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A. P. Zent (NASA Ames), A. Ichimura (San Francisco State Uni.v), R. C. Quinn (SETI Institute)
We have examined the formation and stability of the superoxide radical O2-, which has been hypothesized as a potential Mars oxidant. Rutile (TiO2) was heated to ~ 400 degrees C under vacuum. The samples were tipped off in ampules under 8-9 torr O2, photolyzed with a Hg lamp for 30 minutes; EPR spectra were immediately obtained at 77K. The signature of O2- was clearly observed in the rutile. The sealed ampules were stored at room temperature for up to 2 weeks without any decrease in the number of spins. The same process, applied to rutile that was not baked out yielded superoxide signals that could not be detected once the photolyzing flux was cut off. To examine the effects of partial dehydration, we carried out the same series of experiments on rutile that was baked out at 200 degrees C. This material showed decay of superoxide spins to zero in less than 10 minutes. This qualitative pattern is also observed in experiments on anatase (Attwood, et al., , 2003). We hypothesize that O2- can be stabilized against reaction with H2O and OH by crystalline surface defects. On hydrated surfaces, O2- must compete for stabilizing sites, and the population is quickly extinguished; in dehydrated samples, it can migrate to stabilizing defects. Once sorbed, the O2- radical is stable in the presence of H2O. OMEGA Mars Express data (Poullet et al, 2005) suggest one to several percent adsorbed H2O across the Martian surface, which will significantly decrease O2- lifetime. One possibility for subsurface stabilization of O2- can be postulated based on EPR spectra of anatase, exposed to H2O2 in our lab in 1996, and which in 2005 shows the signature of O2-. Evidently, H2O2 can convert to stable O2- on some surfaces. This hypothesis might allow subsurface diffusion of H2O2, followed by conversion to O2-.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.