Measuring the Stratospheric Temperature of Uranus Using Archival Stellar Occultations

Abstract:

Introduction: Between the discovery of rings in 1977 and the Voyager 2 flyby in 1986, Uranus’ atmosphere was sampled dozens of times by remote sensing. The vast majority of these were earth-based stellar occultations, in which Uranus’ neutral atmosphere is measured by detecting changes in stellar flux as Uranus occults a star. These high-resolution vertical profiles of the atmosphere revealed enigmatic feature referred to as the “energy crisis” [1]: 1) Uranus’ stratosphere and thermosphere are too warm for the amount of sunlight received [2]. 2) Uranus has weaker internal energy than the other giant planets [3]. 3) Despite continuous polar irradiation for 42 years, Uranus’ stratosphere has similar temperatures everywhere [4]. These open questions notwithstanding, the upper atmosphere of Uranus has been virtually unsampled since 1986.

Aims: When the atmosphere of Uranus was last sampled, significant disagreements in temperatures were found between different methods of detection—as much as hundreds of Kelvin. We endeavor to use updated occultation analysis techniques (inversion) [5, 6] on archival data to help resolve the stratospheric temperature uncertainty. By producing higher resolution profiles, we can search for wave action, a potential source of heating that could help move toward resolving the energy crisis [7].

Methods: Starting with a boundary condition and assumptions of atmospheric properties, the inversion procedure performs an Abel transform point-by-point down each occultation light curve. Each point samples a different radius of intervening atmosphere and returns temperature, pressure and neutral density, with errors. We use as many archival datasets as possible to strengthen findings and search for wave activity following [8].

Results: We present high-resolution atmospheric profiles, compare to original analyses as well as other archival remote-sensing results and re-state the stratospheric temperature discrepancy. We outline the stellar occultation observations that should occur in the coming years in advance of potential mission planning for a planetary encounter with the Ice Giants.

[1] Li, C. et al. 2018

[2] Marley & McKay 1999

[3] Pearl et al. 1990

[4] West et al. 1987

[5] Elliot & Young 1992

[6] Elliot, Person, & Qu 2003

[7] Mueller-Wodarg 2008

[8] Saunders, Person, & Withers, in review