Electrical discharges as a possible source of methane on Mars: Lab simulation

Abstract

[1] At a concentration of 10–50 ppbv, methane is suggested to be a trace element in the Martian atmosphere. The sharp variations in its concentration observed are difficult to explain using current theories for sources and sinks (be it biotic or abiotic). Here we propose, and demonstrate with a lab simulation, a new production mechanism for methane based on the effect of electrical discharges over iced surfaces. The discharges, caused by electrification of dust devils and sand storms, ionize gaseous CO₂ and water molecules and their byproducts recombine to produce methane. Our experimental results show that pulsed electrical discharges over ice samples in a synthetic Martian atmosphere produce about 1.41×10¹⁶molecules of methane per joule of applied energy. The results of the electrical discharge experiment were compared with photolysis induced with UV laser radiation and it was found that both produce methane although the efficiency of photolysis is one-third of that of the discharge.

[2] Since 2004, when its discovery was first reported [Krasnopolsky et al., 2004; Formisano et al., 2004], the presumed presence of methane in Mars's atmosphere has been supported by subsequent observations and its concentration been mapped in detail. The concentration, measured with infrared absorption spectroscopy, was found to be time-and-position dependent within the 10–50 ppbv interval. As methane in the Martian atmosphere is fairly stable against known photochemistry (its lifetime is about 300 years) the sharp variations seen in its concentration [Formisano et al., 2004; Mumma et al., 2009; Geminale et al., 2008, 2011] are difficult to explain using current theories for methane sources and sinks (either biotic or abiotic). Among the mechanisms for methane production (sources) being considered are: biological processes [Krasnopolsky et al., 2004; Yung et al., 2010], cometary impact [Krasnopolsky, 2006], and meteorological or geological cycles [Yung et al., 2010; Krasnopolsky, 2006; Mumma et al., 2009]. Destruction mechanisms (sinks) include: dissociation due to UV radiation [Hintze et al., 2010], ion or electron impact from electrification of dust clouds [Atreya et al., 2006; Jackson et al., 2010; Hintze et al., 2010; Farrell et al., 2006], geological sequestration [Lefèvre and Forget, 2009; Zahnle et al., 2011] and oxidation [Atreya et al., 2006; Delory et al., 2006]. Recently, reservations concerning the detection itself and its short-time variability have been voiced by some authors [Zahnle et al., 2011; Krasnopolsky, 2006].

[3] As well as methane, the presence of liquid water and water vapor on Mars has been observed and mapped in the last 10 years [Reiss et al., 2010; Head et al., 2008]. Superficial water ice has been observed on several locations on Mars; not only at the poles, as expected, but also at lower latitudes and at shallow depths [Allen and Kanner, 2007a, 2007b].

[4] Dust devils and dust storms have been spotted on the Martian surface [Thomas and Gierasch, 1985] and were comprehensively mapped in 1999 by Cantor et al. [2001]. As on Earth, the triboelectric charging of particles in dust devils can produce dipolar charge-structures strong enough to trigger electrical discharges [Jackson et al., 2010; Farrell et al., 1999; Yair et al., 2008; Kok and Renno, 2009]. Electrical breakdown can be easily initiated on Mars because the combination of ambient pressure p and the relevant distances d bring the product pd near to CO₂'s Paschen minimum at 0.57 torr∙cm. Estimates of the electric breakdown of Mars' atmosphere vary between 10 to 25 kV/m [Melnik and Parrot, 1998; Atreya et al., 2006; Delory et al., 2006; Sentman, 2004; Eden and Vonnegut, 1973; Yair et al., 2008, and references therein]. The electrostatic phenomena accompanying dust devils on Mars has been successfully simulated in the lab [Eden and Vonnegut, 1973; Krauss et al., 2002] while, concurrently, advances in numerical modeling have shed new light on the effect of the discharges on the chemistry of the atmosphere. These models have been used to, for example, investigate the production of oxidants that degrade organic compounds, including methane [Atreya et al., 2006; Delory et al., 2006; Jackson et al., 2010].

[5] The aim of the present work is to experimentally investigate a new, abiotic mechanism for the production of methane on Mars based on the premise that ice deposited in cracks on the surface can experience electrical discharges caused by dust devils and sand storms. As a consequence of the discharge carbon dioxide and water ice dissociate and their fragments recombine to form methane. Electric fields are not the only ionizing agents for atoms and molecules: UV light can also ionize them, but under the conditions prevailing on Mars' surface, the results must be modest. However, when the photon flux is very intense, as in a laser beam, multiphoton ionization becomes viable. This consists in the simultaneous absorption of several photons to eject an electron. The probability of it happening is a function of both the wave's electric field E and the n-th power of the photon flux and in fact is directly proportional to E²ⁿ [Raizer, 1977]. Here laser photolysis is also investigated in spite of the fact that this is not a realistic mechanism per se (at 1.57×10⁴ Wm⁻² the UV fluency of a typical laser is about 4,000 times higher than that of solar UV [Hintze et al., 2010]) it is nevertheless useful to understand the ionization and recombination mechanisms that produce methane.

