Presentation #415.08 in the session Giant Planet Atmospheres (iPosters).
We have assembled a 41-year record of disk-resolved radio observations of Uranus, from 1981 to 2022. These data span half a uranian year, and track changes in the 1 to 100-bar region of the troposphere. We see features previously reported, including:
A surprisingly low abundance of microwave absorbing species such as NH3 and H2S (Gulkis et al. Icarus 34, 1978), with H2S being more abundant than NH3 (De Pater et al. Icarus 82, 1989). This suggest a composition much different than expected from solar abundances, an unknown process chemically trapping these species much deeper in the planet, or an unknown dynamical process removing species from the upper ~25 bars of the atmosphere.
At all times, polar regions are 5 to 100x more depleted in opacity sources than equatorial latitudes (Hofstadter and Muhleman Icarus 81, 1989). The most likely interpretation of this is the presence of a large-scale atmospheric upwelling from at least ~50 bars at equatorial- to mid-latitudes, poleward transport at pressures less than ~5 bars, and subsidence over the poles. Condensation removes absorbers from the rising air parcels so that the descending air over the poles is clear.
The just-mentioned abundance variation between pole and equator is smaller in midsummer than it is in late summer/early fall, suggesting the circulation pattern weakens near summer solstice (Hofstadter and Butler Icarus 165, 2003).
Variations in opacity of ~30% also exist on scales of ~5˚ in latitude which creates bands on the planet in both the equatorial and polar regions reminiscent of banding seen at Jupiter and Saturn (Molter et al. PSJ 2:3, 2021 and references therein).
Our integrated analysis of images made at wavelengths from 1.4 mm to 21 cm is still underway at the time of this writing, but we will report on the search for additional seasonal variations in the spring-time Northern Hemisphere as it approaches its 2030 summer solstice and on refined values for the NH3 and H2S abundances and their spatial variability. We will also report on modeling of Uranus’ atmospheric dynamics which invokes strong damping at depth of the observed cloud-top zonal winds to explain the proposed equator to pole meridional circulation.