Spatially resolved cloud structure on Uranus: Implications of near-IR adaptive optics imaging

Seven-band near-IR adaptive optics imaging of Uranus by the Keck 11 telescope during 2004, with the assistance of selected Hubble Space Telescope images, provides new constraints on the uranian vertical cloud structure and CH(4) mixing ratio, after tuned deconvolutions are applied to remove significant limb darkening distortions. The most strongly absorbing bands approximately agree with the stratospheric haze model of Rages et al. [Rages, K., Pollack, J.B., Tomasko, M.G., Doose, L.R., 1991. Icarus 89, 359-376]. The next most absorbing bands suggest a CH4 relative humidity of 50-60% above the 1.2-bar condensation level. Window channels imply effective cloud pressures at 12 degrees S that vary from 9 to 3.5 bars, and reflectivity values that vary from 7 to 4%, as the assumed CH4 mixing ratio varies from 0.75 to 4%. The shape of the center-to-limb radiance profile is in best agreement with the deep cloud being translucent, with relatively low optical depth, and is most consistent with low methane mixing ratios (0.75-1 %) if the cloud particles are conservative. Non-conservative particles provide good fits over a wide range of mixing ratios. If C and S are enhanced by the same factor over solar mixing ratios, then the cloud pressures inferred from near-IR observations would be less than H(2)S condensation pressures for methane mixing ratios of similar to 1.3% or greater. The bright band at 45 degrees S must be partly produced by increased particulate scattering at pressures similar to 2 bars to be consistent with its absence in 1.9-mu m images and its presence in 0.619-mu m images. The reflectivity of the lower clouds declines to nearly negligible values in the northern hemisphere, where 1/F observations beyond 50 degrees N are nearly those of a clear atmosphere. The most surprising result is the general lack of scattering originating from the 1.2-bar region where methane is expected to condense. Exceptions occur for discrete features. A large and long-lived discrete feature at 34 degrees S is associated with particulates near 700 mb and similar to 4.5 bars. The highest discrete feature, near 26 degrees N, reached pressures similar to 200 mb and was eleven times brighter than the background atmosphere in K' images. (c) 2007 Elsevier Inc. All rights reserved.