ANOMALOUS EMISSION IN THE 3.28 µm IN THE TITAN UPPER ATMOSPHERE

Earliest Cassini VIMS limb observations at Titan taken in October 26th, 2004 show a strong methane non- LTE limb emission at high atmospheric altitudes. During that pass at Titan, VIMS vertical resolution was about 110 km and the analyzed spectral interval corresponds to the methane emission band centered around 3.33 micron. A detailed analysis of the radiances versus altitudes shows an anomalous emission at altitudes higher than 900 km at wavelengths corresponding to the methane R branch around 3.28 microns. The nature of such emission is under investigation. Different spectral databases and codes both for calculating the methane expected non-LTE emissions and for the retrieval of limb observations have been used. The anomalous emission could not be reproduced using all the available data on methane in the simulations. According to the spectral position it can be guessed that the emission should belong to a molecule containing C-H or C-N bonds. However, different molecules and ions, whose spectral characteristics can be found in literature, have been tested and none could properly fit the data. The observation on Oct. 26th, 2008 was chosen for its very good signal to noise ratio and because of the favorable illumination of the atmosphere as seen from Cassini spacecraft (i.e. low phase angle). Successive observations confirm such a finding