Modeling the CH4 3.3 µm non-LTE emissions in Jupiter and Saturn

International audienceThe homopause divides planetary atmospheres into the high-pressure region where chemical species are well mixed by eddy diffusion and the low-pressure region where molecular diffusion separates species according to their individual masses. In Jupiter and Saturn, the homopause pressure level controls the methane abundance vertical profile, as methane is heavier than the two dominant species, H<SUB>2</SUB> and He. Methane plays a critical role in establishing the thermal structure on these planets. It is the prime near-infrared absorber that warms the Jovian and Kronian upper atmospheres, and its photolysis by solar UV radiation triggers the production of ethane, acetylene, and heavier hydrocarbons that are the prime far-infrared coolants in these upper atmospheres. Hence the homopause level drives the heating rates, and, as ethane is mainly produced by the three-body methyl recombination (2CH<SUB>3</SUB> M -> C<SUB>2</SUB>H<SUB>6</SUB> M) whose reaction rate highly varies with pressure, the homopause level also controls the cooling rates. The determination of the methane homopause level in Jupiter and Saturn through solar occultations has been notoriously difficult as different studies and authors led to different results (see Moses et al. (2004), Fouchet et al. (2009) for reviews). Greathouse et al. (2010) even suggested that the Jovian homopause might vary spatially and/or temporally. Here we present a detailed non-LTE model of CH<SUB>4</SUB> 3.3 µm emissions for Jupiter and Saturn's atmosphere. The model accounts for various mechanisms of non-thermal excitation of CH<SUB>4</SUB> molecules as well as inter- and intra-molecular vibrational-vibrational (VV) and vibrational-translational (VT) energy exchanges. With the help of this model, we studied the sensitivity of CH<SUB>4</SUB> 3.3 µm emissions to the temperature and methane abundance vertical profiles and compared integrated radiances with the corresponding Infrared Space Observatory (ISO) observations. We will discuss implications of these results to the interpretation of the homopause pressure level as well as opportunities the JWST telescope will provide to map the homopause across both planets