[6] The sequence of events according to our model is the following: large electric fields produced by dust devils or sand storms accelerate seed electrons, i. e. naturally occurring electrons, of which on Mars there should be plenty from solar-UV radiation and cosmic rays; these in turn ionize CO₂ molecules and start an avalanche [Jackson et al., 2010]. The slower electrons can participate in the following, well-known dissociative attachment reaction [Atreya et al., 2006; Jackson et al., 2010] with maximum cross section at 4.3 eV [Itikawa, 2002]:

[7] Also, electron impact can dissociate water producing OH and H [Atreya et al., 2006; Jackson et al., 2010] with peaks in the cross section at 6.37 and 8.1 eV [Itikawa and Mason, 2005]:

Finally, from CO and free hydrogen a reaction like the following produces methane [Krasnopolsky, 2006; Muetterties and Stein, 1979]:

From Muetterties and Stein's [1979, Figure 1] paper the activation energy for reaction (3) in the presence of a catalyzer was estimated to be −800 kcal/mole.

[8] The experiments were carried out in a vacuum chamber coupled to a residual gas analyzer (RGA) through a pressure-reduction system. The samples consisted of solid water-ice cylinders 23 mm in diameter and 55 mm in length, fitted with electrodes at both ends. One of them was connected to the high voltage supply and the other was grounded. To prepare the samples two different types of water were employed: a) de-ionized water with conductivity ≤ 1μS/cm, b) saline water with conductivity ∼80 μS/cm. The high voltage was applied in two forms: steady DC or square, fast pulses. In the experiments with UV light only, a pulsed Nd:YAG laser emitting at 266 nm was used as the source. A synthetic Mars mix containing 95.5% CO₂, 2.7% N₂, 1.6% Ar, 0.12% O₂ and 0.08% CO was used in the experiments. The pressure inside the vessel was kept fixed at 5 torr (6.7 mbar). All the experiments were made at room temperature.

[9] Before any experimental run a mass spectrum of a fresh gas load was taken with the sample in position. Afterwards, once the discharge started or the UV laser was applied, the composition of the gas in the chamber was monitored continuously with the RGA. Figure 1 shows an example of the mass spectra of a fresh load of Mars mix (virgin) and of the gas composition after the discharge. The spectrum of the exposed sample shows hydrogen production and also the presence of methane at 15 amu which is one of the main fragments of that gas. (The hydrogen traces in the virgin spectrum come from a slight evaporation of the sample already in place). It was observed that the partial pressure of the residual gases was proportional to the time of exposure to the discharge or UV light.

Figure 1

Mass spectra in a DC discharge. The dotted line corresponds to a fresh load of Marx mix, with the sample in place. The solid line is a spectrum taken two minutes after the discharge started. The peak at 2 amu (atomic mass number) corresponds to hydrogen while the one at 15 amu is one of the main fragments of methane. It can also be seen that as time elapses the sample evaporates, which can be seen from the increment at the peaks 17 and 18 amu that correspond to water vapor.

[10] The yield of methane and hydrogen was obtained from the mass spectra recorded in the experiments. The yield is defined as the number of molecules of a given species per joule of external energy applied (be it electrical or optical). The energy applied by the discharge was obtained from time integration of the applied power P = VIwhile for the photolysis tests the energy was measured using a pyroelectric laser meter. The number of molecules was calculated from the partial pressures of each residual gas. The hydrogen and methane yields obtained with deionized-water ice (conductivity ≤ 1μS/cm) for the two types of discharge employed and UV laser irradiation are shown in Table 1. The figures shown are the average of the yield obtained in about 20 separate experiments. Also shown in Table 1 are the results obtained with UV irradiation. The standard deviation σ for each case is shown between brackets.

Table 1. Yield (molecule/J) for De-ionized Water Ice

	DC	Pulsed	UV
Hydrogen	3.08×10^16 (±2.8×10^16)	1.02×10^17 (±4.4×10^16)	2.26×10^16 (±1.6×10^16)
Methane	2.56×10^15 (±2.6×10^15)	1.41×10^16 (±1.1×10^16)	7.95×10^14 (±2.0×10^14)

[11] From the data in Table 1it is evident that the pulsed-discharge experiment is more efficient in producing methane than the DC one. At 1.41×10¹⁶ molecules/J the yield of methane under pulsed conditions is 5.5 times that obtained with DC and nearly 18 times that of UV irradiation. Large variations in the yield were observed from one individual measurement to another; this is evident from the large standard deviation (up to 100%) obtained. We attribute this dispersion to the fluctuating nature of the discharge itself as this moved constantly around the surface of the sample, never staying in one place. It is interesting to note that the σ for the UV experiment is markedly less than that of the discharge experiments. Discharge experiments made with the Mars mix alone, without the ice sample, did not show (as expected) any trace of methane.

