Contact Electrification of Regolith Particles and Chloride Electrolysis: Synthesis of Perchlorates on Mars

1. Introduction

The Viking 1976 Mars mission experiments (Biemann et al., 1976) led to the conclusion that a highly oxidative soil has effectively eliminated organic compounds on the martian surface. Oxygen detection in the Viking Gas Exchange experiment was explained on the assumption that the martian surface is enriched with hydrogen peroxide. Oyama and Berdahl (1977) concluded that, in the Gas Exchange experiment, O₂ evolution was primarily associated with superoxide, which suggests that the CO₂ liberated in the Labeled Release experiment could be explained by a reaction of hydrogen peroxide in the presence of iron oxide. Although the occurrence of hydrogen peroxide on Mars has been confirmed by ground-based measurements (Clancy et al., 2004; Encrenaz et al., 2004), other oxidants may equally produce the same effect. Recent Mars Phoenix mission observations have provided convincing evidence for the presence of perchlorate in martian soil (Hecht et al., 2009; Kounaves et al., 2010). An intriguing puzzle is how the potent oxidants such as hydrogen peroxide and perchlorate originated in a planetary environment devoid of an oxygenic atmosphere. Even on Earth, only relatively small quantities of non-anthropogenic hydrogen peroxide (Sakugawa et al., 1990; Encrenaz et al., 2004) and perchlorate (Andraski et al., 2014; Calderon et al., 2014) have been detected in natural habitats, and were generated most probably by abiological processes. These compounds have not accumulated to levels as high as those on Mars owing to the presence of reducing organic compounds, microbial activity, or enhanced thermal decomposition in the warmer terrestrial environment. Furthermore, there is no evidence to indicate that the rates of perchlorate and hydrogen peroxide synthesis on Earth are comparable to those on Mars. A slow production rate and faster decomposition under terrestrial conditions could easily account for the observed discrepancy. The highest level of natural perchlorate contamination on Earth, which was detected in the Atacama Desert (Calderon et al., 2014), is below the amounts observed on the surface of Mars. Similarly, minute quantities of hydrogen peroxide detectable in rainwater and aerosols have no significant chemical or biological effects.

Understanding the mechanism of production of oxidative species on Mars is fundamentally important to our overall knowledge of atmospheric and soil chemistry and their influence on any biological processes that existed, or currently exist, on Mars. Further, our understanding of the mechanism of production of oxidative species will undoubtedly impact studies geared toward human missions to Mars and prospects of its colonization.

Several authors have looked into reaction pathways that may lead to synthesis of highly oxidative stable compounds under martian conditions. In the absence of an ozone blanket, atmospheric photolysis of water vapor by intense solar UV radiation generates free radicals such as OH, OH₂, O₂ ⁻. These species could serve as precursors for hydrogen peroxide (Sakugawa et al., 1990) or chlorate and perchlorate in the presence of the chloride ion (Catling et al., 2010). As there is no shielding and the martian atmosphere is tenuous, UV radiation that reaches the surface probably induces photocatalytic reactions with solid particles and airborne dust particles. Such heterogeneous photocatalytic reactions could generate higher concentrations of oxidants (Lefevre et al., 2008; Smith et al., 2014; Carrier and Kounaves, 2015).

Mills (1977) was the first to point out that static electrification of dust may lead to chemical reactions in Mars. He suggested that the glow discharge between oppositely charged grains could generate free radicals and thus deliver oxidative products. Subsequently, many authors have considered chemical effects associated with the electrification of dust particles, notably the creation of oxidative compounds on Mars (Atreya et al., 2006; Delory et al., 2006; Jackson et al., 2010; Farrell et al., 2015). This is interesting because dust devils and dust storms are ubiquitous on Mars, and there is evidence that they are electrified (Renno et al., 2004; Ruf et al., 2009; Renno and Ruf, 2012). During atmospheric activity in arid terrestrial deserts, electrification of sand grains and charge separation give rise to electric discharges. It has been suggested that similar electric discharges in the martian atmosphere have generated oxidative species (Atreya et al., 2006; Delory et al., 2006; Jackson et al., 2010; Farrell et al., 2015). Indirect evidence suggests that electrified dust causes discharges in the martian atmosphere (Renno et al., 2003; Ruf et al., 2009). However, according to Renno and Ruf (2012) they are unlikely to be associated with dust devils. In an electric discharge, chemical reactions are initiated by the electron impact ionization of molecules, energies of which are of the order 10 eV. However, because of the low martian atmospheric pressure, discharges initiate at a much lower voltage gradient (∼3 × 10³ V m⁻¹) buildup, which constrains sufficiently energetic discharges (Kok and Renno, 2009). Glow discharge between oppositely charged grains could also generate free radicals and deliver oxidative products. Recently, it has been suggested that glow discharge and electron avalanche may not be self-quenching and therefore more effectively initiate chemical reactions (Farrell et al., 2015).

