Open AccessHigh‐resolution infrared spectroscopy of ethane in Titan's stratosphere in the Huygens epoch
Author(s) -
Livengood T. A.,
Kostiuk T.,
Sonnabend G.,
Annen J. N.,
Fast K. E.,
Tokunaga A.,
Murakawa K.,
Hewagama T.,
Schmülling F.,
Schieder R.
Publication year - 2006
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2005je002669
Subject(s) - stratopause , stratosphere , mesosphere , atmospheric sciences , titan (rocket family) , infrared , physics , hitran , spectroscopy , mixing ratio , environmental science , astronomy
High‐resolution infrared spectroscopy of ethane (C 2 H 6 ) emission features formed in the stratosphere of Titan was collected on disc center at 11.74 μ m wavelength (851 cm −1 ) on 15 January 2005 UT. The observations were obtained at the Subaru 8.2 m telescope of the National Astronomical Observatory of Japan on Mauna Kea, Hawaii, using the NASA Goddard Space Flight Center Heterodyne Instrument for Planetary Winds and Composition (HIPWAC). Fully resolved rotational‐vibrational transitions of C 2 H 6 were measured with resolving power λ /Δ λ ≥ 10 6 by infrared heterodyne spectroscopy (IRHS). The spectrum is reproduced most effectively by vertical profiles of ethane abundance that are uniform through the stratosphere and enhanced within the mesosphere. Profiles in which there is a significant gradient within the stratosphere are not favored. The retrieved stratospheric ethane mole fraction depends weakly on the form invoked for the mesospheric enhancement. Two forms of the ethane mole fraction profile are found to reproduce the observed spectrum effectively: the best fitting results are obtained with a profile in which the mesospheric ethane concentration increases logarithmically versus decreasing pressure, retrieving a stratospheric ethane concentration of 8.2 ± 2.1 × 10 −6 (1 σ ), increasing proportional to p −1.2 from the stratopause through the mesosphere (p is pressure). A second form of profile, in which the mesospheric ethane concentration is enhanced uniformly by a factor of 9.5, retrieves a stratospheric concentration of 9.7 ± 4.9 × 10 −6 (1 σ ), with the enhancement discontinuity at about one scale height above the stratopause. The retrieved stratospheric mole fraction is consistent with earlier retrievals from IRHS and is somewhat less than contemporaneous retrievals from infrared spectroscopy at lower resolution by the Cassini spacecraft. The retrieved mesospheric concentration is consistent with in situ measurements in Titan's thermosphere made by the Cassini Ion Neutral Mass Spectrometer instrument during the Titan flyby (Waite et al., 2005).