Steady Collapse of Uranus' Exosphere After 1998 to the Present Decade

Uranus' thermospheric temperature decreased from ∼800K in 1986 to ∼450K in 2022 as determined from observations of H3+ and H2 infrared emissions. Spitzer 2007 lower atmosphere observations do not emulate this cooling trend. Here we show that the atomic H Lyman ⍺ emission from the disk of Uranus observed by HST from 2011 to 2022 are not consistent with radiative transfer models based on a constant atmospheric structure retrieved from the Voyager 2 flyby of 1986. Instead, the optical depth of the H column matching the Uranus Lyman ⍺ disk brightness decreased after 1998, consistent with the long-term cooling trend. This decrease is irrespective of auroral activity. While the origin of the cooling is poorly understood, it indicates that the density and extent of the Uranssian exosphere changes on a time scale of years impacting the atmospheric structure, the magnetospheric proton source, and exospheric drag on the inner rings.