The vertical structure of the Uranian atmosphere near equinox as modeled with near-infrared spectroscopic observations

We acquired spectra of Uranus in September 2006 and September 2007, at NASA's infrared Telescope Facility using the spectrograph SpeX. These spectra from 0.8--2.5 mum allow probing the Uranian troposphere due to the low haze opacity and broad range in methane opacity in the near infrared. Our observations occurred close to Uranus' December 2007 equinox, which not only allowed us to simultaneously observe regions of constant latitude due to the near-zero sub-observer latitude, but also provided a rare opportunity to study Uranus in one of the extremes of its 84-year seasonal cycle. We modeled the vertical structure of the Uranian atmosphere with a radiative transfer code designed to generate synthetic spectra based on a given set of atmospheric properties, for comparison with our observations. We employed the band-model methane absorption coefficients of Irwin et al. (2006), determined the effects of collision-induced absorption, and accounted for the large spatial coverage in each observation by calculating the contributions of numerous locations on Uranus to each spectrum. Our models assumed three aerosol layers in the atmosphere of Uranus: a stratospheric haze layer, and two tropospheric cloud layers. We fit optimum values to the optical depth of each aerosol layer in different spectral regions and to the pressure levels of each cloud layer; holding all other parameters constant. Our model results described two overarching regimes on Uranus. In 2006, the spectra from the southern hemisphere were best fit with bright clouds at high altitude, while the northern hemisphere was characterized by a dimmer haze layer, almost nonexistent upper cloud, and a deep lower cloud near 7 bars. However, this changed in the one-year interim between our data sets. In 2007, the high-cloud regions were instead around the equator, and we found the region near 45°S, which in previous years had displayed a bright polar collar, to have taken on the characteristics of the dimmer deeper-cloud regions. However, at 45°N our models detected at 4 bars a prominent upper cloud, indicative of the formation of the anticipated north polar collar.