Hydrogen Escape in Current and Past Mars
Résumé
Mars is today a dry cold desert with a very thin atmosphere preventing the presence of liquid water in its surface. However, the geological record shows that abundant water ran on the Martian surface about 4 billion years ago, indicating the presence of a thicker and probably warmer atmosphere in the distant past. Understanding the fate of the ancient Martian atmosphere and water is a hot topic in planetary research. The thermal (Jeans) Hydrogen escape from Mars is one of the dominant factors shaping the long-term evolution of the planet's atmosphere and climate and is currently the main loss mechanism of water. Recent measurements by different space missions have shown that, contrary to the previously accepted paradigm, water vapour can penetrate into Mars' upper atmosphere, significantly enhancing the Hydrogen escape in short temporal scales. However, many unknowns remain, including the role of the different processes involved in transporting water from the lower to the upper atmosphere and in converting the water molecules into Hydrogen atoms, or the relative importance of the regular seasonal evolution of the escape compared to the sporadic effects of dust storms. A big unknown is the rate of Hydrogen escape in past Martian conditions when the orbital parameters, such as the obliquity or the orbital eccentricity, were different than today. Global Climate Models (GCMs) are sophisticated tools able to describe self-consistently an atmosphere and the important couplings between processes happening at different altitudes. The Mars Planetary Climate Model (M-PCM) is the only GCM incorporating all the processes known to affect the water in the current Martian atmosphere, from the release of water from the polar caps close to the surface to the escape of Hydrogen in the upper atmosphere, including the meridional and vertical transport of water. However, a previous study with the M-PCM showed that the Hydrogen escape rate predicted by the model was significantly lower than observed at
perihelion. We have implemented different upgrades in the M-PCM to improve its ability to simulate water escape. We will describe the improved model and show that it reproduces much better the existing observations. We will then tackle some of the open questions regarding atmospheric escape, such as its interannual variability or the effects of dust storms on the Hydrogen escape. We will also show results of simulations changing the orbital parameters of Mars in order to evaluate the impact of those changes in Hydrogen escape in past Mars conditions and in the accumulated rate of water escape through the history of the planet.