Model, software and database for line-mixing effects in the ν3 and ν4 bands of CH4 and tests using laboratory and planetary measurements-II: H2 (and He) broadening and the atmospheres of Jupiter and Saturn

The absorption shapes of the ν2, ν3 and ν4 infrared bands of CH4 perturbed by H2 in large ranges of pressure and temperature have been measured in the laboratory. In order to model these spectra, the theoretical approach accounting for line-mixing effects proposed for CH4-N2 and CH4-air and successfully tested in the companion paper (I), is used. As before, state-to-state rotational rates are used together with some empirical parameters that are deduced from a fit of a single room temperature spectrum of the ν3 band at about 50 atm. The comparisons between measured and calculated spectra in the ν3 and ν4 regions under a vast variety of conditions (9-300 atm, 80-300 K) then demonstrate the quality and consistency of the proposed model. In the case of the ν2 band, which is of E symmetry, specific parameters, different from those adapted to the ν3 and ν4 transitions of F2 symmetry, are used for proper modeling of the spectral shape. Furthermore, as shown previously, a broad absorption feature grows underneath the ν2 band with increasing H2 density. The latter, for which an empirical model is proposed, is attributed to a collision-induced absorption (CIA) process in methane. From the developed models, a database and associated software are built for the updating of planetary atmospheres radiative transfer codes. The quality of these tools is then further demonstrated using emission measurements of the Jovian and Saturnian atmospheres in the ν4 region (7-10 μm) recorded by the Short Wave Spectrometer of the Infrared Space Observatory and the Composite Infrared Spectrometer on-board Cassini. Comparisons between measured radiances and predictions confirm the failure of the purely Lorentzian approach and the quality of the proposed line-mixing model. Furthermore, it is shown that the methane CIA contribution has a significant influence on the planetary emission beyond 1400 cm−1.