The origins of perchlorate in the Martian soil

Abstract

Abstract Perchlorate (ClO 4 − ) has been detected on Mars, but its production and distribution are unclear. Mechanisms requiring atmospheric chlorine are insufficient for measured concentrations. We conducted studies under Mars conditions using halite (NaCl) alone, soil simulants consisting of silica (SiO 2 ), Fe 2 O 3 , Al 2 O 3 , and TiO 2 . After 170 h irradiation, samples analyzed by ion chromatography (IC) showed ClO 4 − and ClO 3 − present in all samples. When SiO 2 was added, yield increased from 2 to 42 nmol and 0.4 to 2.6 nmol, respectively. We attribute this to SiO 2 and metal oxides acting as photocatalysts, generating O 2 − radicals from O 2 which react with chloride. Results show ClO 4 − and ClO 3 − can be produced photochemically on Cl minerals without atmospheric chlorine or aqueous conditions, and explain high concentration of ClO 4 − and ClO 4 − /Cl − ratios detected by Phoenix. They provide evidence that its distribution on Mars is dictated by distribution of chlorine and provide insight into the oxidizing nature of the soil and its potential effects on organics.