Saturn's tropospheric composition and clouds from Cassini/VIMS 4.6-5.1μm nightside spectroscopy

Leigh N. Fletcher, Kevin H. Baines, Thomas W. Momary, Adam P. Showman, Patrick G.J. Irwin, Glenn S. Orton, Maarten Roos-Serote, C. Merlet

Research output: Contribution to journalArticle

54 Scopus citations

Abstract

The latitudinal variation of Saturn's tropospheric composition (NH3, PH3 and AsH3) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1μm thermal emission spectroscopy on the planet's nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn's cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn's NH3 spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH3 dominates the morphology of the VIMS spectrum, and high-altitude PH3 at p<1.3bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH3 is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH3 shows similar off-equatorial maxima at ±7° with a global abundance of 2-3ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH3 and upper-tropospheric PH3 and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH3, AsH3 and the lower cloud deck).

Original languageEnglish (US)
Pages (from-to)510-533
Number of pages24
JournalIcarus
Volume214
Issue number2
DOIs
StatePublished - Aug 1 2011

Keywords

  • Atmospheres, Composition
  • Atmospheres, Structure
  • Saturn

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Fletcher, L. N., Baines, K. H., Momary, T. W., Showman, A. P., Irwin, P. G. J., Orton, G. S., Roos-Serote, M., & Merlet, C. (2011). Saturn's tropospheric composition and clouds from Cassini/VIMS 4.6-5.1μm nightside spectroscopy. Icarus, 214(2), 510-533. https://doi.org/10.1016/j.icarus.2011.06.006