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Project Progress

Breakdown of methane due to electric discharge: A Laboratory Investigation with Relevance to Mars.

Project Investigators: Inge L. ten Kate (GCA Post-doctoral Fellow) and Paul R. Mahaffy

Methane has been detected in the martian atmosphere both through ground-based observations (Mumma et al. 2003, 2004, 2005, 2008; Villanueva et al. 2008; Krasnopolsky et al. 2004) and Mars-orbit-based observations (Formisano et al. 2004; Geminale et al. 2008). Significant spatial and temporal variations are seen (Mumma et al. 2008, Villanueva et al. 2008, Geminale et al. 2008) Results from the LMD General Circulation Model (Forget et al. 1999) suggest that gases with lifetimes shorter than 10⁷ seconds exhibit strong spatial variability, whereas gases with a longer lifetime are expected to be well mixed after a while (Forget et al. 2005). This spatial variation can also be caused by a not continuous source, because the atmosphere is then not in equilibrium with the gases. Observations with the Planetary Fourier Spectrometer on Mars Express over the course of 2 years confirm these modeled results by showing a decrease in CH₄ from 21 ppbv in the Northern spring to 5 ppbv at the end of the Southern summer (Geminale et al. 2008).

One of the CH₄ removal processes at work in the martian atmosphere is photodissociation by solar ultraviolet (UV) radiation, resulting in a lifetime of CH₄ of 250 to 670 years (Wong et al. 2003; Krasnopolsky et al. 2004). However, the observations of Geminale et al. (2008) are in disagreement with this photochemical lifetime.

A much more effective destruction mechanism for CH4 is breakdown through electric discharge induced chemistry (Farrell et al. 2006; Melnik and Parrot 1998; Hackam 1969). This discharge could be induced on Mars by electric fields generated by dust particle interaction in dust storms and dust devils. Theoretical models describe the processes that could occur in the martian atmosphere and provide estimates of breakdown voltages and chemical reaction schemes of breakdown products (Atreya et al. 2007). Modeling suggests that this dust-induced electrochemistry can significantly increase the destruction rate of methane, both via direct dissociation and the enhanced production of OH- and H- (Delory et al. 2006). At electric fields above 10 kV/m the destruction of methane is more efficient through electrochemistry than photochemistry, with loss times dropping from 1010 seconds (320 year, at 10 kV/m) to less than 1000 seconds (above 25 kV/m) (Farrell et al. 2006).

In order to test these predicted breakdown scenarios, we have built two facilities to simulate these processes, a static discharge facility (Figure 1) and a dust circulation facility (Figure 2).

The static discharge facility consists of two chambers, one of which is kept at hard vacuum (10^-6 mbar) and features a conventional quadrupole mass spectrometer (QMS) to measure neutral and ion abundances. The second chamber — the actual discharge chamber —is mounted inside the QMS chamber and consists of a stainless steel tube capped with a stainless steel lid with a 23 μm hole laser-drilled into it. This chamber is kept at martian near-surface pressure of 6-10 mbar. Discharge processes are induced in this chamber by applying a voltage difference between a high voltage plate inside the chamber and the lid. The hole in the lid is positioned directly in front of the QMS detector, such that breakdown products created by the discharge can be measured directly, avoiding wall and inter-particle interaction.

The dust circulation chamber (Figure 2) is a modification of a chamber built to test dust filters for the SAM instrument. The chamber consists of a 2 liter, ~10 cm diameter cylinder, with 3 gooseneck-shaped tubes integrated into the bottom. Two plates serving as cathode and anode will be mounted inside the chamber in order to enhance the electrification effect to better study the effects on the chemistry if necessary. An electrometer (Jackson and Farrell, 2006) is mounted inside the dust chamber to directly measure the electric field in the dust storm. Furthermore, a mass spectrometer is attached to the system that analyzes high-pressure gas through a capillary system, allowing us to detect methane.

During the experiments in the static chamber, the voltage difference between the plates is slowly increased. This voltage difference and the current between the plates are measured. An oscilloscope is attached to the system and triggers when discharge occurs. Discharge current and voltage are stored and spectra are recorded with the QMS. This QMS is used to detect reaction products, and spectra are taken to estimate breakdown and production rates of species destroyed and formed during the discharge process. Figure 3 shows a spectrum of CO₂ that is ionized by the discharge. When the voltage is below the discharge level the spectrum shows nothing, indicating that the discharge is an effective ionizer.

The dust-chamber is still under development; no dust experiments have been conducted so far. The experimental approach will be as follows: the dust will be cleaned and baked to remove any organic material and residual water, the discharge chamber will be evacuated, the dust will be introduced through the valve system on the top and will accumulate at the bottom, and the chamber will be filled with the desired gas mixture. The gas mixture will be blown through the goosenecks onto the dust layer. By pulsing the gas the dust will be excited and will start moving around in the chamber. Mass spectra will be recorded in various intervals and the electric field will be continuously monitored. For these experiments the most commonly used martian soil analogue JSC-1 (Allen et al. 1998) will be used.

Invited Reviews, Seminars, and Conference Presentations:

Ten Kate gave a poster presentation entitled “Laboratory simulations of discharge in the martian atmosphere” at the Gordon Conference on the Origin of Life (21-24 January 2008, Ventura, CA USA).

Furthermore she participated in the “Cyanobacteria in Lunar environments” workshop (28-30 January 2008, NASA Ames), where she presented the VAPoR instrument in an oral presentation entitled “VAPoR: Analyzing Lunar Regolith by Pyrolysis Mass Spectrometry”.

Ten Kate gave an oral presentation entitled “Laboratory investigations of discharge in the martian atmosphere” at the Astrobiology Science Conference (14-17 April 2008, Santa Clara, CA, USA).

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