Simulations of the martian hydrologic cycle with a general circulation model: Implications for the ancient martian climate

We use a general circulation model (Urata, R.A., Toon, O.B. [2013]. Icarus, submitted for publication) to simulate the martian hydrologic cycle, including the radiative effects of water-ice clouds. We find that the current observed hydrologic cycle can be duplicated by tuning the polar cap albedo. The hydrologic cycle is very sensitive to the size and albedo of the North Polar water-ice cap. The radiative effects of ice clouds on atmospheric temperatures can be significant. Simulations of an ancient climate were performed with a 500 mb CO2 atmosphere and a reduced solar constant. The results show that the climate is highly sensitive to the hydrologic cycle, and can range from cold and dry, to warm and wet depending on initial conditions, cloud particle size, precipitation rates, and cloud cover fraction. A warm climate is obtained by assuming cloud ice particles greater than or equal to 10 μm, and by reducing the efficiency of precipitation to maximize the cloud optical thicknesses. The warm, wet climates have precipitation rates that are 10% of the present day Earth. While carbon dioxide plays only a minor role in creating warm temperatures, it is necessary to have more than 250 mb of carbon dioxide in order to obtain these high temperatures for the conditions assumed in our simulations due to the need for heat transport to the poles.