The oxidation-reduction potential of aqueous soil solutions at the Mars Phoenix landing site

Results from the Mars Phoenix mission Wet Chemistry Laboratory (WCL) are used to determine the oxidation-reduction potential (E(h)) of the Phoenix WCL Rosy Red sample soil solution. The measured E(h) of the Rosy Red sample in the WCL aqueous test solution was 253 +/- 6 mV at a pH of 7.7 +/- 0.1. Measured solution E(h) changes correspond to changes in solution H(+) activity, which is controlled mainly by changes in headspace P(CO2) and solution CO(3)(2-), HCO(3)(-), and CO(2) concentrations. If measured at a P(CO2) of 8 mbar in water, rather than in WCL test solution, the E(h) of the Rosy Red soil solution would be similar to 300 mV. The results of laboratory experiments using analog salt mixtures are compatible with the possible presence of low levels (ppm) of metal peroxides or other oxidants and indicate that levels of readily soluble ferrous iron in the soil are below 1 ppm. Citation: Quinn, R. C., J. D. Chittenden, S. P. Kounaves, and M. H. Hecht (2011), The oxidation-reduction potential of aqueous soil solutions at the Mars Phoenix landing site, Geophys. Res. Lett., 38, L14202, doi: 10.1029/2011GL047671.