  The origin of the martian methane is still poorly understood. A plausible
explanation is that methane could have been produced either by hydrothermal
alteration of basaltic crust or by serpentinization of ultramafic rocks
producing hydrogen and reducing crustal carbon into methane. Once formed,
methane storage on Mars is commonly associated with the presence of hidden
clathrate reservoirs. Here, we alternatively suggest that chabazite and
clinoptilolite, which belong to the family of zeolites, may form a plausible
storage reservoir of methane in the martian subsurface. Because of the
existence of many volcanic terrains, zeolites are expected to be widespread on
Mars and their Global Equivalent Layer may range up to more than $\sim$1 km,
according to the most optimistic estimates. If the martian methane present in
chabazite and clinoptilolite is directly sourced from an abiotic source in the
subsurface, the destabilization of a localized layer of a few millimeters per
year may be sufficient to explain the current observations. The sporadic
release of methane from these zeolites requires that they also remained
isolated from the atmosphere during its evolution. The methane release over the
ages could be due to several mechanisms such as impacts, seismic activity or
erosion. If the methane outgassing from excavated chabazite and/or
clinoptilolite prevails on Mars, then the presence of these zeolites around
Gale Crater could explain the variation of methane level observed by Mars
Science Laboratory.
