The Atmosphere of Uranus


Imaging of Uranus from the visible to the radio, in two groups: near the 2007 equinox (top row), and during mid-northern spring (bottom row). From left to right, we show visible-light images from Hubble WFPC2 and WFC3 (Credit: NASA/ESA/STScI); near-infrared images from Keck (Sromovsky et al. 2015; de Pater et al. 2015); mid-IR images of the stratosphere (13 µm) and troposphere (18.7 µm) from VLT (Roman et al. 2020); millimetre observations from ALMA (Molter et al. 2021); and centimetre-wave observations from VLA (Molter et al. 2021). Uranus has been oriented so the the north pole is at the top.

Uranus provides a unique laboratory to test our understanding of planetary atmospheres under extreme conditions.

Multi-spectral observations from Voyager, ground-based observatories, and space telescopes have revealed a delicately banded atmosphere punctuated by storms, waves, and dark vortices, evolving slowly under the seasonal influence of Uranus' extreme axial tilt. Condensables like methane and hydrogen sulphide play a crucial role in shaping circulation, clouds, and storm phenomena via latent heat release through condensation, strong equator-to-pole gradients suggestive of equatorial upwelling and polar subsidence, and through forming stabilising layers that may decouple different circulation and convective regimes as a function of depth.

Weak vertical mixing and low atmospheric temperatures associated with Uranus' negligible internal heat means that stratospheric methane photochemistry occurs in a unique high-pressure regime, decoupled from the influx of external oxygen. The low homopause also allows for the formation of an extensive ionosphere. Finally, the atmosphere provides a window on the bulk composition of Uranus - the ice-to-rock ratio, supersolar elemental and isotopic enrichments inferred from remote sensing and future \textit{in situ} measurements - providing key insights into its formation and subsequent migration.

This review reveals the state of our knowledge of the time-variable circulation, composition, meteorology, chemistry, and clouds on this enigmatic `Ice Giant,' summarising insights from more than three decades of observations, and highlighting key questions for the next generation of planetary missions. As a hydrogen-dominated, intermediate-sized, and chemically-enriched world, Uranus could be our closest and best example of atmospheric processes on a class of worlds that may dominate the census of planets beyond our own Solar System.

Leigh N. Fletcher

Comments: Invited review submitted for publication in the Oxford Research Encyclopedia of Planetary Science. Oxford University Press. Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2105.06377 [astro-ph.EP] (or arXiv:2105.06377v1 [astro-ph.EP] for this version)
Submission history
From: Leigh Fletcher
[v1] Thu, 13 May 2021 16:10:35 UTC (7,383 KB)
https://arxiv.org/abs/2105.06377

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