Letter to the Editor

Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune's atmospheric temperatures

¹ MS 169-237, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
² LESIA, Observatoire de Paris, CNRS, UPMC, Univ. Paris 7, 5 place Jules Janssen, 91925 Meudon Cedex, France e-mail: Therese.Encrenaz@obspm.fr
³ MS 230-205, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁴ Center for Astrophysics & Space Sciences, CASS 0424, University of California, San Diego, 9500 Gillman Dr., La Jolla, CA, 92093-0424, USA

Received: 13 July 2007
Accepted: 7 August 2007

Abstract

Aims.We studied the distribution of mid-infrared thermal emission from Neptune to determine the spatial variability of temperatures and the distribution of trace constituents, allowing us to determine the relative strengths of radiation and dynamics in its atmosphere.

Methods.Mid-infrared images of the planet were taken at the Very Large Telescope on 1–2 September 2006.

Results.These images reveal strong inhomogeneities in thermal emission. 17.6 and 18.7-μm images exhibit strong seasonally elevated south polar temperatures near Neptune's tropopause. These high temperatures allow tropospheric methane, elsewhere cold-trapped at depth, to escape into the stratosphere. Poleward of 70°S, 8.6- and 12.3-μm emission from stratospheric methane and ethane is enhanced, and a distinct, warm stratospheric feature near 65–70°S latitude is rotating with the neutral atmosphere. This feature may result from a localized wave propagating upward from the troposphere.