 PLASMA CHEMISTRY INDUCED BY MARTIAN DUST STORMS AND DUST DEVILS.  Alian Wang, Yuanchao Yan, Jennifer Houghton, Bradley L. Jolliff, and Kun Wang, Dept. Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University in St. Louis, MO, 63130 (alianw@levee.wustl.edu).  Introduction:  We have shown [1] that a new formation mechanism, multiphase (gas-to-solid) redox plasma chemistry induced by dust storms and dust devils on Mars, is responsible for the abundance of perchlorate observed during the Phoenix mission [2], which cannot be fully interpreted hitherto by photochemistry [3,4]. The link was made using simulation experiments in a Mars atmospheric chamber (the PEACh, [5]), and by scaling our experimental results to the modeled electrostatic discharge (ESD) in Mars dust events and Mars-surface UV radiation levels.  The new mechanism: Lofted sand and dust particles can frictionally electrify at planetary surfaces, which is commonly understood as the tendency of triboelectrical charge to result in negative charges on smaller grains and positive charges on larger grains of similar composition [6,7]. During  convective aeolian processes (e.g., dust devils and dust storms on Mars), lighter, negatively charged grains lift upward while heavier, positively charged grains remain closer to the surface, thus generating large-scale charge separation, i.e., an active electric field (E-field). For example, Efields of up to 60 kv/m and 166 kv/m were detected during the passage of terrestrial dust devils [8,9,10,11] and grain saltation [12], respectively.  Electrostatic discharge (ESD) is expected to occur when a local E-field accumulates beyond the breakdown electric field threshold (BEFT). Mars has a very low BEFT because of its thin atmosphere, i.e., ~ 20-25 kv/m by modeling [13] and ~ 25-34 kv/m by measurements in Mars chambers [14,15]. The Mars' BEFT is ≤ 1% of Earth's BEFT (~ 3000 kv/m); i.e., ESD occurs much more easily on Mars than on Earth. In addition, the ESD on Mars would be more likely in form of Townsend dark discharge (TDD) and normal glow discharge (NGD), but not in form of lightning (which occurs more frequently on Earth).  During ESD, an electron avalanche generates a flux of electrons with a relatively high drift speed. When these electrons collide with Martian atmospheric molecules, CO2, O2, N2, Ar, and H2O, they easily cause molecular ionization or dissociation, resulting in positive and negative ions, plus new neutral species and chain electron avalanches. These charged/neutral particles with high kinetic energy would stimulate redox plasma chemical reactions in the atmosphere and at the planetary surface, the oxidation of chlorides to oxychlorines (chlorates and perchlorates) is one of the phase transformations that we found [1]. Another two potential, and important,  phase transformations induced by martian dust ESD are those involving martian sulfates and martian Fe- phases, which will be reported in a pair of abstracts (this and [16]) at LPSC50.  Experiments, Samples, and Analyses:  The Electrostatic Discharge (ESD) experiments were conducted in a Mars Chamber (the PEACh, [5]). The chamber was first evacuated to 3x10 -2 mbar to remove the air and then was filled with pure CO2. The atmospheric pressure was kept at 3 ± 0.1 mbar. The ESD was generated at about 300 V and 22 mA across the electrodes. Consistent with our early experiments [1], the ESD that we generated was in normal glow discharge (NGD) regime, with a similar electron flux of 1.43x10 20 electrons per (second *m 2), which is about 10 4 times lower than the calculated electron flux for NGD type of ESD that could occur in Martian dust events.  The start phases for martian relevant hydrous and anhydrous sulfates are MgSO4·7H2O, CaSO4·2H2O, Na2SO4, Na2SO3, NaHSO3, FeSO4·7H2O, Na-jarosite and its 1:1 mixture with MgSO4·7H2O.  Each of these samples was grounded and sieved, and a grain size range of 88 µm > d > 63 µm was selected to ensure the comparable results from all sulfates.  The ESD experiments on each sulfates were run at 15 min, 1 hour, 2 hours, 3 hours, 7 hours, and occasionally up to 64 hours.  