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The Upper Atmosphere of Uranus from Stellar Occultations

Presentation #409.08D in the session Uranus and Neptune Systems (Oral Presentation)

Published onOct 23, 2023
The Upper Atmosphere of Uranus from Stellar Occultations

Background: The 1986 Voyager 2 UV stellar and solar occultations at Uranus detected a warm stratosphere and extremely hot thermosphere [1, 2], far in excess of solar irradiance [3, 4] or internal heating [5]. These measurements imply that Uranus has the coldest stratosphere and hottest thermosphere of any Solar System planet [6]. The fundamental lack of understanding about the energy balance of Uranus is an example of the “giant planet energy crisis.”

Motivation: Between 1977 and 1998, many stellar occultations by Uranus were observed from Earth [8]. In an Earth-based stellar occultation, stellar flux is diminished by differential refraction through the occulting atmosphere and is observed as a light curve. The original analyses of these light curves found much cooler temperatures than Voyager 2 found.

Aim: Reprocess archival Earth-based occultations to resolve the tension with Voyager 2 and better understand the upper atmosphere of Uranus.

Methods:

(1) We simulate Earth-based stellar occultation light curves from Voyager 2 atmospheric profiles [9] and determine if they are consistent with observed light curves [10].

(2) We process an observed light curve by first obtaining a boundary condition [11] and then performing numerical inversion. Inversion calculates the bending angle of each incident light ray through the Uranian atmosphere and uses an Abel transform to find pressure, temperature, and density vs. altitude [12]. Recent improvements to this procedure allow for non-isothermal boundary conditions, accurate uncertainties, and much higher vertical resolutions [10, 13].

(3) We fit radiative-convective [14] and conductive models [2] to our temperature profiles from (2).

Results: (1) The Voyager 2 UV occultation profiles are not consistent with Earth-based occultation observations at >3σ statistical confidence. (2) The lower thermosphere of Uranus is much cooler than the Voyager 2 results; we will present a new gestalt Uranus atmospheric profile. (3) Modeling supports our new gestalt profile and we will present trends from the reprocessed temperature profiles [10].

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[2] Stevens, M. et al. (1993). Icarus.

[3] Marley, M. & McKay, C. (1999). Icarus,

[4] Li, C. et al. (2018). JQRST.

[5] Pearl, J. et al. (1990). Icarus.

[6] Mueller-Wodarg, I. et al. (2008). SSR.

[7] Bishop, J. et al. (1995). In Neptune and Triton.

[8] Young, L. et al. (2001). Icarus.

[9] Chamberlain, D. & Elliot, J. (1997). PASP.

[10] Saunders et al. (in review). PSJ.

[11] Elliot, J. & Young, L. (1992). AJ.

[12] Elliot, J. et al. (2003). AJ.

[13] Saunders, W et al. (2021). AJ.

[14] Robinson, T. & Catling, D. (2012). ApJ.

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