Mapping the methane on Mars

Context. In the past few years several authors have reported detection of a very small amount of methane in the martian atmosphere. Suggested hypotheses for this methane production can be linked either to geological or biological activities, and in both cases our perspective on the planet evolution would require a major revision. Aims. It has been suggested that methane is heterogeneously distributed in the atmosphere, but its temporal and spatial variability is still unknown. Our goal is to describe the distribution of the methane concentration in the martian atmosphere and its temporal evolution, providing insight into its sources and constraining its sinks. Methods. Using the extensive data set of the thermal emission spectrometer onboard Mars Global Surveyor and a statistical clustering technique, here we tracked the gas evolution over three martian years using the nu(4) Q-branch methane band at 1306 cm(-1). Results. We identify three localized sources, corresponding to Arabia Terrae, well known for the high content of underground water, and to the two main volcano provinces, Tharsis and Elysium. Our analysis suggests a seasonal cycle of the methane distribution, as well as interannual variations. This discovery potentially constrains the lifetime of the methane molecule in the martian atmosphere to <1 yr, suggesting that there is a process able to remove the methane from the atmosphere much more efficiently than photochemistry (similar to 350 yr). High concentrations of methane are found in the warmest seasons, when relatively high energy could trigger volatile release, which is related either to geological processes, or outbreaks of biological activity. Our results, obtained with independent data in a new spectral range, are quantitatively consistent with previous estimates of methane concentration, building confidence that there is methane on Mars.