A recent Discovery article highlighted one more culprit- comets. In a research paper, NASA's Marc Fries stated:
Individual instances of methane detection on Mars have been reported on five occasions via Earth-based telescopic observations, once by the ESA Mars Express mission over a two-month period, and eight times by the Mars Science Laboratory (MSL) rover. For all instances, Mars experienced a close interaction with the orbit of a periodic comet within 16 days previous to the observation of methane. Moreover, all methane observations correlate with interactions between Mars and seven comets. These comets have all been previously identified as the likely sources of meteor showers on Mars.
An interesting idea. And more scientific speculation about our solar system, similar to guesses about the source of the mysterious bright spots on Ceres (see below), that demonstrates how much we do not yet know about our own neighborhood.