All ESD reaction products were first analyzed by laser Raman spectroscopy, using a Renishaw-inVia Raman system (using 532 nm excitation, ~ 1 µm beam diameter at focus). For some samples, in situ plasma emission spectroscopy was taken in PEACh to detect the release of free radicals from CO2 atmosphere or from the sulfates. XRD analyses of a few samples were taken using Bruker D8 Diffractometer. Ion Chromatograph was used to identify and to quantify the [SO4] 2- generated by ESD from SO3-bearing salts, using an ASupp5-100 anion column (27°C, 3.2 mM Na2CO3/ 1.0 mM NaHCO3/ 2.5% acetone eluent, 0.7 mL/min, with suppression) on a Metrohm 881 Compact IC pro with a conductivity detector. Results: The break-down of MgSO4·7H2O in ESD reaction was very fast (Fig. 1). After 15 min of ESD, most grains turned into amorphous MgSO4·xH2O (x< 3), some kept a crystalline structure but dehydrated to MgSO4·4H2O, and only a few grains remain as MgSO4·6-7H2O.  The break-down of CaSO4·2H2O was slower, but remained crystalline. Fig.2 shows a few typical Raman spectra from a 7 hours' ESD product started with gypsum. It is quite obvious that -CaSO4 was the major phase generated by ESD. Since -CaSO4 is unstable under lab conditions, this ESD product was sealed in a glass bottle immediately after ESD reaction, and the Raman analyses were made through the glass wall of the sample bottle. Another phase generated by ESD was basanite (CaSO4·0.5H2O), with some gypsum remaining, shown as a spectral shoulder in Fig. 2.  It seems that the structure of Na2SO4 is more robust than Mg- and Ca-sulfates, at least under the used electron flux and the known maximum kinetic energy (~ 17.19 eV, [1]) in our experiments. In the spectra of a 7 hours ESD reaction product, there was no obvious change in Raman spectra peak positions, but a broadening of major 1 peak near 992 cm -1 and an increase in the background at 3, 2, 4 peaks and near 100 cm -1 .  Both phenomena suggest a reduction of crystallinity. In contrast, the Raman spectra of the ESD products from both Na2SO3 and NaHSO3 show the appearance of a sharp Raman peak near 997 cm -1 , which does NOT match the Raman features of known SO3- or SO4bearing phases. The XRD analysis made on a 7hrs ESD product from NaHSO3 has a pattern without much change in XRD line positions, but  showing a reduction of line intensities, increase of peak widths, and some combined peaks (Fig. 3), possibly indicating a  reduction in crystallinity.  The IC analysis of the same set of samples revealed a large increase of the SO4 component in the 7 hrs ESD product (Fig.4), consistent with the Raman evidence that the 997 cm -1 peak occurs in all 156 spectra taken from this 7hrs ESD product. However, whether  a SO4bearing, low-crystallinity phase should have a Raman peak shift upward of 3 cm -1 (from standard 993 cm -1 peak of Na2SO4) is not known. Extra Raman peaks occur in the 1200-800 cm -1 range, which could be additional S-bearing phases, consistent with IC  data.  Conclusion: The effects on sulfates by ESD in NGD regime in martian dust events were found to be dehydration, amorphization, and oxidation of S.  The 2 nd abstract [16] presents the results from Fe-sulfates.  Acknowledgement: NASA SSW-80NSSC17K0776  References: [1] Wu et al., (2018) EPSL, 504, 94-105. [2] Hecht et al., (2009), Science, 205, 64-67. [3] Catling et al., (2010), JGR, 115, E00E11. [4] Smith et al., (2014), Icarus, 231, 51-64. [5] Sobron and Wang (2011), JRS, DOI 10.1002/jrs.3017. [6] Forward et al., (2009) GRL, 36, L13201. [7] Krauss et al., (2003) New J. Phys., 5, 70.1-70.9. [8] Esposito et al., (2016) GRL, 43, 5501-5508. [9] Farrell et al. (2004) JGR, 109, E03004. [10] Harrison et al., (2016) Space Sci. Rev. 203, 299-345. [11] Jackson & Farrell (2006) IEEE Trans. Geosci. Rem. Sens. 44, 2942-2949. [12] Schmidt and Schmidt (1998) JGR, 103, 8997-9001. [13] Melink & Parrot (1998), JGR, 103, 29107-29117. [14] Farrell et al., (2015) Icarus, 254, 333337. [15] Yan et al., (2017) LPSC, #2413. [16] Wang et al., (2019), this volume. 
