  HST/STIS observations of Uranus in 2015 show that the depletion of upper
tropospheric methane has been relatively stable and that the polar region has
been brightening over time as a result of increased aerosol scattering. This
interpretation is confirmed by near-IR imaging from HST and from the Keck
telescope using NIRC2 adaptive optics imaging. Our analysis of the 2015
spectra, as well as prior spectra from 2012, shows that there is a factor of
three decrease in the effective upper tropospheric methane mixing ratio between
30\deg N and 70\deg N. The absolute value of the deep methane mixing ratio,
which probably does not vary with latitude, is lower than our previous
estimate, and depends significantly on the style of aerosol model that we
assume, ranging from a high of 3.5$\pm$0.5% for conservative non-spherical
particles with a simple Henyey-Greenstein phase function to a low of
2.7%$\pm$0.3% for conservative spherical particles. Our previous higher
estimate of 4$\pm$0.5% was a result of a forced consistency with occultation
results of Lindal et al. (1987, JGR 92, 14987-15001). That requirement was
abandoned in our new analysis because new work by Orton et al. (2014, Icarus
243, 494-513) and by Lellouch et al. (2015, Astron. & AstroPhys. 579, A121)
called into question the occultation results. For the main cloud layer in our
models we found that both large and small particle solutions are possible for
spherical particle models. At low latitudes the small-particle solution has a
mean particle radius near 0.3 $\mu$m, a real refractive index near 1.65, and a
total column mass of 0.03 mg/cm$^2$, while the large-particle solution has a
particle radius near 1.5 $\mu$m, a real index near 1.24, and a total column
mass 30 times larger. The pressure boundaries of the main cloud layer are
between about 1.1 and 3 bars, within which H$_2$S is the most plausible
condensable.

Earth and Planetary Astrophysics