“…In terrestrial groundwaters in basaltic rocks, the concentrations of Mn exhibit a wide range depending on Eh (Aiuppa et al, ); for instance, low‐Eh groundwaters (e.g., –0.01 V < Eh < 0.1 V) tend to contain higher Mn abundances (e.g., >100–1,000 ppb) due to the stability of Mn 2+ under reducing conditions (Aiuppa et al, ). Thus, if the Mn enrichments found on Mars were Mn 3 O 4 , Mn (OH) 2 , and amorphous Mn (III)/Mn (II), they would have caused coenrichment of Cr as well as Zn, which is inconsistent with the observations (Arvidson et al, ; Lanza et al, ). Based on the geochemical data from terrestrial groundwater in basaltic rocks (e.g., Aiuppa et al, ), we suggest that the chemical form of the Mn oxides found by the Mars rovers is most likely MnO 2 .…”