  We review the reservoirs of methane clathrates that may exist in the
different bodies of the Solar System. Methane was formed in the interstellar
medium prior to having been embedded in the protosolar nebula gas phase. This
molecule was subsequently trapped in clathrates that formed from crystalline
water ice during the cooling of the disk and incorporated in this form in the
building blocks of comets, icy bodies, and giant planets. Methane clathrates
may play an important role in the evolution of planetary atmospheres. On Earth,
the production of methane in clathrates is essentially biological, and these
compounds are mostly found in permafrost regions or in the sediments of
continental shelves. On Mars, methane would more likely derive from
hydrothermal reactions with olivine-rich material. If they do exist, martian
methane clathrates would be stable only at depth in the cryosphere and
sporadically release some methane into the atmosphere via mechanisms that
remain to be determined.
