Why is Uranus giving us the cold shoulder? Analysing planetary scans of H3+ infrared emissions from Uranus's ionosphere

Uranus stands as an enigma within the field of planetary science. What little we know of its ionosphere and aurorae have proven perplexing, whether this be ultraviolet (UV) or infrared (IR) emissions, considerable intensity and column density variability have been observed across the surface of Uranus’s ionosphere [Lamy et al., 2012, 2017, 2020, Lam et al., 1997, Melin et al., 2019 and Thomas et al., 2023 and 2025]. For IR emissions of the molecular ion H₃⁺, a key tracer of energy in Uranus’s ionosphere, only two observations have published spatial intensity maps with a 10 year gap between investigations, this difference prevents a direct comparison as to the variability between emission intensities, temperatures and column densities. To address this, our investigation will study the shorter term variability of Uranus by mapping multiple half planet scans (using Keck-NIRSPEC) in October 2023 and January 2025, an example shown in Fig. 1, to identify ionospheric fluctuations in key auroral and non-auroral regions. Through comparison of these intensity scans with prior JWST observations in December 2021 our discussions will focus on short-term auroral and thermal drivers in Uranus’s ionosphere.

Figure 1. Averaged global projections (~ 5hr) of the spectral radiance of the Q(1,0^-) H₃⁺ emission line from Uranus as observed by Keck-NIRSPEC on the 29^th September 2023. The intensity has been normalised in this figure with the S representing the ULS southern pole of Uranus.