Three-micron extinction of the Titan haze in the 250–700 km altitude range: Possible evidence of a particle-aging process

Abstract

Context. The chemical nature of the Titan haze is poorly understood. The investigation carried out by the Cassini-Huygens suite of instruments is bringing new insights into this question.Aims. This work aims at deriving the vertical variation of the spectral structure of the 3.3–3.4 μ m absorption feature of the Titan haze from Cassini VIMS solar occultation data recorded between 250 and 700 km altitude.Methods. We computed the transmittance of Titan’s atmosphere using a spherical shell model and a radiative transfer code including the influence of CH4 , CH3 D, and C2 H6 , as well as the effects of absorption and scattering by the haze particles. We derived the haze extinction from a comparison of the synthetic spectra with the VIMS solar occultation spectra.Results. We find a marked change in the relative amplitudes of the 3.33 and 3.38 μ m features, which are characteristic of aromatic (double C=C chains or rings) or aliphatic (single C–C chains) structural groups, respectively. The pseudo-ratio of aromatics to aliphatics (uncorrected for the absolute band strengths) varies from 3.3 ± 1.9 at 580−700 km to 0.9 ± 0.1 at 350−450 km, and is 0.5 ± 0.1 around 250 km. The structural change from the aromatic to the aliphatic type between 580 and 480 km appears to correspond to a spontaneous aging of the particles – a transition between unannealed and hardened particles – while the further decrease of the pseudo-ratio of aromatics to aliphatics below 480 km may be related to the coating of the core particles by condensates such as heavy alkanes.