A climatological study of the composition of Titan upper atmosphere from VIMS-IR soundings in limb geometry has been carried out for HCN, C2H2, CH4 and PAH (Polycyclic Aromatic Hydrocarbons) [4,5]. Results of this study are presented here for the period 2004-2012. Ten years ago, on June 2004, NASA's Cassini spacecraft made its Saturn Orbit Insertion (SOI) for an initially scheduled four-year tour of the Saturnian system. At the end of the very successful prime mission all instruments and major spacecraft systems were healthy and NASA Headquarters allocated funding to extend Cassini mission initially for a 2-year period and successively until 2017. The unusually long extent of the Cassini mission has given to all the instrument teams the opportunity to extend their studies to cover an half season of the Saturnian system: short after the winter (SOI) until the summer solstice (end of mission). In the Titan atmosphere above 500 km the mean free path of the molecules is small enough that their emissions due to non-Local Thermal Equilibrium (non-LTE) can be observed. Around 3 ?m the HCN, C2H2, CH4 and PAH molecules have strong transitions and their non-LTE emissions can be used to retrieve the respective Volume Mixing Ratio (VMR) under daylight conditions. The Visible and Infrared Mapping Spectrometer (VIMS) is the only instrument aboard Cassini sounding that IR range of wavelengths. Therefore the non-LTE emission detected by VIMS can be used to study the latitudinal and seasonal variability of these molecules in the thermosphere (500-1200 km). A database of geo-located and time resolved vertical distributions of the gases in the lower thermosphere has been developed from a selection of VIMS limb observations for the 2004-2012 time period. The observations have been selected on the base of long integration times (600 ms) and phase angles lower than 90° [1,2]. To calculate the HCN, C2H2 and CH4 vibrational temperatures the Generic RAdiative traNsfer AnD non-LTE population Algorithm (GRANADA) non-LTE code [3], originally developed for the Earth's atmosphere, has been adapted to Titan's atmosphere [1,2]. Using the computed vibrational temperatures,Titan's limb atmospheric emission has been inverted with the Geofit Broad Band (GBB) radiative transfer code, adapted to Titan atmosphere, upgraded for VIMS instrumental characteristics and non-LTE formalism. More details on the used algorithms can be found in [2]. Starting from the retrieved VMR profile, for each limb vertical scan, a study on the latitudinal and seasonal variability of HCN, C2H2 and CH4 molecules has been carried out, and the results are shown here. For PAH only a study of its spectral contribution has been performed, due to the uncertainty in the spectroscopic data of this ensemble of molecules. Future developments foresee the extension of this study to CO and CH3D molecules.
Climatological Study in the Titan Upper Atmosphere for HCN, C2H2, CH4 and PAH Molecule Concentrations
BM Dinelli;ML Moriconi;F Fabiano
2015
Abstract
A climatological study of the composition of Titan upper atmosphere from VIMS-IR soundings in limb geometry has been carried out for HCN, C2H2, CH4 and PAH (Polycyclic Aromatic Hydrocarbons) [4,5]. Results of this study are presented here for the period 2004-2012. Ten years ago, on June 2004, NASA's Cassini spacecraft made its Saturn Orbit Insertion (SOI) for an initially scheduled four-year tour of the Saturnian system. At the end of the very successful prime mission all instruments and major spacecraft systems were healthy and NASA Headquarters allocated funding to extend Cassini mission initially for a 2-year period and successively until 2017. The unusually long extent of the Cassini mission has given to all the instrument teams the opportunity to extend their studies to cover an half season of the Saturnian system: short after the winter (SOI) until the summer solstice (end of mission). In the Titan atmosphere above 500 km the mean free path of the molecules is small enough that their emissions due to non-Local Thermal Equilibrium (non-LTE) can be observed. Around 3 ?m the HCN, C2H2, CH4 and PAH molecules have strong transitions and their non-LTE emissions can be used to retrieve the respective Volume Mixing Ratio (VMR) under daylight conditions. The Visible and Infrared Mapping Spectrometer (VIMS) is the only instrument aboard Cassini sounding that IR range of wavelengths. Therefore the non-LTE emission detected by VIMS can be used to study the latitudinal and seasonal variability of these molecules in the thermosphere (500-1200 km). A database of geo-located and time resolved vertical distributions of the gases in the lower thermosphere has been developed from a selection of VIMS limb observations for the 2004-2012 time period. The observations have been selected on the base of long integration times (600 ms) and phase angles lower than 90° [1,2]. To calculate the HCN, C2H2 and CH4 vibrational temperatures the Generic RAdiative traNsfer AnD non-LTE population Algorithm (GRANADA) non-LTE code [3], originally developed for the Earth's atmosphere, has been adapted to Titan's atmosphere [1,2]. Using the computed vibrational temperatures,Titan's limb atmospheric emission has been inverted with the Geofit Broad Band (GBB) radiative transfer code, adapted to Titan atmosphere, upgraded for VIMS instrumental characteristics and non-LTE formalism. More details on the used algorithms can be found in [2]. Starting from the retrieved VMR profile, for each limb vertical scan, a study on the latitudinal and seasonal variability of HCN, C2H2 and CH4 molecules has been carried out, and the results are shown here. For PAH only a study of its spectral contribution has been performed, due to the uncertainty in the spectroscopic data of this ensemble of molecules. Future developments foresee the extension of this study to CO and CH3D molecules.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
