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 Casmini-Huvgens 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 mu m absorption feature of the Titan haze from Cassini VMS 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 mu 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 allphatics (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.