Insights into Uranus’ atmosphere from HST FUV observations and radiative transfer modelling

QC 20260114

Abstract

Uranus is one of the extreme worlds in the Solar System. Its large axial tilt of 98^o and orbital period of 84 years lead to unique seasons. It has been visited only by the Voyager 2 spacecraft and remains one of the poorly understood planets in the Solar System. Uranus’ atmosphere is primarily composed of atomic and molecular hydrogen (H and H₂, respectively), helium (He), and methane (CH₄). One of the strongest emission lines from the Sun in the ultraviolet is Lyman alpha (Lyα, 1215.67 Å). It is efficiently scattered by H and H₂, and absorbed by hydrocarbons (mostly, CH₄) in planetary atmospheres. This makes remote sensing observations at Lyα and associated wavelengths an excellent tool to study giant planets’ upper atmospheres. At giant planets, the upper atmosphere plays a key role in various processes such as photochemistry, interaction with the plasma environment and possibly solar wind, magnetosphere-ionosphere coupling, atmospheric escape, and interaction with ring particles. In this thesis, we analysed Hubble Space Telescope (HST) observations of Uranus obtained at Lyα and 1280 Å wavelengths, and performed radiative transfer simulations considering resonant scattering by H, Rayleigh-Raman scattering by H₂, and absorption by CH₄. The results and insights into Uranus’ neutral upper atmosphere gained from the work are presented in a series of papers.

Our analyses of the first spatially resolved images of Uranus’ Lyα emissions, obtained in 1998 and 2011, revealed an extended exosphere of gravitationally bound hot H. The abundance of this hot H varied with time and cannot be explained by production mechanisms involving solar UV radiation alone, pointing to additional energetic processes (Paper I). Further, we analysed Uranus’ Raman-scattered Lyα emissions at 1280 Å, unique among the Solar System giant planets. Using the observed brightness of these emissions, we constrained the vertical distribution of methane in Uranus’ upper atmosphere, providing key inputs for photochemical modelling (Paper II). Our 2024 HST observations revealed a significant increase in exospheric hot H abundance compared to 1998 and 2011, indicating an increase in energetic processes creating this hot H. We also found a persistent azimuthal variation in the exospheric Lyα emissions. Thus, we provide tentative evidence of the role of energetic particles in the Uranian magnetosphere in producing the hot H observed in the exosphere (Paper III).

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