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| Titel |
Constraints on the Bulk Composition of Uranus from Herschel PACS and ISO LWS
Photometry, SOFIA FORCAST Photometry and Spectroscopy, and Ground-Based
Photometry of its Thermal Emission |
| VerfasserIn |
Glenn Orton, Thomas Mueller, Martin Burgdorf, Leigh Fletcher, Imke de Pater, Sushil Atreya, Joseph Adams, Terry Herter, Luke Keller, Sunil Sidher, James Sinclair, Takuya Fujiyoshi |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250125458
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| Publikation (Nr.) |
 EGU/EGU2016-5041.pdf |
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| Zusammenfassung |
| We present thermal infrared observations of the disk of Uranus at 17-200 μm to deduce its
global thermal structure and bulk composition. We combine 17-200 μm filtered
photometric measurements by the Herschel PACS and ISO LWS instruments and 19-35
μm filtered photometry and spectroscopy by the SOFIA FORCAST instrument,
supplemented by 17-25 μm ground-based photometric filtered imaging of Uranus. Previous
analysis of infrared spectroscopic measurements of the disk of Uranus made by the
Spitzer IRS instrument yielded a model for the disk-averaged temperature profile and
stratospheric composition (Orton et al. 2014a Icarus 243,494; 2014b Icarus 243,
471) that were consistent with submillimeter spectroscopy by the Herschel SPIRE
instrument (Swinyard et al. 2014, MNRAS 440, 3658). Our motivation to observe the
17-35 μm spectrum was to place more stringent constraints on the global para-H2 /
ortho-H2 ratio in the upper troposphere and lower stratosphere than the ISO SWS
results of Fouchet et al. (2003, Icarus 161, 127), who examined H2 quadrupole
lines. We will discuss the consistency of these observations with a higher para-H2
fraction than implied by local thermal equilibrium, which would resolve a discrepancy
between the Spitzer-based model and observations of HD lines by the Herschel
PACS experiment (Feuchtgruber et al. 2013 Astron. & Astrophys. 551, A126).
Constraints on the global para-H2 fraction allow for more precise analysis of the
far-infrared spectrum, which is sensitive to the He:H2 ratio, a quantity that was not
constrained by the Spitzer IRS spectra. The derived model, which assumed the
ratio derived by the Voyager-2 IRIS/radio-science occultation experiment (Conrath
et al. 1987 J. Geophys. Res. 92, 15003), is inconsistent with 70-200 μm PACS
photometry (Mueller et al. 2016 Astron. & Astrophys. submittted) and ISO LWS
photometric measurements. However, the model can be made consistent with the
observations if the fraction of He relative to H2 were increased from 15% to 20%.
We will discuss the improved consistency of this He VMR with estimates of its
abundance relative to hydrogen in the protosolar nebula (Asplund et al. 2009. Ann. Rev.
Astron. Astrophys. 47, 481) and the expectation that helium rainout in Uranus and
Neptune should be less important than in the atmospheres of Jupiter and Saturn. |
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