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Uranus Weather Forecast: Early Summer with Weakened Circulation and Strengthened Mysteries

Presentation #409.02 in the session Uranus and Neptune Systems (Oral Presentation)

Published onOct 23, 2023
Uranus Weather Forecast: Early Summer with Weakened Circulation and Strengthened Mysteries

Ground-based radio observations of Uranus have been reported since 1965 and now span most of its 84-year seasonal cycle and include all seasons visible from Earth (winter being always on the far side of the planet). These data are sensitive to conditions in the deep troposphere (pressures from ~1 to 100 bars). We report recent changes in atmospheric composition that mark a return to summertime conditions and confirm speculation (Hofstadter & Butler 2003, Icarus 165) that a transition seen in ~1990 was a seasonal effect.

Uranian latitudes poleward of ±45° are always brighter at centimeter wavelengths than equatorial regions (Briggs & Andrew 1980, Icarus 41). This is most likely due to the poles being depleted in volatile species such as NH3 and H2S (de Pater et al. 1991, Icarus 91). During the fall and spring, polar abundances are depleted by two orders of magnitude when compared to the equator. During summer, the contrast decreases to one order of magnitude. We suspect the depletion is driven by a circulation pattern with rising air at low latitudes, poleward transport at high altitudes, and subsidence over the pole (Hofstadter et al. 1990, Icarus 84). Recent data confirm that this circulation pattern weakens each summer, perhaps due to preferential solar heating inhibiting polar subsidence. We have developed a dynamical model driven by the observed cloud-top winds which recreates the inferred circulation pattern, but it is unclear if the circulation is strong enough to maintain the depletion of volatiles over the poles and it does not predict significant seasonal variability. One can invoke other mechanisms to explain the unusually bright poles such as mushballs (Guillot et al. 2020, JGR Plan. 25) or true kinetic temperature contrasts, but these will also require strong atmospheric motions that weaken during summer. Our data suggest the weakening of atmospheric circulation is not symmetric about solstice but begins ~15 years prior and ends ~5 years after. Some of the mysteries from this analysis are: Does the large-scale circulation or small-scale convection create the differences between pole and equator? How does sunlight influence the atmosphere as deep as ~50 bars? Why do seasonal effects seem to lead rather than lag solar insolation?

We will conclude our presentation with recent work analyzing Uranus’s polar vortex (Molter et al. 2021, PSJ 2; Akins et al. 2023, GRL 50) and revisiting an older mystery: does the non-solar N/S ratio in the atmosphere indicate unknown chemical and dynamical processes are active in the deep planet or might they indicate Uranus formed from a different reservoir of volatiles than did Jupiter?

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