Bibliography - D. McGown
- Onstott, T. C., D. McGown, J. Kessler, B. S. Lollar, K. K. Lehman, and S. M. Clifford, 2006: Martian CH4: sources, flux and detection. Astrobiology, 6(2), doi:10.1089/ast.2006.6.377 377-395
[ Abstract ]Recent observations have detected trace amounts of CH4 heterogeneously distributed in the
martian atmosphere, which indicated a subsurface CH4 flux of ˜2 X 105 to 2 X 109 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, H2,
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 H2 generation by H2O
radiolysis. For a He flux of ˜105 cm-2 s-1 this model yields an abiogenic CH4 flux and a microbial
CH4 flux of ˜106 and ˜109 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/H2/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.
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