A Non-LTE model for the atmosphere of Titan and implications for the Cassini CIRS experiment

Details A Non-LTE model for the atmosphere of Titan and implications for the Cassini CIRS experiment A Non-LTE model for the atmosphere of Titan and implications for the Cassini CIRS experiment

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7023m&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7023

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The Composite Infrared Spectrometer (CIRS) on the Cassini/Huygens mission is a pair of interferometers designed to measure infrared emissions over wavelengths from 7 to 1000 micrometers (1400 to 10 cm-1). CIRS will address a wide variety of science objectives for the atmospheres of Saturn and Titan, and for Saturn's icy satellites and rings. In particular, CIRS measurements of the limb emission spectra from the atmosphere of Titan in the mid- and near-infrared will enable the retrieval of temperature, pressure and species abundances. They can also be used to study the energetics of radiatively active species in regions of Titan's atmosphere where Local Thermodynamic Equilibrium (LTE) is expected to break down. The principle of atmospheric sounding is based on the assumption that the atmospheric compounds emit according to the Planck function at the local kinetic temperature. However many of the ro-vibrational states of atmospheric constituents responsible for infrared emissions have excitation temperatures that differ from the local kinetic temperature. These non-LTE emissions must be taken into account in the inversion of infrared measurements taken by spaceborne instrumentation. While non-LTE situations have been studied extensively for remote sensing of the Earth (see Lopez-Puertas and Taylor, 2001), only one study, that by Yelle (1991), has been performed for the case of the atmosphere of Titan. In this paper, that work is re-assessed and extended to consider the potential impact of non-LTE effects in CH_4, C_2H_2, C_2H_6, HCN, and other 10 species in the Titan atmosphere spectra to be measured by the CIRS instrument. References: R. Yelle, Astrophysical Journal, 1991, vol. 383, 380-400. M. Lopez-Puertas, and F. W. Taylor, 2001, "Non-local thermodynamic equilibrium in atmospheres". World Scientific Publishing.

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