Methane for Atmospheric and Planetary Applications: What Spectroscopic Parameters Do We Need?

The spectroscopic knowledge of methane is important to study the greenhouse effect and some planetary atmospheres. The analysis of atmospheric and planetary spectra can be performed through line-by-line computations or through cross-sections data. When available with enough accuracy, line parameters allow calculations under any conditions of pressure, temperature and absorption path. Many works have been published on methane, leading to a huge line list (more than 250,000 transitions) in the HITRAN 2004 database. However, for most of the transitions between 1 and 1.65 µm), energies of the lower levels (E00) are unknown. Consequently, line intensities known at the standard temperature (296K) cannot be calculated for the temperatures of planetary atmospheres or of high layers of our atmosphere. On the other hand, cross sections can be used with enough accuracy only when the experimental conditions are close enough to those of the studied atmosphere. This requires a set of cross sections for each experiment and atmospheric conditions. As example, new measurements of methane cross sections had to be recently performed1 under Titan surface similar conditions and matching the experimental recording conditions of the DISR (DISR-Descent Imager/Spectral Radiometer) of the Huygens probe landed in January 2005 on Titan. The experimental device used to record these cross sections is presented, together with some typical results and their use to deduce methane amount on Titan surface. To get more versatile data, we propose a method to deduce E00 values from absorption spectra, without knowing complete assignments, but precise enough to allow calculation of line intensities at any temperatures of interest. To test the feasibility of this method, two spectra (at 120K and 300K) are simultaneously adjusted using a multispectrum fitting procedure in which E00 is a free parameter. Some examples of E00 retrievals for lines already assigned in HITRAN show the efficiency of the method.