Gcm Simulations of Martian Methane

Introduction: The spectroscopic detection of methane in the atmosphere of Mars is the first observation of an organic compound on that planet [1-3]. CH4 has a lifetime of a few centuries, which suggests a young or current source. On the other hand, this lifetime is long enough for the atmospheric circulation to yield a uniform distribution of CH4 across the planet, which is not what is observed. Indeed an intriguing aspect of Martian methane is that it appears to vary with time and location. Earth-based and satellite observations report the presence of strong local enhancements, as well as a substantial seasonal evolution [4-6]. To understand better how such variations can occur and what are the implications on the nature of the source, we performed multiple general circulation model simulations including various scenarios of emission. Method: We used the LMD general circulation model which has been shown to represent the details of Martian climate [7], tracer transport [8] and photochemistry [9] with good accuracy. The methane chemistry has been implemented in our photochemical module. Loss of methane occurs by photolysis above 80 km, and by oxidation by OH and O(D) at lower altitudes. These constituents are produced respectively by the photolysis of H2O and O3. The fact that the LMD GCM reproduces well their observed climatology, subject to large seasonal and geographical variations, is an important advantage for a precise estimate of the fate of methane in the Martian atmosphere. Results: we first considered a simulation which does not include any active source of methane. Thus, the experiment reproduces a scenario which assumes that methane was released sometime in the Martian past, and is currently only subject to atmospheric transport and photochemical loss. The model is initialized with a uniform methane mixing ratio of 14 ppbv, corresponding to the yearly-averaged value measured by the Planetary Fourier Spectrometer (PFS) on board Mars-Express [6]. Figure 1 displays the results obtained at vernal equinox after a quasi-stationary state is reached. It can be seen that methane remains almost uniformly mixed up to about 70 km at low to mid latitudes. Above that altitude, its mixing ratio decreases rapidly as a consequence of fast loss by photolysis, whereas the meridional circulation brings low values down towards the poles. Results obtained at other seasons, as well as the enrichment/depletion of methane caused by CO2 condensation/sublimation [10], will be presented in the workshop.