In the present study, it is shown that even well below the threshold voltage gradient for electric discharge under martian conditions, contact-electrified and chloride-impregnated regolith particles are capable of initiating chemical reactions. When oppositely charged soil grains generated by eolian agitation coalesce by their mutual attraction, water condensation will electrolyze brine and produce hypochlorite, chlorite, chlorate, perchlorate at the positively charged grain and hydrogen at the negatively charged grain. Even if water condenses as ice (a real situation under martian conditions), the reaction will proceed because of chloride ionization on the ice surface (Bolton and Pettersson, 2001; Kim et al., 2007). Limitations of the potential difference needed for electrolysis and discharge dissipation above breakdown electric fields enable an estimation of the required soil grain dimensions, and a simple calculation yields the optimum rate of production of perchlorate.

2. Proposed Hypothesis

Observations based on desert sandstorms and laboratory simulation experiments indicate that frictional collisions electrify soil grains (Sickafoose et al., 2001; Sternovsky et al., 2002; McCown et al., 2006; Williams et al., 2009; Callis, 2011; Zheng, 2013). The sign of charge depends mainly on grain size. During the collisions, the larger particles tend to assume positive charges, which is in contrast to the smaller ones that acquire negative charges. In dust storms on Earth, negative charges carried by lighter particles reside aloft. Theoretical arguments have also been presented to explain this observation (Lacks and Levandovsky, 2007; Forward et al., 2009). The frictional collisions between soil grains that cause electrification occur mostly near the soil surface, where the particle concentration is high. If A and B denote the particle species that acquire positive and negative charges, respectively, the collisions, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} A + B \to A \left( {{N^ + }} \right) + B \left( {{N^ - }} \right) \tag{1} \end{align*} \end{document}

leading to electrification, occur with a frequency F represented as (Maxwell, 1867) \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} F \sim {n_A}{n_B}{ \pi}{ \left( {{r_A} + {r_B}} \right) ^2}v \tag{2} \end{align*} \end{document}

where N is the number of electronic charges in a grain and n_A, n_B and r_A , r_B are the number densities and radii of the two types of grains, and v is the speed of collision.

After collisions, positively and negatively charged grains are attracted to each other, and a significant fraction of them coalesce and remain in contact as A(N ⁺)B(N ⁻). Such segregations are favored if their electrostatic binding energy exceeds the kinetic energy imparted to a grain. Again when oppositely charged particles loose kinetic energy, they segregate. In sandstorms, larger grains are very sluggish in movement due to their inertia (Kok et al., 2012). Therefore, most of the larger oppositely charged grains coalesce. If the grains are insulating and the atmosphere is dry, charges in the grains do not recombine readily, and grains remain segregated. If e = charge of the electron and ɛ ₀ = vacuum permittivity, the magnitudes of the electric fields at the surfaces of the individual particles and at the contact region between oppositely charged particles are (Ne/4πɛ₀)r_A ⁻², (Ne/4πɛ₀)r_B ⁻², (Ne/4πɛ₀)(r_A ⁻² + r_B ⁻²), respectively. Assuming that both particles have radii of the similar order of magnitude r, the condition that their electric charges will not undergo dissipation due to gaseous breakdown is \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} Ne / 4{ \pi}{ \varepsilon _0}{r^2} < {E_B} \tag{3} \end{align*} \end{document}

where E_B  = breakdown field strength. It is important to emphasize that insulating oppositely charged particles in contact do not readily recombine in a dry atmosphere. In fact, the static electrification during rubbing of two surfaces indeed happens because of this reason. In ordinary circumstances, charges are lost largely due to moisture effects, which make surfaces conducting. Early electrostatic experimenters noted that, in dry air, the main static charge loss mechanism is air ionization (de Angelis, 2012). If the air pressure is reduced, a slower decay is observed (Crookes, 1878) because collisions of ionizing radiation with air molecules are lesser at lower gas densities. Therefore, at potentials significantly below the breakdown threshold, charges are retained longer at lower air pressures.

