A review of the results of the Viking Biology experiments suggests that heterogeneous chemistry between the regolith and photochemically-produced oxidants best explains the data. Laboratory and numerical studies suggested that atmospherically-derived oxidants would neither survive long, nor diffuse deeply, into the martian regolith. Even including mechanical mixing, the total depth of the superoxidizing zone is likely to be no more than a few meters. Review of additional literature suggests that some of the species responsible for the Viking experiments may also have formed in situ, directly on the regolith material. These complexes form rapidly and abundantly when stimulated with UV photons, but significantly they can apparently form in lower abundances without UV stimulation from species known to be present in the martian atmosphere. This may help explain the small amounts of oxidant seen in the subsurface sample acquired by Viking. Critical laboratory data must now be gathered on the surface diffusion of chemisorbed O_2~- radicals in multimineralic fines, in order to assess the potential mobility of this strongly oxidizing species at depth. In addition, adsorbents above and beyond TiO_2 must be examined for their interactions with a Mars-like chemical environment. If the mobility of the chemisorbed oxidants is low enough, relict organics could persist in regolith materials that would appear superoxidizing in Viking-like tests.
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