Probing methane in Uranus’ upper stratosphere using HST observations of the 1280 Å Raman feature

We analysed far-ultraviolet (FUV) spectra of Uranus obtained by the HST STIS and COS instruments in 2012 and 2014, respectively, to determine the brightness of Raman-scattered Lyman-alpha (Lyα) emissions centred at 1280 Å (hereafter, the Raman feature). The Raman feature is unique among the Solar System’s giant planets and forms in Uranus’ atmosphere due to weak vertical mixing of hydrocarbons with H2, leading to efficient Rayleigh–Raman scattering. Methane is the dominant hydrocarbon species on Uranus, and since it absorbs FUV radiation, it affects the Rayleigh–Raman scattering of Lyα photons by H2 and, eventually, the brightness of the Raman feature. We derive a brightness of 20−6+1 R from the STIS data, which is similar to the brightness measured by Voyager 2 UVS during the 1986 flyby of Uranus, when considering the suggested recalibration of UVS measurements by a factor of ∼0.5. Based on the observed brightness, we constrain the upper altitude (pressure) level for the abundance of methane in the upper atmosphere using radiative transfer simulations that include resonant scattering by H, Rayleigh–Raman scattering by H2, and absorption by CH4. We considered the solar Lyα flux as the source of Lyα radiation at Uranus. We find that resonant scattering by H significantly affects Rayleigh–Raman scattering by H2 and thus the modelled brightness of the Raman feature. We derive methane profiles by obtaining the simultaneous fit to the observed Lyα, as well as the 1280 Å brightness of Uranus. Methane appears to be depleted (number density becomes less than 1 cm−3) above the altitude (pressure) range of ∼478–515 km (4 × 10−3–2.4 × 10−3 mbar), while the Lyα absorption optical depth reaches unity for methane in the altitude (pressure) range of ∼237–257 km (2.54 × 10−1–1.65 × 10−1 mbar). When neglecting resonant scattering by H, the methane depletion must be deeper in the atmosphere at an altitude (pressure) of ∼395 km (1.4 × 10−2 mbar), similar to previous findings based on Voyager 2 observations of the feature. The analysis of the Raman feature provides independent CH4 constraints in the upper atmosphere for detailed photochemistry modelling and highlights the importance of UV instruments for the future Uranus Orbiter and Probe (UOP) mission.