Oppositely charged grains A ⁺ and B ⁻ coalesce to form structures A ⁺ B ⁻, and moisture condenses over them as well as over any individual grains A⁺ and B ⁻ . Most of the moisture condensed over A ⁺ and B ⁻ grains will also finally coalesce. Recently, a decrease in the atmospheric moisture immediately after a storm has also been observed, which implies water absorption on dust grains released to the atmosphere (Trokhimovskiy et al., 2015). Naturally, chlorides (e.g., NaCl) contaminate martian dust because of their abundance. Chloride and water will make the contact between A ⁺ and B ⁻ in the A ⁺B⁻ structure conducting, which will lead to charge separation and electrolysis. Even if moisture condenses as ice, a peculiar property of chlorides will render the system ionically conducting. It has been found that chloride ionizes on the surface of ice at temperatures above ∼140 K (Bolton and Pettersson, 2001; Kim et al., 2007; Park et al., 2012). In NaCl-doped ice crystallites, the spontaneously ionized mobile Cl⁻, OH⁻, H⁺ reside on the surface, and Na⁺ diffuses to the interior (Kim et al., 2007). Thus, with A ⁺ B ⁻ incorporating condensed water, the ions Cl⁻(H⁺) move toward positively (negatively) charged grains (Fig. 1) and discharge just as occurs in the electrolysis of brine. This is the crucial new idea basic to the model proposed here, and this is testable in a Mars simulation chamber where suitable salt-impregnated terrestrial regolith particles are agitated to induce electrification. The potential difference involved is [(Ne)/r_A  + (Ne)/r_B ]∼(Ne)/r, where N is the number of electronic charges in the positively and negatively charged grains and r_A ≃r_B  = r. Consequently, at a positively charged grain A ⁺, the transfer of positive charges would initiate oxidative reactions given below, where h⁺ denotes a positive charge of one unit (electronic charge) derived from A ⁺. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{C}}{{ \rm{l}}^ - } + {{ \rm{H}}_2}{ \rm{O }} + 2{{ \rm{h}}^ + } \to { \rm{ Cl}}{{ \rm{O}}^ - } + 2{{ \rm{H}}^ + } \left( {1.71} \right) \tag{4} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{Cl}}{{ \rm{O}}^ - } + {{ \rm{H}}_2}{ \rm{O }} + 2{{ \rm{h}}^{ \rm{ + }}} \to { \rm{ Cl}}{{ \rm{O}}_2}^ - + 2{{ \rm{H}}^{ \rm{ + }}} \left( {1.42} \right) \tag{5} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{Cl}}{{ \rm{O}}_2}^ - + {{ \rm{H}}_2}{ \rm{O }} + 2{{ \rm{h}}^ + } \to { \rm{ Cl}}{{ \rm{O}}_3}^ - + 2{{ \rm{H}}^ + } \left( {1.16} \right) \tag{6} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{Cl}}{{ \rm{O}}_3}^ - + {{ \rm{H}}_2}{ \rm{O }} + 2{{ \rm{h}}^ + } \to { \rm{ Cl}}{{ \rm{O}}_4}^ - + 2{{ \rm{H}}^ + } \left( {1.19} \right) \tag{7} \end{align*} \end{document}

OPEN IN VIEWER

FIG. 1.

Schematic diagram illustrating electrolytic process that occurs when a pair of chloride-contaminated positively and negatively charged soil grains coalesce and condense moisture.

The final reaction that occurs on positively charged grains, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{C}}{{ \rm{l}}^ - } + 4{{ \rm{H}}_2}{ \rm{O }} + 8{{ \rm{h}}^ + } \to { \rm{ Cl}}{{ \rm{O}}_4}^ - + 8{{ \rm{H}}^ + } \left( {1.39} \right) \tag{8} \end{align*} \end{document}

is initiated in steps of two electron transfers via Reactions 4–7. The intermediates ClO⁻ (hypochlorite), ClO₂ ⁻ (chlorite), ClO₃ ⁻ (chlorate) are sequentially oxidized in two electron transfers that are more probable compared to four, six, and eight transfers that would shortcut the oxidation process.

At the negatively charged grains, B ⁻, H⁺ ions (occurring as H₃O⁺) are discharged, liberating hydrogen. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} 2{{ \rm{H}}^ + } + 2{{ \rm{e}}^ - } \left( B \right) \to {{ \rm{H}}_2} \tag{9} \end{align*} \end{document}

The over-all of (8) and (9) is \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{C}}{{ \rm{l}}^ - } + 4{{ \rm{H}}_2}{ \rm{O }} \to { \rm{ Cl}}{{ \rm{O}}_4}^ - + 4{{ \rm{H}}_2}, \Delta G = 112.1 \ { \rm{ kJ / mol}} \tag{10} \end{align*} \end{document}

The chloride ion oxidizes to perchlorate, extracting oxygen from water and liberating hydrogen, which eventually escape the planet. Some utilization of electrons in negatively charged particles for other reduction processes cannot be ruled out. As ferric oxide is invariably a component of martian soil, electrons in negatively charged particles may also be utilized to reduce iron oxides (e.g., Fe₂O₃ to Fe₃O₄).

