Characterising Saturn's vertical temperature structure from Cassini/CIRS

L. N. Fletcher*, P. G. J. Irwin, N. A. Teanby, G. S. Orton, P. D. Parrish, R. de Kok, C. Howett, S. B. Calcutt, N. Bowles, F. W. Taylor

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

63 Citations (Scopus)

Abstract

Thermal infrared spectra of Saturn from 10-1400 cm(-1) at 15 cm(-1) spectral resolution and a spatial resolution of 1 degrees-2 degrees latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and Southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-comective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified.

Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydro.-en fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40 degrees N.

The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to pora-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the Summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution. (C) 2007 Elsevier Inc. All rights reserved.

Original languageEnglish
Pages (from-to)457-478
Number of pages22
JournalIcarus
Volume189
Issue number2
DOIs
Publication statusPublished - Aug 2007

Keywords

  • CLOUD STRUCTURE
  • HUBBLE-SPACE-TELESCOPE
  • structure
  • EQUATORIAL REGION
  • VOYAGER INFRARED MEASUREMENTS
  • ROTOTRANSLATIONAL ABSORPTION-SPECTRA
  • PARA-HYDROGEN FRACTION
  • UPPER TROPOSPHERE
  • OUTER PLANETS
  • Saturn
  • HELIUM ABUNDANCE
  • THERMAL STRUCTURE
  • atmospheres
  • composition

Cite this

Fletcher, L. N., Irwin, P. G. J., Teanby, N. A., Orton, G. S., Parrish, P. D., de Kok, R., Howett, C., Calcutt, S. B., Bowles, N., & Taylor, F. W. (2007). Characterising Saturn's vertical temperature structure from Cassini/CIRS. Icarus, 189(2), 457-478. https://doi.org/10.1016/j.icarus.2007.02.006