Search for H2- N2 Dimer in Titan's Atmosphere

Details Search for H2- N2 Dimer in Titan's Atmosphere Search for H2- N2 Dimer in Titan's Atmosphere

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.3607t&link_type=abstract

American Astronomical Society, DPS meeting #38, #36.07; Bulletin of the American Astronomical Society, Vol. 38, p.550

Physics

Optics

Scientific paper

Molecular dimers (van der Waals bound associations of two molecules) involving H2 are useful for investigating the dynamics of planetary atmospheres because they can provide the ratio of ortho-H2 and para-H2 even for uncertain temperature. Because of the long transition time to achieve equilibrium between these states compared to the characteristic times of convection and advection, differences in the ratio of these species vs position over a planetary disk can provide information on dynamical phenomena. Since Voyager, dimers have now been detected for all of the outer planets and Titan, except Uranus and Titan, using ground-based near-IR vibrational-band spectra. We report our search for the H2-N2 dimer in Titan's near-IR spectrum ( 2.122 μm) using the new Gemini Integral Field Spectrograph (NIFS) on 7 and 8 Feb 2006 with adaptive optics to resolve Titan's 0.9" disk with a resolution of about 0."1. This is an improvement upon previous searches, which necessarily averaged Titan's whole disk, because clear regions where the surface is visible can be spatially resolved from clouds and haze. Our search strategy was to sort the spectra for all the disk pixels in order of increasing pressure-induced (PIA) H2- N2 absorption. This facilitates locating the maximum clear absorption path through the atmosphere because local haze or clouds raises the albedo, and PIA absorption is weighted more to lower altitudes than CH4 is by virtue of its pressure-squared dependence. The PIA H2 S(1) band "turns on" between 2.02 μm (where the surface is least attenuated) and 2.121 μm (peaking near the quadrupole line, where the surface is essentially obscured). One unanticipated problem is the presence of weak CH4 lines spanning the broad dimer wavelengths, which are enhanced at the longer paths. It is necessary to model these in order to detect and reveal the dimer structure. We describe our progress.

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