On the Lifetime and Extent of Methane Plumes on Mars

The recent discovery of methane in the martian atmosphere hints at a planet not nearly as dormant as originally assumed. Being a reduced gas in an oxidizing atmosphere, its presence requires an active, or at least recent, emission source, as the photochemical lifetime of methane is only ~350 years. Recent spectroscopic observations have positively identified methane emissions in the 3.3 μm band, having both a spatial and temporal variability, which suggests both local source regions and seasonal variations in source strength. Peak methane plume source strength is estimated to be ≥0.6 kg/s, producing a mixing ratio of ~45 ppb. Interannual variations in plume strength show the rapid loss of this methane signal, on timescales of months to years, which is in stark contrast to the previously assumed photochemical decay rate for methane in the martian atmosphere. We apply the Mars Weather Research and Forecasting (MarsWRF) general circulation model to the question of the origin of this methane in the martian atmosphere. Due to the limited capabilities of ground-based observations in general, we use this general circulation model to simulate the development of passive atmospheric plumes over several time periods and for several different species lifetimes and initial conditions. Methane is treated as a passive tracer, which does not interact with other atmospheric species. The MarsWRF model allows for either a prescribed mass injection or surface flux of methane, so we are able to replicate the strength of observed plumes well. The MarsWRF plume physics are also more realistic than simpler diffusion models, and show plume evolution substantially different than prior efforts. We find that the observed signals do not necessarily require an extremely short lifetime for methane as previously suggested, and that the source of the observed signals must have been either active at the time of the observations, or very recently extinguished. Furthermore, the observed signals are more consistent with regional emission than a tightly localized plume. These results shed further insight into the possible size and origin of the sources that may be responsible for the observed methane signals.