some of the npOx. In addition, remote sensing observations of atmospheric dust (which shows slight compositional and grain size differences from surface dust), indicates that the bulk volume of dust grains consists of plagioclase feldspar and zeolite, along with minor pyroxene and olivine components. Such fine material can be generated easily via mechanical erosion from feldspar-rich basalts, such as rocks in the southern highlands on Mars. Collectively, these observations indicate that any chemical alteration of dust by aqueous activity has been very minor. The occurrence of nanophase ferric oxides (npOx) in dust There are several processes that can yield npOx as an oxidation product without the involvement of free oxygen (O2). One or more of those processes may have dominated on Mars, since atmospheric modeling over geologic time scales indicates that free O2 (generated mostly via the photodissociation of water (H2O)) may have always been a trace component with a partial pressure not exceeding 0.1 micropascal (μPa). One oxygen-(O2)-independent process involves a direct chemical reaction of ferrous iron (Fe2+) (commonly present in typical igneous minerals) or metallic iron (Fe) with water (H2O) to produce ferric iron (Fe3+(aq)), which typically leads to hydroxides such as goethite (FeO•OH) under experimental conditions. While this reaction with water (H2O) is thermodynamically disfavored, it may be sustained nevertheless, by the rapid loss of the molecular hydrogen (H2) byproduct. The reaction can be further facilitated by dissolved carbon dioxide (CO2) and sulfur dioxide (SO2), which lower the pH of brine films increasing the concentration of the more oxidative hydrogen ions (H+). However, higher temperatures (c. 300 °C) are usually needed to decompose Fe3+ (oxy)hydroxides such as goethite into hematite. The formation of palagonitic tephra on the upper slopes of the Mauna Kea volcano may mirror such processes, as consistent with the intriguing spectral and magnetic similarities between palagonitic tephra and Martian dust. In spite of the need for such kinetic conditions, prolonged arid and low pH conditions on Mars (such as diurnal brine films) may lead to the eventual transformation of goethite into hematite given the thermodynamic stability of the latter. Fe and Fe2+ may also be oxidized by the activity of hydrogen peroxide (H2O2). Even though the H2O2 abundance in the Martian atmosphere is very low, it is temporally persistent and a much stronger oxidant than H2O. H2O2-driven oxidation to Fe3+ (usually as hydrated minerals), has been observed experimentally. In addition, the pervasiveness of the