Recent spectroscopic detections of CH4 in the atmosphere of Mars are the
first definitive observations of an organic compound on that planet. The
relatively short photochemical lifetime of CH4 (~300 years) argues for a
geologically young source. We demonstrate here that low-temperature
alteration of basaltic crust by carbon-bearing hydrothermal fluid can
produce the required CH4 flux of 1 × 107 moles year-1, assuming conservative
values for crustal permeability and oxygen fugacity as implied by martian
basaltic meteorites. The crustal thermal disturbance due to a single dike ~
10 × 1 × 10 km during the past 104 years is capable of driving the
alteration, if all carbon is supplied by magmatic degassing from a dike with
only 50 ppm C. Atmospheric methane strongly suggests ongoing magmatism and
hydrothermal alteration on Mars.
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