Mid-infrared spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 1. Determination of the mean temperature structure of the upper troposphere and stratosphere
On 2007 December 16–17, spectra were acquired of the disk of Uranus by the Spitzer Infrared Spectrometer (IRS), ten days after the planet’s equinox, when its equator was close to the sub-Earth point. This spectrum provides the highest-resolution broad-band spectrum ever obtained for Uranus from space, allowing a determination of the disk-averaged temperature and molecule composition to a greater degree of accuracy than ever before. The temperature profiles derived from the Voyager radio occultation experiment by Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987–15001) and revisions suggested by Sromovsky et al. (Sromovsky, L.A., Fry, P.A., Kim, J.H. [2011]. Icarus 215, 292–312) that match these data best are those that assume a high abundance of methane in the deep atmosphere. However, none of these model profiles provides a satisfactory fit over the full spectral range sampled. This result could be the result of spatial differences between global and low-latitudinal regions, changes in time, missing continuum opacity sources such as stratospheric hazes or unknown tropospheric constituents, or undiagnosed systematic problems with either the Voyager radio-occultation or the Spitzer IRS data sets. The spectrum is compatible with the stratospheric temperatures derived from the Voyager ultraviolet occultations measurements by Herbert et al. (Herbert, F. et al. [1987]. J. Geophys. Res. 92, 15093–15109), but it is incompatible with the hot stratospheric temperatures derived from the same data by Stevens et al. (Stevens, M.H., Strobel, D.F., Herbert, F.H. [1993]. Icarus 101, 45–63). Thermospheric temperatures determined from the analysis of the observed H2 quadrupole emission features are colder than those derived by Herbert et al. at pressures less than ∼1 μbar. Extrapolation of the nominal model spectrum to far-infrared through millimeter wavelengths shows that the spectrum arising solely from H2 collision-induced absorption is too warm to reproduce observations between wavelengths of 0.8 and 3.3 mm. Adding an additional absorber such as H2S provides a reasonable match to the spectrum, although a unique identification of the responsible absorber is not yet possible with available data. An immediate practical use for the spectrum resulting from this model is to establish a high-precision continuum flux model for use as an absolute radiometric standard for future astronomical observations.
Funding
We thank NASA’s Spitzer Space Telescope program for initial support of the data acquisition, reduction and its initial analysis, and we thank Tom Soifer for Director’s Discretionary Time on Spitzer (program #467). This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work from the Spitzer program was provided by NASA through an award issued by JPL/Caltech. Another portion of our support was provided to JPL/Caltech, from NASA’s Planetary Atmospheres program. J. Moses acknowledges support from NASA grants 25 NNX13AH81G, as well as older grants from the NASA Planetary Atmospheres program. L. Fletcher acknowledges the Oak Ridge Association of Universities for its support during his tenure at the Jet Propulsion Laboratory in the NASA Postdoctoral Program (NPP), together with the Glasstone and Royal Society Research Fellowships during his current tenure at the University of Oxford. F. J. Martin-Torres acknowledges support from the Spanish Economy and Competitivity Ministry (AYA2011-25720 and AYA2012-38707). During his contribution to this work, M. Line was supported by NASA’s Undergraduate Student Research Program (USRP). This research made use of Tiny Tim/Spitzer, developed by John Krist for the Spitzer Science Center. The Center is managed by the California Institute of Technology under a contract with NASA. The radiative-transfer calculations were primarily performed on JPL supercomputer facilities, which were provided by funding from the JPL Office of the Chief Information Officer. We thank Linda Brown, Helmut Feuchtgruber, Tristan Guillot, Mark Hofstadter, Kathy Rages, Larry Sromovsky and Larry Trafton for helpful and illuminating conversations, J. Schaefer for help in implementing dimer contributions into the H2 collision-induced opacity calculations, Emmanuel Lellouch and an anonymous reviewer for helpful co
History
Citation
ICARUS, 2014, 243, pp. 494-513 (20)Author affiliation
/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and AstronomyVersion
- AM (Accepted Manuscript)
