Deciphering conflicting observations of methane on Mars

Recent Mars missions have reported ostensibly conflicting results for methane concentration in the atmosphere of Mars. The Curiosity rover revealed a baseline level of ~0.4 ppbv near the surface at Gale crater, whereas the ExoMars Trace Gas Orbiter constrained methane concentration above 5 km altitude to no more than ~0.02 ppbv. Given its ~300-year lifetime informed by standard photochemical models, methane should be uniformly mixed throughout the atmosphere, making it difficult to understand the large contrast between the near-surface and the high-altitude concentrations. Moreover, the Curiosity rover has detected diurnal and seasonal variabilities in the near-surface concentration and a number of transient, strong methane signals (“methane spikes”). What modulates the near-surface concentration remains obscure.

We hypothesize that a methane emission hot spot exists near the Curiosity rover and is episodically emitting methane, and this has produced the high baseline concentration at the site of Curiosity and the transient strong signals. Thus, near-surface methane concentration can be modulated by diurnal and seasonal variability of wind and the planetary boundary layer. The small emission flux permitted by the hypothesis can also explain the low methane abundance in the bulk atmosphere.

We use a detector-oriented, Lagrangian transport model to test this hypothesis. Particles representing methane parcels are released from the location of Curiosity (the “detector”) at the time of methane signal detection, transported backwards in time by reversed, simulated bulk wind, and dispersed by parameterized turbulence. Based on the computed back-trajectories, we quantify the influence of emission at any location and time on the detected methane signals, and back out the minimum emission rate at any possible emission site. Our findings show that a nearby emission site, most likely in northwestern Gale crater, is indeed able to cause the large difference between the methane concentrations near the surface and in the bulk atmosphere. Additionally, a nearby emission site can also explain the diurnal and seasonal variabilities of the near-surface methane concentration as well as the methane spikes. Sensitivity tests against a range of assumed methane lifetime demonstrate the robustness of our conclusions. For future missions, we design a strategy for deploying a sensor network that is aimed at constraining the location of the methane emission site down to a spatial scale of a few kilometers, which could enable future rovers to directly probe the methane emission site.