Forming perchlorates on Mars through plasma chemistry during dust events

Abstract

We report experimental evidences to support a new formation mechanism, multiphase redox plasma chemistry, for perchlorate on Mars observed during the Phoenix mission, whose high concentrations and high ClO4/Cl ratio cannot be fully interpreted by photochemistry. This chemical reaction occurs between Cl-bearing minerals on the Mars surface and free radicals generated by electrostatic discharge (ESD) during Mars dust events (dust storms, dust devils, and grain saltation).We conducted simulated ESD experiments in a Mars chamber with pure CO2, CO2 + H2O(g), and Mars Simulate Gas Mixture at Martian atmospheric pressure. We directly observed (1) the instantaneous generation of atmospheric free radicals CO+2, CO+, OI, HIII, HII, OH, ArI, N2, and N+2 in normal glow discharge (NGD), detected by in situ plasma emission spectroscopy, and O3 by UV and Mid-IR spectroscopy; (2) the fast transformation of NaCl to NaClO3 and NaClO4 detected by laser Raman spectroscopy, with oxychlorine enrichment at the sample surfaces confirmed by ion chromatography. Through two sets of experimental comparison, we found that the oxidation power of ESD-electron is three orders of magnitude higher than that of UVC-photon. We scaled our experimental results to the modeled ESD in Mars dust events and Mars surface UV radiation level, and concluded that plasma chemistry occurred during Mars dust events can be an additional important formation mechanism for the large amounts of perchlorates observed during various missions to Mars.