Localizing Putative Methane Sources on Mars from Back-Trajectory
Modeling Techniques
Abstract
A variety of measurements of methane in the Martian atmosphere have been
made over the past 15 years, showing wildly varying indications of
methane abundance, location and lifetime in the Martian atmosphere.
Attempts have been made to use numerical tools 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. Here we present results
using a novel, complementary method of localizing methane surface
sources by modeling passive tracer trajectories backwards in time from
the locations where observations of atmospheric methane have been made.
Such back-trajectory modeling employs both GCM modeled winds and a
Lagrangian particle dispersion model to isolate potential upwind sources
of the observed signals. This approach avoids many of the pitfalls
inherent in forward-trajectory modeling approaches such as numerical
diffusion and subgrid-scale motion which cannot be captured in the
Eulerian framework of a GCM. We have chosen to focus on localization of
the detection of methane by the Planetary Fourier Spectrometer near Gale
crater around Ls=336° in MY 31. This observation is consistent with a
near-coincident enhanced methane ‘spike’ observed by the Mars Science
Laboratory TLS instrument. We have chosen to use the Stochastic
Time-Inverted Lagrangian Transport (STILT) particle dispersion model in
conjunction with the Mars Weather Research and Forecasting (MarsWRF) GCM
for our back-trajectory modeling. To date, we have combined MarsWRF
output with a more basic trajectory model, which advects particles based
on bulk winds, and have found areas of enhanced tracer density to the
north of Gale crater at prior times. Incorporation of turbulent
processes in the planetary boundary layer will subject these preliminary
results into test. And geological context will also be used to constrain
the likelihood of these methane source locations.