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A. S. Yen, B. C. Murray (Caltech)
The origin of the maghemite inferred to be present in the martian soil by the Pathfinder and Viking magnetic properties experiments remains uncertain. Recent experimental results show that metallic iron can be oxidized into maghemite and hematite under conditions similar to the current martian environment. These data suggest that non-aqueous weathering of meteoritic iron could be the source of the pigmenting oxides in the soils on Mars.
The discovery of 1% to 7% of a magnetic mineral, believed to be maghemite, by the Viking Landers [1] was confirmed by Pathfinder [2]. The conventional explanation for the origin of the ferric oxides on Mars is based on the dissolution and subsequent oxidation of ferrous iron from silicate minerals in aqueous environments [e.g., 3]. We present the idea that oxides of exogenic, rather than locally derived, iron could be responsible for the Mars surface spectra. This concept is not new [4], but here we introduce a pathway for oxidation without liquid water. New experiments which monitor the conductivity of electron-beam evaporated Fe show that metallic iron can be efficiently converted to oxide in the presence of ultraviolet radiation and oxygen. Analyses of the resulting oxide phases by FTIR techniques show both maghemite and hematite. A test of the meteoritic origin of at least some of the iron at the martian surface can be conducted with a measurement of the Ni content of the soil. Assuming an average of 7% Ni in iron meteorites and assuming roughly a quarter of the Fe at the martian surface is exogenic, 0.1% to 0.5% Ni should be present in the surface soil.
[1] Hargraves et al., JGR, 82, 4547-4558, 1977. [2] Hviid et al., Science, 278, 1768-1770, 1997. [3] Burns, Geochimica et Cosmochem. Acta, 57, 4555-4574, 1993. [4] Gibson, Icarus, 13, 96-99, 1970.