Large abundances of PAHs in Titan's Upper Atmosphere

In this paper we present the analysis of the Cassini/VIMS limb daytime observations of Titan upper atmosphere in the 3.3 um region. This spectral region is dominated by the strong CH4 bands. However, we have found that, in addition to CH4, there is an unidentified emission peaking near 3.28 um. This emission is very intense, comparable to that from the strong bands of CH4, with a maximum limb intensity at about 950 km and extending from about 600 km up to about 1250 km. The analysis of this emission shows that it cannot be explained by emission from CH4 nor from the major hydrocarbons compounds of Titan upper atmosphere. Many other potential candidates have been discarded and we suggest that it is caused by aromatic hydrocarbons. The emission has then been analyzed by using the NASA Ames PAH IR Spectroscopic Database and found that the PAH spectral emission fit very well the unidentified feature. Based on excitation models for PAHs we have been able to explain very well the observed spectral feature and to derive the vertical distribution of the PAHs abundances in Titan upper atmosphere. The derived large concentrations of PAHs (2-3 10^4 particles/cm^3 at about 1000 km) together with its vertical structure suggest that they are the neutral counterpart of the small concentrations of the heavy ions measured by Cassini/CAPS. This finding is very important since these PAHs represents the missing link in the theory that the origin of Titan stratospheric main haze layer is in the upper atmosphere.