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

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Abstract

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, which is confirmed by near-IR imaging from HST and from Keck/NIRC2 adaptive optics imaging. Our analysis of the 2015 spectra, as well as prior spectra from 2012, shows that there is a X 3 decrease in the effective upper tropospheric methane mixing ratio between 30 deg N and 70 deg N. The absolute value of the deep CH4 mixing ratio, likely independent of 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+/-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 previous higher estimate of 4+/-0.5% was a due to 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 of Orton et al. (2014, Icarus 243, 494-513) and Lellouch et al. (2015, Astron. & AstroPhys. 579, A121) called into question the occultation results. For the main cloud layer we found that both large and small particle solutions are possible for spherical particle models. The small-particle solution has a mean particle radius near 0.3 microns, a real refractive index of 1.7-1.9, and a total column mass of 0.03 mg/cm^2, while the large-particle solution has a particle radius near 1.5 microns, a real index near 1.24, and a total column mass 30 X larger. The main cloud layer extends between about 1.1 and 3 bars, within which H2S is the only plausible condensable. Since its real refractive index is 1.9, the small-particle solution provides the closest physical match to the most plausible cloud constituent.

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Most cited references 21

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Rain, winds and haze during the Huygens probe's descent to Titan's surface.

M G Tomasko, B Archinal, T. Becker … (2005)

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.