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

摘要

Lindal et al. (1987, J. Geophys. Res. 92, 14987-15001) presented a range oftemperature and CH4 profiles for Uranus that were consistent with 1986 Voyagerradio occultation measurements. A localized refractivity slope variation near1.2 bars was interpreted to be the result of a condensed CH4 cloud layer.However, models fit to near-IR spectra found particle concentrations in the1.5-3 bar range (Sromovsky et al. 2006, Icarus 182, 577-593, Sromovsky and Fry2008, Icarus 193, 211-229, Irwin et al. 2010, Icarus 208, 913-926), and arecent analysis of STIS spectra argued that aerosol particles formed diffuselydistributed hazes, with no compact condensation layer (Karkoschka and Tomasko2009, Icarus 202, 287-309). Trying to reconcile these results, we reanalyzedthe occultation observations with a He volume mixing ratio reduced from 0.15 to0.116, 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 mixingratios within the putative cloud layer and to reach above-cloud and deep CH4mixing ratios compatible with STIS spectral constraints. Using a 5-layervertical aerosol model with two compact cloud layers in the 1-3 bar region, wefind that the best fit pressure for the upper layer is virtually identical tothe pressure range inferred from the occultation analysis for a methane mixingratio near 4% at 5 deg S, arguing that Uranus does indeed have a compactmethane cloud layer. While our cloud model can fit the latitudinal variationsin spectra between 30 deg S and 20 deg N using the same temperature and CH4profiles, closer to the pole, the model requires the introduction of anincreasingly 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).