Localizing putative methane sources on Mars from spacecraft observations and back-trajectory modeling techniques

A variety of measurements of methane in the martian atmosphere have been made over the past 15 years, showing inconsistencies in methane abundance, location and lifetime. These observations may be broken down into two categories, remote and in situ. Remote data consists of both Earth-based and orbital observations; these have been highly disparate in time, season and region. In situ data consists solely of observations made by the Mars Science Laboratory Tunable Laser Spectrometer. Together, these data have yielded wildly varying indications of methane in the martian atmosphere. Attempts have been made to apply numerical models such as general circulation models (GCMs) to identify source locations and timing of methane releases, but these remain inconclusive under the current approach of forward-trajectory plume modeling. Under this approach, passive 'tracers' are introduced into the GCM at specific locations and seasons, and are allowed to evolve with time, dispersing with the atmospheric circulation. Relationships are then sought between the evolved methane plume and observations. Results using this approach are qualitative at best, and have yet to provide convincing evidence of localized methane surface sources.

In the present work, we shall demonstrate the alternative approach of back-trajectory plume modeling for source localization. Back-trajectory modeling uses observations at known locations and times as the initial condition, and introduces tracers into the model at that location. By stepping backwards in time, and using GCM-modeled meteorological conditions as a guide, the trajectory of these tracers may be mapped back in time. We employ the Caltech Lagrangian chemistry transport model (CTM) and modeled 3D winds from the MarsWRF GCM in our study, which also includes atmospheric photochemistry. GCM simulations are optimized for the times of the considered methane measurements using data from the Mars Climate Sounder. At present, the major photochemical pathways in the CTM are inconsistent with the apparent short lifetime of Mars methane, but new heterogeneous chemical pathways, including interaction with surface olivine minerals may be easily included in the model. Results will be presented demonstrating the technique on the ground-based observations of Mumma et al., (2009).