Simulation of Titan haze formation using a photochemical flow reactor: The optical constants of the polymer

Abstract

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn's magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real (n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6±0.1 throughout the 0.2–2.5 μm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k( λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119–130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185–190). The slopes of the k( λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127–137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515–529) were also comparable to Titan's. These finding show that the k( λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2–2.5 microns). There is a large difference between the k( λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127–137; Coll et al., 1999, Planet. Space Sci. 47, 1331–1340; Khare et al., 2002, Icarus 160, 172–182). The C/N ratio in the haze analog prepared by discharges is in the 2–11 range while that of the photochemical analogs is in the 18–24 range. The use of discharges and UV light for initiating the chemistry in Titan's atmosphere is discussed.