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Project Progress

The search for methane in the Martian atmosscaldwell@arc.nasa.govphere has been implemented from Mars orbit (Formisano et al. 2004) as well as with Earth-based telescopes (Krasnopolsky et al. 2004; Mumma et al. 2004). Methane concentrations in the Martian atmosphere are highly variable, from 10 ppbv globally (Formisano et al. 2004; Krasnopolsky et al. 2004) to 250 ppbv locally (Mumma et al. 2004). Different theories have been proposed to explain this variability. One proposed 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).

In order to test these predicted breakdown scenarios, we have built a laboratory setup to simulate these processes (Figure 1). The facility consists of two chambers, one of which is kept at hard vacuum (10-6 mbar) and features a quadrupole mass spectrometer (QMS) to measure neutral and ion abundances.

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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. Figure 2 shows a schematic drawing of the setup, with an exploded view and detailed picture of the discharge apparatus.
During the experiments, the voltage difference of the plates and the current are measured. The 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. The experimental setup was completed in June 2007 and discharge experiments have been initiated. Figure 3 shows a typical mass spectrum as recorded by the QMS.

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Invited Reviews, Seminars, and Conference Presentations:
Ten Kate presented a poster entitled “Laboratory simulations on discharge in the Martian atmosphere” at the Conference “BioAstronomy 2007” (16 -20 July 2007, San Juan, Puerto Rico) and an oral presentation with the same title at the preceding AbGradCon (14 – 15 July, San Juan).