3d Modeling Of The Early Martian Climate And Water Cycle

Mathematics – Logic

Scientific paper

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Scientific paper

Missions to Mars regularly reveal new evidence suggesting that the early environmental conditions were very different from today, with liquid water flowing on the surface. Which climatic or geophysical processes enabled such conditions? Were the conditions episodically suitable for liquid water, or stable on long time-scales? Can we explain the distribution of the valley networks and other ancient landforms? To help understand these key issues, we have developed a new 3D global climate model (GCM). We wish to understand the possible climate that would occur on Mars if 1) the solar luminosity is decreased by 25%, as was the case 3.8 billion years ago, and 2) the surface pressure is increased up to several bars (no other greenhouse gases than CO2 and H2O are assumed to be present). We paid particular attention to the radiative transfer in dense CO2 atmospheres, where collision-induced absorption can be significant. We found that previous parameterisation of this phenomenon overestimated the greenhouse effect, and derived a new approach based on recent studies.
We analyse the effects of clouds and water vapour on the surface temperature and discuss the likely nature of the early hydrological cycle. CO2 ice clouds form in the middle atmosphere above 10 km. They cause significant surface warming through their scattering greenhouse effect. However, their effect is partly counterbalanced by the the albedo effect of the water ice clouds, which form much lower. Overall, it is difficult to achieve annual mean surface temperature significantly above 0°C anywhere on the planet for pressures below 2 bar. Nevertheless, temperatures above freezing can occur, especially in the lower plains, due to atmospheric adiabatic warming. On such a planet, the water cycle and precipitation strongly depend on the amount of water available at the surface, the location of the main surface reservoirs and the obliquity.

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