The color centers in halite induced by Martian dust activities

Abstract

The electrostatic discharge (ESD) induced by Martian dust activities (dust storms, dust devils, and grain saltation) has been recently proposed as a powerful geological agent to modify the structure and chemistry of Martian salts. Here, we performed new experiments to simulate gas discharge in Martian dust activity in a Mars chamber to investigate the modifications of chloride exposed to a medium strength ESD process. The structural, chemical and spectral features of NaCl before and after ESD exposure were systematically characterized with multiple analysis methods. On the basis of the experimental results, we found that 1) the colorless NaCl precursor subjected to ESD exposure subsequently displayed violet/blue color, indicating the presence of color centers; 2) the color centers (VK centers and Na colloids) in NaCl cause strong and characteristic spectral features in the visible to near-infrared reflectance spectra and Raman spectra; and 3) the relative density of color centers (VK centers and Na colloids) in NaCl gradually changes with increasing ESD duration. Our results also suggest that preferential Cl was released within NaCl crystals during the ESD process, and new phases ClO3−, ClO4−, and CO32− formed, which is consistent with previous studies (Wang et al., 2020b; Wu et al., 2018). We interpreted that preferential Cl release within NaCl crystals should be associated with the decomposition of VK centers and the sputtering process. We also proposed that the chemically active Cl species released from solid NaCl crystals is an essential part of the overall reaction pathway of perchlorate formation on Mars. Chloride exposed to the radiation environment at the Martian surface may also contain color centers, shedding light on the detection of normally featureless chlorides on Mars.