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 (1.47–2.38 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°S. In 2011 our observations focused on a moderately bright discrete cloud feature in the middle of the bright circumpolar band at 45°N. The spatial and spectral resolution of our data allow us to retrieve atmospheric parameters between ∼65°S and 75°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° to above 2 bars by ±60°. 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°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 ∼1.3 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.
© 2015 Elsevier. Received 1 December 2014, Revised 3 April 2015, Accepted 13 April 2015, Available online 20 April 2015. The authors would like to thank L. Sromovsky for helpful discussion and suggestions at various stages of this work. The near-infrared data were obtained with the W.M. Keck Observatory, which is operated by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented would have been possible. This research was supported in part by NASA's Planetary Astronomy Program under Grant NNX07AK70G to the University of California, Berkeley. K. de Kleer is additionally supported by the National Science Foundation Graduate Research Fellowship under Grant DGE-1106400. S. Luszcz-Cook is supported by the Kalbfleisch Postodoctoral Fellowship at the American Museum of Natural History. This research has also made use of the SIMBAD database, operated at CDS, Strasbourg, France.