For simplicity, we have confined the above discussion to a coalesced pair of positively and negatively charged grains. There will also be segregations constituted of many particles (e.g., a large positively charged particle covered with a number of smaller negatively charged particles), but here again, the same argument applies.

Instead of (1)–(5), water oxidation could also occur at the surface of positively charged grains and generate oxygen or hydrogen peroxide, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} 2{{ \rm{H}}_2}{ \rm{O + 4}}{{ \rm{h}}^ + } = {{ \rm{O}}_2} + 4{{ \rm{H}}^ + } \left( {1.23} \right) \tag{11} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} 2{{ \rm{H}}_2}{ \rm{O }} + 2{{ \rm{h}}^ + } = {{ \rm{H}}_2}{{ \rm{O}}_2} + 2{{ \rm{H}}^ + } \left( {1.77} \right) \tag{12} \end{align*} \end{document}

The numbers given inside the parentheses in Eqs. 4 –8 and 11–12 are the respective oxidation potentials (E _o /V) as measured in customary electrochemical laboratory experiments, under ambient conditions. Since chloride is completely ionized on the ice surface, potentials involved are expected to be of the same order of magnitude or less.

The limits on the sizes of the dust particles required for initiation above electrostatic chemical reactions can be estimated as follows: For ion separation and electrolysis to occur, the potential V between the centers of the two grains A ⁺ and B ⁻ should be at least of the order of 1 volt; that is, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} V = Ne / 4{ \pi}{ \varepsilon _0}r > 1 \tag{13} \end{align*} \end{document}

As the breakdown potential of the martian atmosphere is ∼3 × 10³ V m⁻¹, the constraint (3) can be expressed as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} Ne / 4{ \pi}{ \varepsilon _0}{r^2} < 3 \times {10^3} \tag{14} \end{align*} \end{document}

The constraints (13) and (14) are always satisfied provided that r ≥ 330 μm, N ≤ 2.1 × 10⁵. It is encouraging to note that experiments on contact charging of granular soil materials yield the correct order of magnitude of grain sizes. Particles that have sizes of the order of 100 μm are those that are involved in saltation (Kok et al., 2012). Saltation leads to intergranular friction and mixing of regolith with air, the former effect leading to static electrification and the latter promoting moisture absorption. In a fluidized bed setup with a martian regolith stimulant, the mean diameters of positively and negatively charged particles were found to be 400 and 296 μm, respectively (Forward et al., 2009). Similarly, in a wind tunnel experiment conducted to study static electrification of wind-blown sand and saltation, it has been found that particles of diameter ∼500 μm predominantly acquire positive charges, whereas those less than 250 μm are negative (Zheng et al., 2003). In a detailed study of the electrical properties of Mars analog dust, Merrison et al. (2004) observed that wind tunnel–blown grains acquire charges of the order 10⁵e, which is consistent with the estimation obtained from the proposed mechanism.

The grain size of martian dust particles associated with dust storms has been estimated to be in the range 0.01–10 μm; however, a significant fraction of soil particles are expected to have larger dimensions (Weitz et al., 2006) that exceed the above limit of 330 μm and the median size of ∼300 μm (Zeng et al., 2015). Although such particles would not fly far into the atmosphere, they are violently agitated during a storm, which causes static electrification. A collision that produces two positively and negatively charged dust particles, each carrying approximately N electronic charges, could generate a maximum of N/8 molecules of NaClO₄ (transfer of 8 electronic charges is necessary to create one ClO₄ ⁻ ion). Therefore, using (2), the optimum rate of production R (moles per unit volume) of perchlorate production can be written as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} R \sim \left[ {N{n_A}{n_B}{ \pi}{{ \left( {{r_A} + {r_B}} \right) }^2}v} \right] / 8A \tag{15} \end{align*} \end{document}

where A = Avogadro's number. Assuming r_A ∼400 μm, r_B ∼300 μm, a grain density of ∼2000 kg m⁻³, a bulk soil density of 1400 kg m⁻³ corresponding to a number density n_A  = n_B ∼ 10¹⁰ in the soil, N = 2 × 10⁵ (upper limit estimated earlier), and a nominal impact speed v = 1 ms⁻¹, we obtain R = 5.7 × 10⁻² mol ClO₄ per hour per m³. This is clearly an upper limit because the optimum values of the parameters are used in the estimation without taking into account the rates of back reactions in the individual steps. The back reactions are diffusion-controlled and expected to have smaller rate constants because of the low temperature. In the real situation, the grains are not always charged equally and opposite, and grain agglomerations of more than two particles would also be formed. These effects will not influence the above estimation significantly.

