Publications of the MPIfR
Optical & Infrared
Interferometry Group
Livengood, T. A.; Kostiuk, T.;
Sonnabend, G.; Annen, J. N.; Fast, K. E.; Tokunaga, A.; Murakawa, K.;
Hewagama, T.; Schmülling, F.; Schieder, R.
High-resolution infrared spectroscopy of ethane
in Titan's stratosphere in the Huygens epoch
Journal of Geophysical Research, Volume 111, Issue
E11
Abstract
High-resolution infrared spectroscopy of ethane (C2H6) emission
features formed in the stratosphere of Titan was collected on disc
center at 11.74 μm 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 C2H6 were measured with resolving
power λ/Δλ >= 106 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 (1σ), 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 (1σ), 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).
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