Possibility of H2O2 decomposition in thin liquid films on Mars

H2O2 is an important and agressive oxidant on Mars. Today liquid interfacial water on Mars is ephemerally present between the water ice and the mineral surfaces, where slow chemical changes might happen, including H2O2 decompositon. The most effective way for this is catalyzed by Fe ions, which could decompose it under Ph<4.5 with half life of 1–2 days. This process might be important during volcanically influenced periods when sulphur release produces acidic pH. Under current conditions, using the value of 200 K as the temperature in interfacial water (at the souther hemisphere), and applying Phoenix lander’s wet chemistry laboratory results, the pH is not favourable for Fe mobility and this kind of decomposition. Despite these conditions, microscopic scale interfacial liquid water still might support the process by the reaction called heterogeneous catalysis, without acidic pH and mobile Fe, but with minerals surfaces containing Fe decomposition of H2O2 with half life of 20 days can happen. This duration is still longer but not several orders than the existence of springtime interfacial liquid water on Mars today. This estimation is relevant for activation energy controlled reaction rates. Although the available tests and theoretical calculations do not provide firm values for the diffusion speed in such a “2 dimensional” environment, using relevant estimations this parameter in the interfacial liquid layer is smaller than in bulk water. But the 20 days duration mentioned above is still relevant, as the activation energy driven reaction rate is the main limiting factor in the decomposition and not the diffusion speed. Bulk liquid water is not expected in the Amazonian period, but interfacial liquid water probably appeared regularly, and its locations, especially during volcanically active periods might make certain sites than others more interesting for astrobiology with the lower concentration of oxidizing H2O2.