Constraints on Uranus' Methane Distribution at Equinox

Jim Norwood

Methane is the third most abundant component in Uranus' atmosphere and plays a major role in the thermodynamics, structure, and photochemistry therein. However, the methane abundance itself is poorly known, with post-Voyager estimates ranging from a mixing ratio of 2.3% to below 1% (Lindal et al. 1987, JGR 92, 14987-15001; Sromovsky and Fry 2006, BAAS, 28, 489). Further complicating matters is the possibility of temporal variations in the methane abundance due to Uranus' dramatic seasonal changes.

The primary goal of this project is to determine the vertical profile of methane at Uranus' present equinoctial state. A radiative transfer code will simulate the Uranian atmosphere, employing the recently calculated methane absorption coefficients of Irwin et al. (2006, Icarus 181, 309-319). Center-to-limb brightness profiles and near-infrared spectra will be generated and compared with spectra acquired at NASA's Infrared Telescope Facility. The properties of the model Uranian atmosphere, including the vertical methane profile, will be varied to determine which model atmosphere best reproduces observations. Horizontal variations in Uranus' methane abundance will be investigated using IRTF spectra covering eighteen latitudes across the planetary disk. The near-infrared spectra of Fink and Larson (1979, ApJ 223, 1021-1040) will be similarly analyzed to investigate seasonal evolution in Uranus' methane abundance.