Martian CH4:: Sources, flux, and detection

Recent observations have detected trace amounts of CH4 heterogeneously distributed in the martian atmosphere, which indicated a subsurface CH4 flux of similar to 2 x 10(5) to 2 X 10(9) cm(-2) s(-1). Four different origins for this CH4 were considered: (1) volcanogenic; (2) sublimation of hydrate-rich ice; (3) diffusive transport through hydrate-saturated cryosphere; and (4) microbial CH4 generation above the cryosphere. A diffusive flux model of the martian crust for He, H-2, and CH4 was developed based upon measurements of deep fracture water samples from South Africa. This model distinguishes between abiogenic and microbial CH4 sources based upon their isotopic composition, and couples microbial CH4 production to H-2 generation by H2O radiolysis. For a He flux of -10(5) cm(-2) s(-1) this model yields an abiogenic CH4 flux and a microbial CH4 flux of similar to 10(6) and similar to 10(9) cm(-2) s(-1), respectively. This flux will only reach the martian surface if CH4 hydrate is saturated in the cryosphere; otherwise it will be captured within the cryosphere. The sublimation of a hydrate-rich cryosphere could generate the observed CH4 flux, whereas microbial CH4 production in a hypersaline environment above the hydrate stability zone only seems capable of supplying -105 cm(-2) s(-1) of CH4. The model predicts that He/H-2/CH4/C2H6 abundances and the C and H isotopic values of CH4 and the C isotopic composition of C2H6 could reveal the different sources. Cavity ring-down spectrometers represent the instrument type that would be most capable of performing the C and H measurements of CH4 on near future rover missions and pinpointing the cause and source of the CH4 emissions.