The debate of life on Mars centers around the source of the globular, micrometer-sized mineral carbonates in the ALH84001 meteorite; consequently, the identification of Martian processes that form carbonates is critical. This paper reports a previously undescribed carbonate formation process that occurs on Earth and, likely, on Mars. We identified micrometer-sized carbonates in terrestrial aerosols that possess excess 17 O (0.4-3.9‰). The unique O-isotopic composition mechanistically describes the atmospheric heterogeneous chemical reaction on aerosol surfaces. Concomitant laboratory experiments define the transfer of ozone isotopic anomaly to carbonates via hydrogen peroxide formation when O 3 reacts with surface adsorbed water. This previously unidentified chemical reaction scenario provides an explanation for production of the isotopically anomalous carbonates found in the SNC (shergottites, nakhlaites, chassignites) Martian meteorites and terrestrial atmospheric carbonates. The anomalous hydrogen peroxide formed on the aerosol surfaces may transfer its O-isotopic signature to the water reservoir, thus producing mass independently fractionated secondary mineral evaporites. The formation of peroxide via heterogeneous chemistry on aerosol surfaces also reveals a previously undescribed oxidative process of utility in understanding ozone and oxygen chemistry, both on Mars and Earth.nanoparticles | mineral dust | heterogeneous chemical transformation | surface chemistry | mass-independent fractionation T he search for life beyond Earth is pursued based upon the requirement of liquid water both as a solvent and transport medium, and its biochemically unique role in providing support to cell structure (1). Central to the question of potential life on Mars is whether liquid water ever existed on the surface of Mars and, if so, under what climatic conditions (2). Apart from satellite observations of valleys and channels formed by aqueous activity, there are few quantitative measures (e.g., Thermal and Electrical Conductivity probe on Phoenix Lander; Compact Reconnaissance Imaging Spectrometer for Mars; High Energy Neutron Detector on board Mars Odyssey spacecraft) of significant amounts of liquid water at the surface of Mars today (3-5). Secondary minerals such as carbonates and sulfates record physicalchemical settings of the environment in which they are formed and geochemical relations between the atmosphere and the Martian surface (6). Unlike terrestrial carbonate sediments, the Martian surface lacks large amounts of carbonates despite a CO 2 -rich (95% vol∕vol) atmosphere, though, there is evidence of Mg-Fe rich carbonates (16-34 wt %) in the Columbia Hills and <5% CaCO 3 in Martian dust and soils (3,(7)(8)(9). In contrast to most Martian meteorites, ALH84001 contained substantial amounts of secondary carbonate minerals with an average age of 3.90 AE 0.04 billion years, contemporaneous with a wet period in Martian history (10). The formation of well-defined globular structures of a definite size range and their a...

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