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 (C 2 H 2 ) on cosmic dust analogues over the temperature range encountered in Titan's atmosphere below 600 km. The uptake coefficient, γ , 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 C 2 H 2 on Mg 2 SiO 4 , MgFeSiO 4 and Fe 2 SiO 4 , respectively, indicating that γ is independent of Mg or Fe active sites. The uptake of C 2 H 2 was also measured on SiO 2 and SiC as analogues for meteoric smoke particles in Titan's atmosphere, but was found to be below the detection limit (γ −8 and -7 , respectively). The rate of cyclo-trimerization of C 2 H 2 to C 6 H 6 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 µm) entering Titan's atmosphere do not ablate ( 6 H 6 via uptake of C 2 H 2 on cosmic dust, followed by cyclo-trimerization and desorption, is probably competitive with gas-phase production of C 6 H 6 between 80 and 120 km.