High spectral resolution infrared studies of Titan: Winds, temperature, and composition

Abstract

The Cassini Huygens mission provides a unique opportunity to combine ground-based and spacecraft investigations to increase our understanding of chemical and dynamical processes in Titan’s atmosphere. Spectroscopic measurements from both vantage points enable retrieving global wind structure, temperature structure, and atmospheric composition. An updated analysis of Titan data obtained with the NASA Goddard Space Flight Center’s Infrared Heterodyne Spectrometer (IRHS) and Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) prior to and during the Cassini Huygens mission is compared to retrievals from measurements with the Cassini Composite Infrared Spectrometer (CIRS). IRHS/HIPWAC results include the first direct stratospheric wind measurements on Titan, constraints on stratospheric temperature, and the study of atmospheric molecular composition. These results are compared to CIRS retrievals of wind and temperature profile from thermal mapping data and ethane abundance at 10–15° South latitude, near the equatorial region. IRHS/HIPWAC wind results are combined with other direct techniques, stellar occultation measurements, and CIRS results to explore seasonal variability over nearly one Titan year and to provide an empirical altitude profile of stratospheric winds, varying from ∼50 to 210 m/s prograde. The advantage of fully resolved line spectra in species abundance measurements is illustrated by comparing the possible effect on retrieved ethane abundance by blended spectral features of other molecular constituents, e.g., acetylene (C 2H 2), ethylene (C 2H 4), allene (C 3H 4), and propane (C 3H 8), which overlap the ν 9 band of ethane, and are not resolved at lower spectral resolution. IR heterodyne spectral resolution can discriminate weak spectral features that overlap the ν 9 band of ethane, enabling ethane lines alone to be used to retrieve abundance. Titan’s stratospheric mean ethane mole fraction (8.6±3 ppmv) retrieved from IRHS/HIPWAC emission line profiles (resolving power λ⧸Δ λ∼10 6) is compared to past values obtained from lower resolution spectra and from CIRS measurements (resolving power λ⧸Δ λ∼2×10 3) and more compatible recent analysis. Results illustrate how high spectral resolution ground-based studies complement the spectral and spatial coverage and resolution of moderate spectral resolution space-borne spectrometers.