Non-LTE analysis of Uranus Observations from Spitzer

Spitzer Infrared Spectrometer (IRS) observations of the disk of Uranus between 5.2 and 32 microns (1920 cm-1-270 cm-1) contain a wealth of information about the its cold atmosphere. In particular, they enable the retrieval of temperature and the abundances of several gaseous species as a function of pressure. They can also be used to study the energetics of radiatively active species in regions of Uranus’ atmosphere where Local Thermodynamic Equilibrium (LTE) is expected to break down. Care must be taken in atmospheric sounding not to assume that the atmospheric compounds emit according to the Planck function at the local kinetic temperature. Many of the ro-vibrational states of atmospheric constituents responsible for infrared emissions have excitation temperatures that differ from the local kinetic temperature. While non-LTE emission has been extensively considered for remote sensing of the Earth, only one study by Appleby (Icarus, 85, p355-379, 1990), who examined the radiative equilibrium temperatures of methane (CH4) in the upper atmospheres of Jupiter, Saturn, Uranus, and Neptune, has estimated the influence of non-LTE effects in Uranus' upper atmosphere. Uranus is composed mainly of hydrogen and helium. Helium is radiatively inactive, and hydrogen is active only in its weak collision- induced absorption, and its quadrupole lines. At low pressures, non-LTE processes involve the quantum levels of the more abundant minor constituents become important. In the case of Uranus the most important gas in the latter category is CH4, which has a role rather analogous to that of carbon dioxide in the Earth’s atmosphere. Since the work by Appleby, the spectroscopic and kinetics information of methane and other hydrocarbons has greatly improved and computer capabilities allow avoiding previous simplifications. For example, the full coupling between CH4 v4 and the higher-energy vibrational states emitting/absorbing in the near-IR is now possible. In this presentation we analyze the non-LTE effects in Spitzer data using a new non-local thermodynamic equilibrium radiative transfer for the infrared emissions of CH4, C2H2, and C2H6 which are observed in its spectrum.