Dongyu Liu and Samuel P. Kounaves*… – ACS Earth Space Chem. July 12, 2019
Abstract
The widespread presence of perchlorate (ClO4–) on Mars has significant implications for the alteration or destruction of indigenous organic compounds that may have been or still be present on Mars. The intermediary products of the UV-driven production of ClO4– include oxychlorines (ClOx) such as chlorite (ClO2–), chlorate (ClO3–), and chlorine-dioxide (ClO2) gas. The objective of this study was to start with ClO2– or ClO3– under Mars ambient and vary temperature, humidity, and UV wavelengths in order to isolate the reaction pathways leading to ClO4–. We also investigated the role of titanium dioxide (TiO2) as a catalyst for these reactions. We show here that the production of ClO4– from ClO2– and ClO3– proceeds through different pathways. The ClO2– is rapidly converted to stable levels of ClO3– and Cl–, suggesting that the amount present on Mars will likely be very low compared to other ClOx. We also observed that temperature does not affect ClO4– production when starting with NaClO2 but causes a decrease in ClO4– production when starting with NaClO3, and production of ClO4– using UV > 300 nm with O2 present does not involve an ozone (O3) pathway. We have also shown that adding TiO2 to the SiO2/ClOx mixture has a catalytic effect in the production of ClO4– under terrestrial conditions but shows primarily a shielding effect under Mars ambient; ClO3– is stable under Mars ambient even in the presence of TiO2 and is not affected by temperature or humidity. Finally, it was shown that water is necessary for generation of ClO2(g) during perchlorate production from either NaClO2 or NaClO3. This suggests that production of ClO2 might be occurring on Mars in areas where ice can provide increased humidity levels.
