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Ice Giant Circulation Patterns: Implications for Atmospheric Probes

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Version 2 2020-04-28, 15:29
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journal contribution
posted on 2020-04-28, 15:29 authored by LN Fletcher, ID Pater, GS Orton, MD Hofstadter, PGJ Irwin, M Roman, D Toledo

Atmospheric circulation patterns derived from multi-spectral remote sensing can serve as a guide for choosing a suitable entry location for a future in situ probe mission to the Ice Giants. Since the Voyager-2 flybys in the 1980s, three decades of observations from ground- and space-based observatories have generated a picture of Ice Giant circulation that is complex, perplexing, and altogether unlike that seen on the Gas Giants. This review seeks to reconcile the various competing circulation patterns from an observational perspective, accounting for spatially-resolved measurements of: zonal albedo contrasts and banded appearances; cloud-tracked zonal winds; temperature and para-H2 measurements above the condensate clouds; and equator-to-pole contrasts in condensable volatiles (methane, ammonia, and hydrogen sulphide) in the deeper troposphere. These observations identify three distinct latitude domains: an equatorial domain of deep upwelling and upper-tropospheric subsidence, potentially bounded by peaks in the retrograde zonal jet and analogous to Jovian cyclonic belts; a mid-latitude transitional domain of upper-tropospheric upwelling, vigorous cloud activity, analogous to Jovian anticyclonic zones; and a polar domain of strong subsidence, volatile depletion, and small-scale (and potentially seasonally-variable) convective activity. Taken together, the multi-wavelength observations suggest a tiered structure of stacked circulation cells (at least two in the troposphere and one in the stratosphere), potentially separated in the vertical by (i) strong molecular weight gradients associated with cloud condensation, and by (ii) transitions from a thermally-direct circulation regime at depth to a wave- and radiative-driven circulation regime at high altitude. The inferred circulation can be tested in the coming decade by 3D numerical simulations of the atmosphere, and by observations from future world-class facilities. The carrier spacecraft for any probe entry mission must ultimately carry a suite of remote-sensing instruments capable of fully constraining the atmospheric motions at the probe descent location.


Fletcher was supported by a Royal Society Research Fellowship at the University of Leicester. Fletcher and Roman also received funding from a European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement No. 723890). A portion of this work was performed by Orton at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. de Pater was in part supported by the National Science Foundation, NSF grant AST-1615004 and NASA Grant NNX16AK14G through the Solar System Observations (SSO) program to the University of California, Berkeley. Irwin and Toledo acknowledge support from the UK Science and Technologies Facilities Council (STFC). We are grateful for support from the In Situ Exploration of the Ice Giants: Science and Technology workshop held in February 2019 at the Laboratoire d’Astrophysique de Marseille for inspiring this review.



Fletcher, L.N., de Pater, I., Orton, G.S. et al. Ice Giant Circulation Patterns: Implications for Atmospheric Probes. Space Sci Rev 216, 21 (2020).

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