Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations--particularly the suggested seasonal and latitudinal variations--and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet's surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

The 17O and 18O isotope fractionation associated with photolysis of O3 in the Chappuis band was determined using a broadband light source with cutoff filters at 455, 550, and 620 nm and narrowband light sources at 530, 617, and 660 nm. The isotope effects follow a mass‐dependent fractionation pattern (δ17O/δ18O = 0.53). Contrary to theoretical predictions, fractionations are negative for all wavelength ranges investigated and do not change signs at the absorption cross‐section maximum. Our measurements differ from theoretical calculations by as much as 34‰ in εO3+hν18 = (18J/16J − 1). The wavelength dependence is also weaker than predicted. Photo‐induced fractionation is strongest when using a low‐wavelength cutoff at 620 nm with εO3+hν18 = −26.9(±1.4)‰. With decreasing wavelength, fractionation values diminish to εO3+hν18 = −12.9(±1.3)‰ at 530 nm. Results from an atmospheric model demonstrate that visible light photolysis is the most important tropospheric sink of O3, which thus contributes about one sixth to the ozone enrichment.

Investigation of isotope effects in ozone (O 3 ) photolysis and its contribution to the overall ozone isotope composition is difficult since photolysis always leads to secondary O 3 formation and O 3 decomposition by reactions with O( 3 P). Here we use a large excess of carbon monoxide (CO) as O( 3 P) quencher to suppress O( 3 P) + O 3 . This allows disentangling the isotope effects in photolysis and chemical removal when the data are evaluated with a kinetic model. The largest systematic uncertainty arises from an unidentified O 3 removal reaction, which is responsible for an unaccounted 20% of the total removal rate. Assuming no isotope fractionation in this reaction, we find 18 ε O3þhν = ( 16 J/ 18 J À 1) = À16.1 (±1.4)‰ and 17 ε O3þhν = À8.05 (±0.7)‰ for O 3 photolysis and 18 ε OþO3 = ( 16 k/ 18 k À 1) = À11.9 (±1.4)‰ and 17 ε OþO3 = À5.95 (±0.7)‰ for chemical removal via O( 3 P) + O 3 . Allowing for isotope fractionation in the unidentified reaction results in lower fractionation values for photolysis and higher fractionations for chemical removal. Several fractionation scenarios are examined, which constrain the fractionation in photolysis to 18 ε O3þhν > À9.4‰ and 17 ε O3þhν > À4.7‰ and in the chemical removal to 18 ε OþO3 < À18.6‰ and 17 ε OþO3 < À9.3‰. Both fractionations are thus significant and of similar magnitude. Because our measurements are dominated by photolysis in the peak region of the Chappuis band, isotope fractionation of atmospheric O 3 by visible photons should also be in the same range. The isotope fractionation factor for O + O 3 directly bears on ozone chemistry in the lower thermosphere.