The Atmosphere of Uranus. (arXiv:2105.06377v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fletcher_L/0/1/0/all/0/1">Leigh N. Fletcher</a>
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.
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.
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