The Depths of Saturn Revealed: Discrete Cloud Systems and Winds at the 2-Bar Level as Imaged by Cassini/VIMS

Details The Depths of Saturn Revealed: Discrete Cloud Systems and Winds at the 2-Bar Level as Imaged by Cassini/VIMS The Depths of Saturn Revealed: Discrete Cloud Systems and Winds at the 2-Bar Level as Imaged by Cassini/VIMS

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p23d..08b&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P23D-08

Physics

5700 Planetary Sciences: Fluid Planets, 5704 Atmospheres (0343, 1060), 5739 Meteorology (3346)

Scientific paper

We present imagery and analysis of discrete, deep-level systems of clouds as revealed near the 2-bar level in 5.1-micron thermal-infrared images obtained by the Visual Infrared Mapping Spectrometer onboard the Cassini/Huygens orbiter. At this wavelength, the clouds are seen silhouetted against the indigenous thermal radiation upwelling from depth. The thermal flux dominates reflected sunlight, so that the clouds and their movements can be readily observed on both the day and night sides of the planet. During four Saturn passes from February to July, 2005, high-spatial-resolution images were obtained at spatial scales from 225 to 750 km per VIMS pixel, revealing a variety of intriguing deep cloud features over a large range of spatial scales. Axisymmetric structures are significantly more heterogeneous and narrower in latitudinal breadth than observed at higher altitudes. Various shapes and wave-like structures are observed in the largest features with longitudinal dimensions exceeding 3000 km, including circular and oval features, donut-shaped structures, and swirls. Center-to-limb spectral analysis of several features as observed on both the day and night sides at diagnostic hydrogen and methane-absorbing wavelengths places these 5-micron-discrete clouds near the 2-bar level. Tracking such thermally-backlit clouds enables the determination of winds at near the two-bar level near where relatively large particles (> ~5 micron radius) reside within a major cloud layer, possibly composed of ammonium hydrosulfide or water condensates, but not ammonia. Comparison of these winds with those determined by tracking sunlit cloudtops residing near the 1-bar-level enables the estimation of vertical windshears. Specifically, direct comparison with contemporary cloudtop wind measurements by Cassini/ISS (Porco et al., 2005, Science 307, 1243-1247) indicates a strong wind shear exists at depth in the Equatorial Region, with winds increasing with depth from the stratosphere above the 100-mbar level downward to at least the 2-bar level. At the equator, when compared to 1-bar-level results of Porco et al. (2005), we find a tropospheric windshear of ~ 1.8 m/(s-km) in the 1-2-bar region, close to the 1.5 m/(s-km) value observed in the 1-2 bar region of Jupiter by the Galileo probe (Atkinson et al., 1998, JGR 103, 22,911-22,928). Outside the Equatorial Region, shears are significantly less.

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