[12] It is possible that superficial ice could come from water that once flowed over the Martian surface and in doing that it might have become saline. In order to investigate whether the salinity of the water might have an effect on the production of methane and hydrogen, a limited series of tests were carried out using ice samples made with saline water. For these experiments a small amount of NaCl was added to the water to increase its conductivity to ∼80 μS/cm. Using this water to prepare new samples, the yields of methane and hydrogen were measured again using DC and pulsed power. The new yields, shown in Table 2, were found to be of the same order of magnitude as those obtained with deionized water ice. The methane production is not markedly affected by the salinity of ice; its yield in Table 2 falls within one standard deviation of that of Table 1.

Table 2. Yield (molecule/J) for Saline Water Ice

	DC	Pulsed
Hydrogen	3.16×10^16	2.23×10^17
Methane	1.97×10^15	1.61×10^16

[13] The differences in efficiency between pulsed and DC power are attributed to the presence of water vapor. Being an electronegative gas, water vapor captures electrons and in this way it reduces the availability of free electrons for further ionization. With DC power the water vapor pressure increased at a faster rate than in the other two experiments, thus the reduced efficiency of DC discharges in comparison with the pulsed experiment. Another factor might be the discharge's voltage drop. With DC the voltage across the arc is low: ∼200 V, as the arc is very conductive. The high resistance of the thin filaments in the pulsed discharge, on the contrary, produced a higher voltage drop and, consequently, a higher electric field. The enhanced field propitiates avalanche production and consequently the yield.

[14] In the UV-laser irradiation experiments ionization takes place through the multi-photon mechanism, i. e. several photons are required to dislodge a single electron. The photon flux required to ionize with laser light is an inverse function of the sample density: for given beam diameter and pulse length the number of electrons released is proportional toρSⁿ, where ρ is the substance's density, S the laser fluency and nthe n-th power of the photon flux [Raizer, 1977]. For that reason rarified gases require higher photon fluxes to achieve ionization than more dense substances. Due to its low pressure, ionization of CO₂ molecules is modest, whereas for ice, being a solid, ionization is more efficient. This helps to explain why the yield of methane in the laser experiments is low, in spite of a high hydrogen availability. There are simply not enough CO molecules to match the numbers of hydrogen so that reaction (3) can proceed at a faster rate.

[15] This view was corroborated by an experiment on UV irradiation of water samples under Martian conditions performed by Bar-Nun and Dimitrov [2006] who found that no significant amount of methane was obtained unless extra CO was added. In the experiment 6.55 eV light was used to illuminate the water molecules, unfortunately this energy is not hard enough to directly ionize CO₂, whose ionization threshold is at 13.8 eV. Consequently little CO was released. Having photon energy below the threshold is not an obstacle for laser light which, thanks to its high photon flux, has the possibility to ionize both CO₂ and H₂O through the multiphoton mechanism.

[16] At this stage it is pertinent to pose the following question: granted, a dust devil can produce electrification, but can it achieve breakdown on the ice surface? In order to answer this question a simulation was carried out using the program COMSOL. The geometry employed is shown in Figure 2 and consists of the following elements: an infinitely long, rectangular trench, 10 cm wide, is filled with water ice up to a certain depth h. Above it there is a layer of electrified dust 1.1 m thick with charge density 4×10⁻⁷ Cm⁻³. This density is of the order of the maximum charge concentration reported in previous work on Martian dust storms and also of the order of the maximum density in terrestrial dust devils. Farrell et al. [2004] for example, measured the charge density in a terrestrial dust devil using separate electric and magnetic methods and found it to be ∼1.6×10⁻⁷ and 8×10⁻⁷ Cm⁻³, respectively. The simulations of a Martian dust storm encountering a hill carried out by Melnik and Parrot [1998] show that the charge concentration at the top of the hill can reach a maximum density of 4.18×10⁻⁷ Cm⁻³. From the in-situ measurement of the field produced by a dust devil in New Mexico in 1962, Crozier [1964] obtained a space charge density of 1.7×10⁻⁷ Cm⁻³.

Figure 2

Schematics of the geometry employed in the 2D electric field simulations (cross section, not at scale).

