Near-IR methane absorption in outer planet atmospheres: Improved models of temperature dependence and implications for Uranus cloud structure

Near-IR absorption of methane in the 2000-9500 cm⁻¹ spectral region plays a major role in outer planet atmospheres. However, the theoretical basis for modeling the observations of reflectivity and emission in these regions has had serious uncertainties at temperatures needed for interpreting observations of the colder outer planets. A lack of line parameter information, including ground-state energies and the absence of weak lines, limit the applicability of line-by-line calculations at low temperatures and for long path lengths, requiring the use of band models. However, prior band models have parameterized the temperature dependence in a way that cannot be accurately extrapolated to low temperatures. Here we use simulations to show how a new parameterization of temperature dependence can greatly improve band model accuracy and allow extension of band models to the much lower temperatures that are needed to interpret observations of Uranus, Neptune, Titan, and Saturn. Use of this new parameterization by Irwin et al. Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Bowles, N., Calcutt, S.B., 2005b. Icarus. In press] has verified improved fits to laboratory observations of Strong et al. Strong, K., Taylor, F.W., Calcutt, S.B., Remedios, J.J., Ballard, J., 1993. J. Quant. Spectrosc. Radiat. Trans. 50, 363-429] and Sihra 1998. Ph.D. Thesis, Univ. of Oxford], which cover the temperature range from 100 to 340 K. Here we compare model predictions to 77 K laboratory observations and to Uranus spectra, which show much improved agreement between observed and modeled spectral features, allowing tighter constraints on pressure levels of Uranus cloud particles, implying that most scattering contributions arise from pressures near 2 bars and 6 bars rather than expected pressures near 1.25 and 3.1 bars. Between visible and near-IR wavelengths, both cloud layers exhibit strong decreases in reflectivity that are indicative of low opacity and submicron particle sizes.