The methane abundance and structure of Uranus' cloud bands inferred from spatially resolved 2006 Keck grism spectra

Abstract

Grism spectra of Uranus obtained at the Keck Observatory in 2006, using the NIRC2 instrument and adaptive optics, provide new constraints on the vertical structure of Uranus' cloud bands and on the volume mixing ratio of methane. The best model fits to H-band spectra (1.49–1.635 μm) are found for a methane volume mixing ratio of 1.0 ± 0.25% for latitudes near 43° S and 1–1.6% for latitudes of 12° S and 33° N. Analysis of the J-band spectra are confused by discrepancies between short-wave and long-wave sides of the 1.28 μm window region. The short-wave side of the window (1.23–1.30 μm) is best fit with 1.6% CH 4, but if the fitted spectral range is extended to include the long-wave side of the window (1.2–1.34 μm), the best fit CH 4 mixing ratio is 4% or more, although many small scale spectral features are poorly fit over this range even at high methane mixing ratios, suggesting that models of methane opacity may be inconsistent in this spectral region. Most of the latitudinal variability of the H-band spectra can be fit with clouds near 2–3 and 6–8 bar, with cloud reflectivity of the deeper layer increasing from ∼2% at 33° N to 3–4% in the southern hemisphere. This layer is most likely made of H 2S particles and appears weakly reflective because it is optically thin and possibly also contaminated by absorbing materials. The reflectivity of the 2–3-bar cloud increases from 0.5% at 33° N to ∼1% at the bright band centered near 43° S, where the upper cloud is a little higher (pressure is 10% lower) and ∼25% more reflective than at nearby latitudes. The bright band is also associated with lowering of the deep cloud pressure, by ∼1.4 bar. The bright band parameters are roughly consistent with those obtained from 1975 disk-averaged spectra, obtained when the southern hemisphere was more exposed to the Sun. The lack of significant cloud particle contributions near 1.2 bar, where occultation results suggested a methane cloud, is confirmed by both spectra and HST imaging observations.