The observed perchlorate concentration in martian soil depends on its reduction and degradation as well as processes that remove it from the reaction site. Here, formation of hygroscopic liquidized perchlorate during warmer weather may play an important role. Similarly, the intermediate products (hypochlorite, chlorite, and chlorate) should also have their equilibrium concentrations. Sodium hypochlorite (NaClO), even in small concentration formed via Reaction 4, will increase the soil pH, owing to very poor dissociation of HClO (dissociation constant = 3.5 × 10⁻⁸). NaClO concentration of ∼3.5 × 10⁻⁶ will raise the pH of water to 8 and increase the alkalinity. However, the pH of the Phoenix site was consistent with carbonate-saturated solution, possibly because ClO⁻ is scavenged by other system components (Quinn et al., 2011). Brine electrolysis also generates chlorine and sodium hydroxide. However, in the present situation chlorine and sodium hydroxide are more likely to react to form hypochlorite. The Viking result can also be explained as a result of the presence of hypochlorite instead of hydrogen peroxide (Quinn et al., 2013).

There is experimental evidence for initiation of chemical reactions by statically charged objects via direct charge transfer rather than electric discharging in a gaseous phase. Many redox reactions and hydrogen evolution from water have been observed when electrified insulators come into contact with water (Liu and Bard, 2008). Oxide powders (Cu₂O, NiO, Fe₂O₃) and pure water agitated in glass containers in the absence of oxygen and light split water into oxygen and hydrogen. Triboelectric charging has been suggested as the cause of this phenomenon (Domen et al., 2000). The conversion of mechanical energy for water splitting in these processes has been reported as ∼4.3% (Ikeda et al., 1999). The result is amazing, because in this experiment, grains are agitated in water, and most of the separated charge dissipates via the conduction. In the mechanism we have conjectured, conductivity necessary for electrolysis triggers after buildup of the charge, and consequently much higher efficiencies can be expected.

The hypothesis we have proposed could be rigorously tested in a Mars simulation chamber, where mixtures of grains of different sizes and materials (sand, basalt, ferric oxide, etc.) are agitated, providing an arrangement for intermittent injection of moisture.

3. Summary and Conclusion

A plausible mechanism for synthesis of the oxidative species hypochlorite, chlorite, chlorate, and perchlorate, that is, the end product of chloride oxidation in martian soil, has been suggested. The idea is based on contact-electrification of sodium chloride-contaminated regolith grains agitated by dust storms and devils. Naturally, oppositely charged grains coalesce into pairs or agglomerates of several particles, with a spatially separated negative and positive charge distribution. Initially, the grain charges do not recombine readily, as they are insulating and the soil environment is generally dry. When wind agitation mixes up coalesced grains and air, water vapor absorption is enhanced. This effect ensues ionic discharge and initiates reactions parallel to those that occur during electrolysis of brine. The electrolysis generates hypochlorite, chlorite, chlorate, and perchlorate with a concomitant reduction of water to hydrogen. Even if water is condensed as ice, electrolytic reactions will progress as chloride ionizes on the surface of ice. The potential differences needed for redox reactions involved are of the order of 1 V. The other constraint is the requirement of grain surface electric fields that do not exceed the breakdown potential of the martian atmosphere. It is shown that these conditions are easily satisfied, provided that soil particle dimensions exceed 330 μm, and such grain sizes are abundant in martian soil. This process does not require large-scale charge separation that leads to macroscopic electric discharges or microscopic glow discharges between oppositely charged grains. The hypothesis can be readily tested in a Mars simulation chamber.

The same reaction scheme described above, which occurs in the martian atmosphere, cannot be ruled out. Less heavy negative-positive particle segregations that reach saturated regions of the atmosphere will also get deposited with condensed water. Again individual negatively and positively charged particles in the atmosphere could also participate in chemical reactions (Phillips, 2013). However, reactions at a single positively or negatively charged particle are more susceptible to recombination. An isolated, positively charged grain can initiate Reactions 4–8 via transfer of the positive charge. However, the protons liberated will also recombine and reverse the reaction, unlike what occurs in the two-particle system where the electric field-driven ionic discharge occurs at a shorter distance. The proton derived from Reactions 4–8 (always hydrated) may be scavenged by a negatively charged particle or ions. Redox reactions between charged dust particles in gaseous phase could also have important astrophysical implications, especially in the interstellar medium (Caruana and Holt, 2010).