Recovery and characterization of Neptune's near-polar stratospheric hot spot
Images of Neptune obtained in 2006 at ESO's Very Large Telescope (Orton et al., 2007, Astronomy & Astrophysics 473, L5) revealed a near-polar hot spot near 70°S latitude that was detectable in filters sampling both stratospheric methane (7 μm) and ethane (∼12 μm) emission. Such a feature was not present in 2003 Keck and 2005 Gemini North observations, which showed only a general warming trend toward Neptune's pole that was longitudinally homogeneous. Because of the paucity of longitudinal sampling in the 2003, 2005 and 2006 images, it was not clear whether the failure to see this phenomenon in 2003 and 2005 was simply the result of insufficient longitudinal sampling or whether the phenomenon was truly variable in time. To unravel these two possibilities, we made follow-up observations on large telescopes that were capable of resolving Neptune at thermal-infrared wavelengths: Gemini South in 2007 and 2010 using the T-ReCS instrument, Subaru in 2008 using the COMICS instrument and VLT in 2008 and 2009 using the VISIR instrument. Two serendipitous T-ReCS images of Neptune were also obtained in 2007 using a broad N-band (8–14 μm) filter, whose radiance is dominated by stratospheric emission from both methane and ethane. The feature was recovered (i) in 2007 with T-ReCS in the broad N-band image and (ii) in 2008 with COMICS in a 12.5-μm image. However, T-ReCS observations in 2010 that covered up to 250° of longitude did not show evidence of an off-polar hot spot. Although we have not definitively ruled out the possibility that various observers have simply missed a semi-permanent feature, it seems statistically very unlikely to be the case. With only 3 sightings in 13 independent observing epochs, it is likely that the phenomenon is ephemeral in time. A possible origin for the phenomenon is a large planetary wave that is dynamically confined to the high-latitude regions characterized by prograde zonal winds. It may be episodically excited by dynamical activity deeper in the atmosphere. This must be coupled with mixing near the poles that destroys or at least substantially attenuates the hot spot over the south pole that leads to an appearance of the typical polar stratospheric hot spot being offset in latitude.
© 2011 Elsevier. Received 4 December 2010. Received in revised form 15 June 2011. Accepted 18 June 2011. Available online 30 June 2011. We thank Erich Karkoschka for helpful comments. Orton and Yanamandra-Fisher conducted a portion of this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. They thank Mark Hofstadter, the Principal Investigator of the relevant program, for this support. De Pater was supported by NSF Grant AST-0908575. Fletcher was supported by a Glasstone Science Fellowship at the University of Oxford. Schneider and Liu acknowledge support by a David and Lucile Packard Fellowship and by the NASA Outer Planets Research Program (Grant NNX10AQ05G).Fujiyoshi and Fuse were supported by the National Astronomical Observatory of Japan. Edwards and Geballe were supported by the Gemini Observatory, on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministrio da Cinica e Tecnologia (Brazil) and Ministerio de Cienica, Tecnologia e Innovacin Productiva (Argentina). The simulations on which Fig. 5 is based were performed on Caltech's Division of Geological and Planetary Sciences Dell cluster. The new results presented in this paper were based, in part, on observations made at the European Southern Observatory telescopes, in programs 081C-0496(A) and 083C-0163(A) and (B), obtained from the ESO/ST-ECF Science Archive Facility; on data collected at Subaru Telescope in program IDS08-032, which is operated by the National Astronomical Observatory of Japan; and on data obtained from the Gemini North Telescope in program GN-2007B-A-105 and the Gemini South Telescope in programs GS-2007B-Q-47, GS-2010B-A-42, which are operated by the Association of Universities for Research in Astronomy.