[17] It is well known that at the interface between high-permittivity solids and (low-permittivity) gases the interface guides an external electric field allowing penetration into cavities and crevices. This is the case for the simulation implemented here as the ice's relative permittivity is a high 95.7 while that of the soil is 3.0. For the atmosphere the relative permittivity employed was 1.0. The results of the simulation are shown inFigure 3. The curves shown are the field intensities on the ice surface at three different depths relative to the surrounding terrain. Also shown in the figure are the relative positions of the trench's walls. The plot labeled E0 in Figure 3 corresponds to zero depth, i.e., when the surface of the ice is flush with the surrounding terrain. The other two plots correspond to the case when the ice surface is 10 cm (E10) or 30 cm (E30) below the ground. As expected, the field at the center (x = 0) is stronger for zero depth and as this increases the field at the center diminishes. But at the edges, near the wall, interesting effects are observed: the field experiences an enhancement as the walls are approached. In fact, the field at the walls for depth h = 10 cm surpasses the field obtained for zero depth. For the 30 cm depth the field at the walls is similar to that obtained at zero depth.

Figure 3

Electric field calculations as a function of position x along the surface of ice in a trench. The field was calculated using 3 different depths: 0, 10 and 30 cm relative to the ground's surface. Shown in the plot are the position of the trench's walls. Key: E0, zero depth; E10, depth 10 cm; E30, depth 30 cm.

[18] The electronic mean free path λ between ionizing collisions is a function of the local electric field E. In a dust devil it has been calculated to be around 60 m at E = 10 kV/m and for E > 15 kV/m it drops below one meter [Jackson et al., 2010] furthermore, for E ∼ 25 kV/m λ is just a few centimeters long (∼3 cm) [Farrell et al., 2006]. For the parameters employed in the calculations of Figure 3 it is evident that the conditions for avalanche initiation are just met. Assuming a threshold of 20 kV/m, breakdown is achieved at the corners of the trench for the three depths employed. From the results of Figure 3 it is evident that the field produced by the space charge cloud not only penetrates the trench but is, in fact, amplified. So the field produced by a dust devil can not only overcome the weak dielectric strength of the Martian atmosphere but also penetrate into cracks on the soil and so reach the ice lying at the bottom, with added strength, due to the topography of the terrain. Clearly a more realistic model with a thicker cloud of space charge would give higher fields and penetrate deeper.

[19] In their survey of Mars' northern hemisphere, Allen and Kanner [2007a, 2007b] found that water ice is exposed both at the poles and at mid latitudes (30°–65°N) while the observations of Head et al. [2008] show that water gullies and their associated ice and snow deposits have a distribution concentrated in the 30–50° latitude bands on each hemisphere. Additionally, there is the model by Forget et al. [2005] that predicts that most of the ice should deposit at high latitudes, with very little ice between −35° to +35° latitude. On the other hand, in the mapping of dust storms carried out by Cantor et al. [2001] it is shown that these are more frequent in mid to high latitudes (from ±30° to ±80°). A common trend in these variables is evident: they overlap at a band extending from mid to high latitudes that excludes the equatorial areas. This trend is more or less on step with the methane's seasonal distribution of Geminale et al. [2011] who found that the maxima in methane concentration extends from ±20° latitude onwards to the poles. The methane distribution is consistent with the production requirements assumed here: concurrent presence of exposed ice and sand storm (or dust devil) activity.

[20] In their report, Mumma et al. [2009] found that one of the large methane plumes released in 2003, covering an area of 60°×60°, should have come from a source of strength ≥39 mol/s. Using the methane yield for pulsed discharges in Table 1 this release corresponds to 1.66×10⁹ W of power needed to generate that amount of gas through a discharge. This translates into a required power density of 1.32×10⁻⁴ Wm⁻². So, a fraction 2.24×10⁻⁷ of the total solar irradiation on Mars' surface is required to produce the observed concentration. Once the plume fades out, one of the sink mechanisms discussed above should restore the methane concentration to its normal level.

[21] Besides the on-site methane readings the Curiosity rover may send later this year it would be very interesting if numerical modeling of the present experiment could be implemented. A model such as that ofAtreya et al. [2006] and Jackson et al. [2010] incorporating a source term for methane would be most welcome. Most models do include methane in the calculations but only through a loss term, never as source term.

[22] In this work it was assumed that methane is produced via the dissociation of H₂O and CO₂ (be it from ionization or UV irradiation) followed by combination of CO and H, as posited in equation (3). The present mechanism may be acting in parallel with other proposed sources but its main advantage is that it can generate methane very quickly and thus explain the generation of plumes. In the discharge and UV laser experiments the end result is the same (save for differences in efficiency) even considering that they might follow a different reaction path. Even though UV photolysis does not play a role in the production of methane on Mars, in the lab it was useful to understand the reaction mechanisms as it has the same effect as an electrical discharge.

Acknowledgments

[23] The authors are grateful to the Mexican Science Council (CONACyT) for financial support (grant 155673) and DGAPA-UNAM: IN-101212.

[24] The Editor thanks Jasper Kok and an anonymous reviewer for assisting in the evaluation of this paper.