Methane on Uranus: The case for a compact CH4 cloud layer at low latitudes and a severe CH4 depletion at high-latitudes based on re-analysis of Voyager occultation measurements and STIS spectroscopy

Abstract

Lindal et al. (1987, J. Geophys. Res. 92, 14987-15001) presented a range of temperature and CH4 profiles for Uranus that were consistent with 1986 Voyager radio occultation measurements. A localized refractivity slope variation near 1.2 bars was interpreted to be the result of a condensed CH4 cloud layer. However, models fit to near-IR spectra found particle concentrations in the 1.5-3 bar range (Sromovsky et al. 2006, Icarus 182, 577-593, Sromovsky and Fry 2008, Icarus 193, 211-229, Irwin et al. 2010, Icarus 208, 913-926), and a recent analysis of STIS spectra argued that aerosol particles formed diffusely distributed hazes, with no compact condensation layer (Karkoschka and Tomasko 2009, Icarus 202, 287-309). Trying to reconcile these results, we reanalyzed the occultation observations with a He volume mixing ratio reduced from 0.15 to 0.116, which is near the edge of the 0.033 range given by Conrath et al. (1987, J. Geophys. Res., 15003-10). This allowed us to obtain saturated CH4 mixing ratios within the putative cloud layer and to reach above-cloud and deep CH4 mixing ratios compatible with STIS spectral constraints. Using a 5-layer vertical aerosol model with two compact cloud layers in the 1-3 bar region, we find that the best fit pressure for the upper layer is virtually identical to the pressure range inferred from the occultation analysis for a methane mixing ratio near 4% at 5 deg S, arguing that Uranus does indeed have a compact methane cloud layer. While our cloud model can fit the latitudinal variations in spectra between 30 deg S and 20 deg N using the same temperature and CH4 profiles, closer to the pole, the model requires the introduction of an increasingly strong upper tropospheric depletion of CH4 at increased latitudes, in rough agreement with the trend identified by Karkoschka and Tomasko (2009, Icarus 202, 287-309).