Analyzing the role of interfacial water on sulfate formation on present Mars

Abstract

This work investigates a realistic scenario for sulfate formation on Mars that may have occurred under conditions representative to the whole Amazonian period. The process, according to reaction kinetics, works without the necessity of elevated temperature and is due to volcanic eruptions. Theoretical modeling utilizing a heterogeneous pathway leading to sulfates in the presence of microscopic subzero-temperature liquid H2O layer is presented for the first time. The process takes place via the following steps: 1. explosive volcanic eruption with gas, vapor and ash particle release; 2. adsorption of volcanic SO2 and H2O on grains in the volcanic cloud, and formation of interfacial H2O layer between the ice–grain boundary; 3. chemical reactions in the microscopic liquid H2O film forming HSO3–(aq); 4. oxidation to HSO4–(aq); 5. acid-sulfate reaction yielding SO4–-bearing minerals. Steps 1 and 2 happen in the atmosphere, steps 3 and 4 may occur both in the atmosphere and after deposition on the surface, while step 5 takes place on the surface. According to the findings the heterogeneous pathway is an important one, therefore future models should consider them besides homogeneous gas-phase reactions. Furthermore, microscopic liquid H2O could produce specific zonal distribution of sulfate content on the surface.