudes) and the characteristics of an aqua planet in terms of climate (wet low latitudes). Given the fact that air in low latitudes becomes wet for high permeabilities, we suggest that land planets do not necessary have a wide habitable zone, depending on geological conditions of the surface and shallow subsurface.