Uranus's Northern Polar Cap in 2014

In October and November 2014, spectra covering the 1.436 to 1.863-m wavelength range from the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope showed the presence of a vast bright north polar cap on Uranus, extending northward from about 40 degrees N and at all longitudes observed. The feature, first detected in August 2014 from Keck telescope images, has a morphology very similar to the southern polar cap that was seen to fade before the 2007 equinox. At strong methane-absorbing wavelengths (for which only the high troposphere or stratosphere is sampled) the feature is not visible, indicating that it is not a stratospheric phenomenon. We show that the observed northern bright polar cap results mainly from a decrease in the tropospheric methane mixing ratio, rather than from a possible latitudinal variation of the optical properties or abundance of aerosol, implying an increase in polar downwelling near the tropopause level. Plain Language Summary In this paper we show that Uranus's northern polar cap in 2014 results from a hole-like depletion of methane rather than a formation or accumulation of aerosols as was previously thought. In this regard, our results provide key insights on the Uranus postequinox circulation as well as on the formation and nature of the different bright features observed before and after 2007 equinox in both hemispheres.