Mass-independent Fractionation of Oxygen Isotopes in the Martian Atmosphere

Details Mass-independent Fractionation of Oxygen Isotopes in the Martian Atmosphere Mass-independent Fractionation of Oxygen Isotopes in the Martian Atmosphere

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.3109l&link_type=abstract

American Astronomical Society, DPS meeting #39, #31.09; Bulletin of the American Astronomical Society, Vol. 39, p.473

Astronomy and Astrophysics

Astronomy

Scientific paper

Several SNC meteorites exhibit 17O excesses in secondary minerals (carbonate and sulfate) and water. Relative to the SNC silicate fractionation line, measured Δ17O values range from +0.3 to +1.0 ‰ (1). Two possible sources of this MIF have been suggested: 1) non-thermal escape of O from the atmosphere2, and 2) mass-independent photochemistry in the atmosphere1. In either case exchange of atmospheric CO2 and H2O with near-surface ice reservoirs deposited the MIF signature into crustal fluids from which secondary minerals were precipitated. Here I evaluate the MIF possible from atmospheric chemistry.
The largest source of MIF in the martian atmosphere is the spin-forbidden photolysis of CO2 from 167 to 200 nm. Experiments3 at 185 nm have demonstrated a yield of nearly pure 17O with Δ17O 50 ‰, a result of a hyperfine interaction involving the non-zero nuclear spin of 17O. Assuming that similar MIF signatures apply at wavelengths > 185 nm, the MIF signatures of CO and O will be 50 ‰ at altitudes below 60 km. The principal CO2 formation reaction, CO + OH --> CO2 + H, also produces a small MIF of 2 ‰ at low pressure4, but is negligible compared to CO2 photolysis. Mass-dependent photolysis of H2O, and subsequent reaction of OH with O, yields O2 with Δ17O 20 ‰. MSL may be able to measure 33O2, but 33O2 has a relative abundance near the lower limit of the dynamic range ( 10-7) of the SAM GCMS. Ozone formation will also introduce MIF, but is not the dominant MIF source in the martian atmosphere. The implications of the oxygen isotope model results for the source of MIF in the SNC secondary minerals will be discussed.
1Farquhar and Thiemens 2000.
2Jakosky 1993.
3Bhattacharya et al. 2000.
4Rockmann et al. 1998.

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