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

• Published in: Icarus
• Main Authors: Fletcher, L, de Pater, I, Orton, G, Hammel, H, Sitko, M, Irwin, P
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: South Pole; Gemini; South pole
• Online Access: https://doi.org/10.1016/j.icarus.2013.11.035; https://ora.ox.ac.uk/objects/uuid:8e36322b-38ba-430e-a4b9-115b9dd0bb8e

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 (158-164K) 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 <±5K at 1mbar and <±3K at 0.1mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two (from 500 to 1200ppb at 1mbar). 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 25-50μm 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 ...