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Neptune at summer solstice: Zonal mean temperatures from ground-based observations, 2003-2007

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journal contribution
posted on 2019-10-15, 14:04 authored by Leigh N. Fletcher, Imkede de Pater, Glenn S. Orton, Heidi B. Hammel, Michael L. Sitko, Patrick G. J. Irwin
Imaging and spectroscopy of Neptune’s thermal infrared emission from Keck/LWS (2003), Gemini-N/MICHELLE (2005); VLT/VISIR (2006) and Gemini-S/TReCS (2007) is used to assess seasonal changes in Neptune’s zonal mean temperatures between Voyager-2 observations (1989, heliocentric longitude Ls = 236◦ ) and southern summer solstice (2005, Ls = 270◦ ). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158-164 K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is < ±5 K at 1 mbar and < ±3 K at 0.1 mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two (from 500 to 1200 ppb at 1 mbar). The retrieved C2H6 abundances are extremely sensitive to the details of the T(p) derivation, although the underlying cause of the variable ethane emission remains unidentified. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane), with no large seasonal hemispheric asymmetries evident at solstice. At low and mid-latitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50 µm mapping of tropospheric temperatures and para-hydrogen disequilibrium (a tracer for vertical motions) suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H2 conditions). The most significant atmospheric changes have occurred at high southern latitudes, where zonal temperatures retrieved from 2003 images suggest a polar enhancement of 7-8 K above the tropopause, and an increase of 5-6 K throughout the 70 − 90◦S region between 0.1 and 200 mbar. Such a large perturbation, if present in 1989, would have been detectable by Voyager/IRIS in a single scan despite its long-wavelength sensitivity, and we conclude that Neptune’s south polar cyclonic vortex increased in strength significantly from Voyager to solstice.


Fletcher was supported during this research by a Royal Society Research Fellowship at the University of Oxford. Some of the data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. This investigation was partially based on VLT/VISIR observations acquired at the Paranal UT3/Melipal Observatory under ID 077.C-0571; and on Gemini MICHELLE and TReCS observations acquired under ID GN-2005ADD-10 and ID GS-2007B-Q-47 at the Gemini Observatory, which is operated by AURA, Inc., under an NSF cooperative agreement on behalf of the Gemini partnership. This work has been supported in part by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST 9876783, and by NSF Grant AST-0908575 to UC Berkeley. Orton was supported by grants fro NASA to the Jet Propulsion Laboratory, California Institute of Technology. The UK authors acknowledge the support of the Science and Technology Facilities Council (STFC). We thank T. Greathouse for providing the Neptune T(p) structure derived for the October 2007 TEXES observations, and for interesting discussions about the nature of these results. We also thank R. Campbell at Keck user support for providing details of the historical filters on Keck/LWS; and M. Gustafsson for providing collision-induced absorption coefficients for a range of para-H2 fractions pertinent to Neptune’s atmosphere. Finally, we thank two anonymous reviewers for helping to improve the quality of this manuscript. The UK authors acknowledge the support of the Science and Technology Facilities Council (STFC).



Icarus, 2014, 231, pp. 146-167 (22)

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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