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

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 μm 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, CH3D, and C2H6, 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 μm 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