Uranus' cloud structure and seasonal variability from Gemini-North and UKIRT observations

Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIES and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1 ''. NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH(4)(long), and long slit spectroscopy measurements were also made (1.49-1.79 mu m) with the entrance slit aligned on Uranus' central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3 '' x 3 '' field of view stepped across Uranus' disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803 mu m. The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006-2008 and show that the north polar zone at 45 degrees N has continued to steadily brighten while that at 45 S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45 degrees N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH(4) ice, rather than H(2)S or NH(3) ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST 1FU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties. Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3 years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15 degrees N found to lie near the 500 mb level, while another at 30 degrees N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH(4) ice. (C) 2011 Elsevier Inc. All rights reserved,