New constraints on the CH₄ vertical profile in Uranus and Neptune from Herschel observations^⋆

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: emmanuel.lellouch@obspm.fr
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive Pasadena, CA 91109, USA
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ Space Science Institute, 4750 Walnut St., Suite 205, Boulder, CO 80301, USA
⁶ Faculty of Science, Kyoto Sangyo University. Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan

Received: 12 May 2015
Accepted: 12 June 2015

Abstract

Dedicated line observations of CH₄ rotational lines performed with Herschel/PACS and HIFI in 2009–2011 provide new inferences of the mean methane profile in the upper tropospheres and stratospheres of Uranus and Neptune. At Uranus, CH₄ is found to be near saturation, with a ~9 × 10^-4 tropopause/lower stratosphere mole fraction. This is nominally six times larger than inferred from Spitzer in 2007, although reconciliation may be possible if the CH₄ abundance decreases sharply from ~100 to 2 mbar. This unexpected situation might reflect heterogeneous conditions in Uranus’ stratosphere, with local CH₄ depletions and heating associated with downwelling motions. Higher CH₄ abundances compared to values inferred under solstitial conditions by Voyager in 1989 suggest that atmospheric mixing is effectively subdued at high latitudes and/or is time-variable. At Neptune, the mid-stratosphere CH₄ abundance is (1.15 ± 0.10) × 10^-3, in agreement with earlier determinations and indicative of either leakage through a warmer polar region or upwelling at low or middle latitudes. On both planets, spatially resolved observations of temperature and methane in the stratosphere are needed to further identify the physical processes at work.