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 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 CH 4 , CH 3 D, and C 2 H 6 , 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.

Experimental Simulation of Titan's Stratospheric Photochemistry: Benzene (C6H6) Ices

“…In addition, Courtin et al. (2015) demonstrated the coating of condensates below 300 km could impact the ratio between aromatics and aliphatics. In addition, experimental works performed by Couturier‐Tamburelli et al.…”

“…In our study, the benzene residue formed in our experiments results from photo-processed ices without considering further aging processes such as that induced by the accretion of condensates at their surface. In addition, Courtin et al (2015) demonstrated the coating of condensates below 300 km could impact the ratio between aromatics and aliphatics. In addition, experimental works performed by Couturier-Tamburelli et al (2018) highlighted a possibility of photo-insertion of hydrogen atoms into a nitrile-based residue coated with HCN ice.…”

Titan's atmosphere and climate

“…They identify CH 4 , C 2 H 6 , CH 3 CN, C 5 H 12 , C 6 H 12 , and C 6 H 14 as possible constituents based on the VIMS measurements from ∼60 to 500 km [ Kim et al , ; Sim et al , ; Kim and Courtin , ]. Analysis of VIMS solar occultation measurements from 250 to 700 km show composition variations that may indicate aerosol aging and/or condensation onto particles as the particles become more aliphatic in character with decreasing altitude [ Courtin et al , ].…”

The 3.4μm absorption in Titan’s stratosphere: Contribution of ethane, propane, butane and complex hydrogenated organics

“…But this explanation is not completely satisfactory, mainly because we should find absorptions in the wavelength range 3.38-3.48 µm in high altitude occultation spectra which is not the case (Courtin et al, 2015). In this latter reference, the authors do not observe strong features (in extinction) in this wavelength range above 500 km but rather below 480 km (Courtin et al, 2015 , Fig. 2).…”

“…This ratio is about 3 at 700 km and decreases to about 0.5 below 300 km. Then the conclusion of Courtin et al (2015) is a growth of molecules from PAHs to more complex organics by particle-aging and coating process when the altitude decreases. So, even if the explanation in Dartois et al (2004Dartois et al ( , 2005 about the HACs production in the ISM is not completely relevant for Titan's upper atmosphere, it is possible to have HACs at low altitude, by particle-aging and coating process.…”