Clouds and aerosols on Uranus: Radiative transfer modeling of spatially-resolved near-infrared Keck spectra

We observed Uranus in the near-infrared H and K' bands (1.47-238 mu m) in 2010 and 2011 with the OSIRIS imaging spectrograph on the Keck II telescope with adaptive optics. In 2010, three years past the equinox, we had a good view of the north polar region while still having access to southern latitudes down to 70 degrees S. In 2011 our observations focused on a moderately bright discrete cloud feature in the middle of the bright circumpolar band at 45 degrees N. The spatial and spectral resolution of our data allow us to retrieve atmospheric parameters between similar to 65 S and 75 degrees N via radiative transfer modeling. We test vertical aerosol profiles with combinations of diffuse and compact scattering layers, and find that we can reproduce our equatorial data for a range of cases, provided the deepest detectable aerosol layer is compact and located between 2 and 3 bars, with the higher cloud altitudes corresponding to models with higher methane deep volume mixing ratios. Using a parameterized atmosphere with a diffuse upper haze and a moderately compact lower cloud, we find that both the haze and the cloud reach their maximal optical depth just north of the equator and thin toward the poles. When we fix the abundance of methane with latitude, we find that the bottom cloud shifts to shallower depths at higher latitudes in both hemispheres; for a methane profile with a deep volume mixing ratio of 2.22%, the cloud rises from the 3-bar level equatorward of +/- 20 degrees to above 2 bars by +/- 60 degrees. However, when we allow the tropospheric methane abundance to vary according to a parameterized vertical profile, we find that the lower cloud depth is stable in latitude while the methane becomes increasingly depleted toward both poles. In both cases, we find denser aerosol layers and higher methane abundances in the northern hemisphere than the southern, consistent with a seasonal post-equinox trend. In particular, the bright band near 45 degrees N is relatively undepleted in methane, and represents a local peak in the opacity and altitude of the lower cloud. The cloud feature we detected in 2011 falls in the middle of this band. This feature extends from a depth of similar to 3 bars up to the 0.5-bar level. Both CH4 and H2S are expected to condense below this level; if the cloud has formed as the result of a convective upwelling event, these are the most likely condensation species. (C) 2015 Elsevier Inc. All rights reserved.