Titan’s atmosphere: An optimal gas mixture for aerosol production?

Different types of plasma experiments have been developed to study Titan’s chemistry directly from N2–CH4 gas mixtures. Most of these plasma experiments produce tholins by deposition as thin films onto solid surfaces instead of producing them in the gas phase only, as in Titan’s atmosphere. It is not the case however of the plasma experiment called PAMPRE (French acronym for Production d’Aérosols en Microgravité par Plasma Réactif).

In PAMPRE, tholins are produced in the gas phase using a low pressure Radio Frequency Capacitively Coupled Plasma (RF CCP) discharge produced between two electrodes.

In the PAMPRE setup, the chemistry between N2 and CH4 is induced by the plasma discharge. Previous studies have shown that the solid particles, or tholins, produced in PAMPRE are spherical in shape with mean diameters varying from 90 nm to 1 lm depending on the experimental conditions.

The PAMPRE experiment was used to produce tholins in different N2–CH4 gas mixtures in order to study the influence of the methane concentration on the production rates and composition of the tholins.

Evolution of the CH4 concentration in the gas phase with time in gas mixtures with [CH4]0 = 1%, 2%, 4%, 6%, 8%, and 10%, at p = 0.9 mbar. The plasma was turned on after 50 s. The steady state was reached after 150 s for all conditions, giving the steady state CH4 concentration, [CH4]SS, which corresponds to the CH4 concentration in the gas phase during the tholin production.

Mass of CH4 consumed, MCH4cons , (in mg h_1) in the gas phase, as a function of initial CH4 concentration for p = 0.9 mbar (black diamonds) and p = 1.7 mbar (white diamonds). The CH4 consumption is linear up to [CH4]0 = 6%, for p = 0.9 mbar, and up to [CH4]0 = 8% for p = 1.7 mbar. A linear regression taking into account the uncertainty on MCH4cons gives slopes of 17.21 ± 0.54 mg h_1 and 15.91 ± 0.40 mg h_1 for p = 0.9 mbar and 1.7 mbar, respectively.

CH4 consumption efficiency as a function of initial CH4 concentration for p = 0.9 mbar (black diamonds) and p = 1.7 mbar (white diamonds).

Tholin production rate as a function of initial CH4 concentration, for p = 0.9 mbar (black diamonds) and p = 1.7 mbar (white diamonds).

Elemental composition of tholins as a function of initial (bottom horizontal axis) and steady state (top horizontal axis) CH4 concentrations, for p = 0.9 mbar: H, black diamonds; C, crosses; N, triangles; O, squares.

Elemental composition of tholins as a function of initial (bottom horizontal axis) and steady state (top horizontal axis) CH4 concentrations, for p = 1.7 mbar: H, black diamonds; C, crosses; N, triangles; O, squares.

Thermogravimetry analysis of tholins produced in a 10% CH4 gas mixture, at p = 0.9 mbar.

C/N ratio as a function of initial CH4 concentration for tholins produced at p = 0.9 mbar (black diamonds) and p = 1.7 mbar (white diamonds).

Carbon gas to solid conversion rate as a function of the initial (bottom horizontal axis) and steady state (top horizontal axis) CH4 concentrations for p = 0.9 mbar and p = 1.7 mbar.