Revisiting PAHs contribution to Titan’s upper atmosphere.

Motivation. The atmosphere of Titan is a key environment for studying complex organic chemistry in a cold, oxygen-poor and nitrogen-rich environment. Photochemistry of CH₄ triggers a rich chemistry, leading to the detection of several organic molecules, including benzene [1]. In addition, Polycyclic Aromatic Hydrocarbons (PAH) molecules have been invoked to explain the signal leftover from the non-LTE modelling of CH₄ in VIMS limb data [2]. Nevertheless, questions remain about PAH formation and evolution on Titan.

Methodology. In this work, we reanalyse Cassini VIMS limb spectra using a new extended version of the NASA Ames PAH IR Spectroscopic Database [3], containing more than 3000 PAH molecules, with sizes up to 169 rings. We model the PAH emission mechanism, considering the solar irradiance spectrum at the date of the observations and extending the photo-absorption cross-section down to the near infrared. A Non-Negative Least Square (NNLS) fitting is used to obtain information on average PAH size that best fits the VIMS data at 1000, 950 and 900 km and uncertainty on this value is evaluated using a Monte Carlo technique. We included both neutral and anions PAHs. We follow up with Non-Negative Least Chi-square minimisation [4] fitting to gain insight on the type of single PAHs that could reproduce the VIMS residual features.

Results. Preliminary results show that, even considering a larger database of PAHs with different sizes, the average number of aromatic rings of the best-fitting PAHs is only slightly higher than the 10-11 rings found in the earlier study [1]. Also, surprisingly, the largest PAHs are found at higher altitude, up to 1000 km, which is counterintuitive given that PAHs are believed to be the seeds for aerosols formation and their size should increase as the altitude decreases. This could be explained if new formation routes, for example ion-molecule reactions, currently not included in the models, are available at these altitudes. Averaging data over different dates and geolocations could also have an effect. When looking at which single PAH molecule best fit the data, we find the asymmetric, catacondensed PAHs are preferred. These findings challenge existing models of PAH growth and survival in Titan’s atmosphere and call for new modelling efforts.

References:

[1]  Coustenis A. et al. 2003 Icar 161 383

[2] M. López-Puertas et al 2013 ApJ  770 132

[3] C.W. Bauschlicher Jr. et al 2018 ApJS 234 32

[4] P Désesquelles et al 2009 J. Phys. G: Nucl. Part. Phys. 36 037001