Chlorine Release From Common Chlorides by Martian Dust Activity

Abstract

Chlorine is one of the highly mobile elements that participated in early aqueous chemistry and later alteration in Mars history. Our new experimental results suggest that chlorine could cycle on present‐day Mars between the atmosphere and surface, driven by multiphase redox plasma chemistry induced by current Martian dust activity (dust storms, dust devils, and grain saltation). We present two sets of experimental results that demonstrate the instantaneous release of chlorine from seven common chlorides during a medium strength electrostatic discharge (ESD) process that induced plasma chemistry in a Mars environmental chamber. Results include (1) the direct detection of a plasma emission line at 837.8 nm of the first excited state of the Cl atom (Cl‐I) by in situ plasma spectroscopy during the ESD process for MgCl2, FeCl2, and AlCl3 and (2) the characterization of Cl‐bearing phases in the films deposited on the upper electrode after 7 hr of ESD exposure on each of seven chlorides (NaCl, KCl, CaCl2, MgCl2, FeCl2, AlCl3, and FeCl3), using Raman spectroscopy, X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy‐dispersive X‐ray (EDX) spectroscopy, and X‐ray photoelectron spectroscopy (XPS). This study is part of a series of laboratory investigations on the Martian atmosphere and surface interaction induced by electrochemistry.

Amorphization of S, Cl‐Salts Induced by Martian Dust Activities

“…The rate of amorphization (i.e., the ease of damaging the crystalline structure) of common chlorides appears to be more related to the type of cations present, and reflected by a grouping of chlorides mentioned in Section 4.2.2, that is, higher rates in Fe, Mg, and Al chlorides than in Ca, Na, and K chlorides. The same grouping was found in experimentally observed rates of Cl‐release induced by ESD from Mg, Fe, Al, Ca, Na, and K chlorides (A. Wang, Yan, Jolliff et al., 2020), which was correlated with the degree of M‐Cl bond covalence that is usually quantified by the difference of electronegativity of M and Cl (Allred, 1961). These electronegativity differences range from 2.34 to 2.16 for KCl, NaCl, and CaCl 2 , and from 1.2 to 1.85 for FeCl 3 , AlCl 3 , FeCl 2 , and MgCl 2 (A. Wang, Yan, Jolliff et al., 2020).…”

“…When electrostatic discharge is induced by martian dust activity, it exerts two additional effects on surface minerals: physically impacting them with energetic electrons and chemically attacking them with free radicals and electrons. The potential effects in chemistry and biology of ESD induced by Mars dust activities have been investigated and reported by Wu et al (2018), and Wang et al (2020a, b and this study) and by another group in Denmark (Bak et al, 2016(Bak et al, , 2018Knakjensen et al, 2014;Thoegersen et al, 2019).…”

“…(2018), and A. Wang, Yan, Jolliff et al. (2020), that is, with the same electron flux shown in Table .…”

“…On the basis of the studies referenced above and our previous experimental investigations (Wu et al., 2018; A. Wang, Bradley, Yan, 2020; A. Wang, Yan, Jolliff et al., 2020), we hypothesize that S‐ and Cl‐rich X‐ray amorphous materials at the surface of Mars may be very common, and one mechanism to produce them is by multiphase redox plasma chemistry (or simply, electrochemistry ) induced by electrostatic discharge (ESD) that occurred during Martian dust activities during the Amazonian period. In this manuscript, we report the results of 75 sets of ESD experiments on 22 Mars‐relevant minerals under Martian atmospheric conditions to explore and test our hypothesis.…”

“…(2018), A. Wang, Yan, Jolliff et al. (2020), and A. Wang, Yan, Dyar et al., (2020) (and this study), A. Wang, Bradley, Yan (2020) and by another group in Denmark (Bak et al., 2016, 2018; Bak, Larsen et al., 2017; Bak, Zafirov et al., 2017; Knak Jensen et al., 2014; Thoegersen et al., 2019).…”