Uptake of acetylene on cosmic dust and production of benzene in Titan's atmosphere

Abstract

A low-temperature flow tube and ultra-high vacuum apparatus were used to explore the uptake and heterogeneous chemistry of acetylene (C2H2) on cosmic dust analogues over the temperature range encountered T below 600 km. The uptake coefficient, , was measured at 181 K to be (1.6 ± 0.4) × 10 -4 , (1.9 ± 0.4) × 10 -4 and (1.5 ± 0.4) × 10 -4 for the uptake of C2H2 on Mg2SiO4, MgFeSiO4 and Fe2SiO4, respectively, indicating that  is independent of Mg or Fe active sites. The uptake of C2H2 was also measured on SiO2 and SiC as analogues for meteoric smoke particles T but was found to be below the detection limit ( < 6 × 10 -8 and < 4 × 10 -7 , respectively). The rate of cyclo-trimerization of C2H2 to C6H6 was found to be 2.6 × 10 -5 exp(-741/T) s -1 , with an uncertainty ranging from ± 27 % at 115 K to ± 49 % at 181 K. A chemical ablation model was used to show that the bulk of cosmic dust particles (radius 0.02 -10  T mass loss through sputtering), thereby providing a significant surface for heterogeneous 2 chemistry. A 1D model of dust sedimentation shows that the production of C6H6 via uptake of C2H2 on cosmic dust, followed by cyclo-trimerization and desorption, is probably competitive with gas-phase production of C6H6 between 80 and 120 km.

Impacts of Cosmic Dust on Planetary Atmospheres and Surfaces

“…The main ionospheric peak in Titan was located at 1180 ± 150 km during the Voyager I fly-by (Bird et al 1997). More recently, Cassini radio occultation measurements show that there is a secondary intermittent layer between 500 and 600 km, which coincides with the region where ablation is predicted to occur (Frankland et al 2016).…”

“…There has been a great deal of work in the past decade on meteoric smoke particles. This has included laboratory investigations into MSP formation (Saunders and Plane 2006) and ice-nucleating ability (Nachbar et al 2016); detection in the terrestrial atmosphere by rocket payloads , radar ) and optical extinction (Hervig et al 2009); and modelling impacts through providing a surface for heterogeneous chemistry (Frankland et al 2016;James et al 2017b) and ice nuclei for the formation of mesospheric clouds on Earth (Rapp and Thomas 2006;Hervig et al 2012) and Mars (Määttänen et al 2013;Listowski et al 2014). The role of MSPs in the formation of sulphuric acid particles in Venus, and polar stratospheric clouds in the terrestrial atmosphere, still needs to be better understood.…”

“…2, there is a relative lack of measurements in the outer solar system which has necessitated the use of simplifying assumptions regarding the particle mass/velocity distributions; however, the dust model of Poppe (2016) now provides these distributions for ablation modelling. This was recently applied to Saturn's moon Titan (Frankland et al 2016). Due to the large masses and hence deep gravitational wells of the giant planets, the minimum meteoroid entry velocities are 59.5, 35.5, 21.3, and 23.5 km s for Jupiter, Saturn, Uranus, and Neptune, respectively.…”

“…As further concluded by Moses and Poppe (2017), interplanetary dust plays a minor role in the delivery of oxygen and other exogenous species to Saturn's atmosphere. Frankland et al (2016) investigated the role of meteoric material in converting acetylene (C 2 H 2 ) into C 6 H 6 in Titan's atmosphere. Because of the relatively small mass of the moon, dust particles enter at low speeds (<29 km s −1 ) and encounter an atmosphere with a large scale height (∼40 km, so the pressure increases gradually compared to the Earth or Venus).…”

“…Hence, a much smaller fraction of the particles ablate compared with the terrestrial planets, and unablated meteoroids-rather than MSPs-provide most of the surface area for heterogeneous chemistry. The kinetics of C 2 H 2 uptake and cyclo-trimerization into C 6 H 6 were measured at low temperatures in the laboratory and input into a 1D model, which shows that the heterogeneous synthesis of C 6 H 6 on cosmic dust is likely to be competitive with the gas-phase production of C 6 H 6 between 80 and 120 km (Frankland et al 2016).…”