The methane distribution and polar brightening on Uranus based on HST/STIS, Keck/NIRC2, and IRTF/SpeX observations through 2015

Abstract Space Telescope Imaging Spectrograph (STIS) observations of Uranus in 2002 and 2012 revealed that both polar regions of Uranus were depleted in upper tropospheric methane relative to equatorial regions. Similar observations in 2015 confirm the relative stability of the north polar methane depletion, but show that the north polar region was becoming significantly brighter at wavelengths of intermediate methane absorption. This is not due to decreases in the amount of methane in the north polar region because the brightening also occurred at wavelengths dominated by hydrogen absorption, as observed by STIS and confirmed by near-IR imaging from HST and from the Keck telescope using NIRC2 adaptive optics imaging. Radiation transfer models also confirmed that increased aerosol scattering is responsible for the temporal change, while a persistent negative latitudinal gradient in upper tropospheric methane helps to make the north polar region brighter. Our prior quantitative analysis of STIS spectra (Sromovsky et al. 2014, Icarus 238, 137–155), which was constrained to be consistent with occultation results of Lindal et al. (1987, JGR 92, 14987–15001), found that the spectra were most consistent with a deep methane volume mixing ratio of 4.0 ± 0.5%. A revised approach to our analysis was suggested by new temperature and methane profiles derived from Spitzer spectra by Orton et al. (2014, Icarus 243, 494–513) and new methane mixing ratio estimates based on far IR and sub-mm observations by Lellouch et al. (2015, Astron. & AstroPhys. 579, A121), implying that methane might be saturated or even supersaturated in some regions of the stratosphere. Because both of these results are inconsistent with occultation results of Lindal et al. (1987, JGR 92, 14987–15001), we revised our analysis to allow STIS spectra to constrain our models without regard to occultation consistency. We also simplified our vertical structure models for aerosols to the minimum complexity needed to match the spectral observations. Our new analysis of the 2015 spectra shows that methane’s relative variation from low to high latitudes is similar to prior results, implying about a factor of three decrease in the effective upper tropospheric methane mixing ratio between 30° N and 70° N, which is accompanied by an effective depletion depth that increases with latitude over the same range. However, we find that the absolute value of the deep methane mixing ratio 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 ± 0.5% for conservative non-spherical particles with a simple Henyey–Greenstein phase function to a low of 2.7% ± 0.3% for conservative spherical particles. Our methane distribution results are based on fitting the 730–900 nm portion of the STIS spectra, which is the region most sensitive to the methane to hydrogen ratio. But for Mie scattering models we had to add some absorption in this region to allow boosting the optical depth enough to match the spectrum at shorter wavelengths. For these absorbing particles, inferred methane mixing ratios are up to 12% smaller. For non-spherical particle models, we did not need to add absorption, but were able to use a wavelength-dependent optical depth to extend the spectral match to shorter wavelengths. We find that both large and small particle solutions are possible for spherical particle models. Both can be made to fit not only STIS spectra, but can also be extended to fit near-IR groundbased spectra (obtained with the SpeX instrument at NASA’s Infrared Telescope Facility) out to 1.6 µm by adjusting the particle’s imaginary index. The small-particle solution has a mean particle radius near 0.3 µm, a real refractive index near 1.65, and a total column mass of 0.03 mg/cm2, while the large-particle solution has a particle radius near 1.5 µm, 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 H2S is the most plausible condensable. However, too little is known about the imaginary index spectrum of H2S to determine whether it matches the other characteristics that seem to be required of the cloud particles. It is also possible that photochemical products might play a role as contaminant or as a primary constituent of aerosols. We also find evidence for a deep cloud layer, possibly composed of NH4SH and located near 10 bars if optically thick.